A mid-infrared imaging survey of star forming regions containing methanol and water maser emissions

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Title:
A mid-infrared imaging survey of star forming regions containing methanol and water maser emissions
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xv, 197 leaves : ill. ; 29 cm.
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De Buizer, James Michael
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Astronomy thesis, Ph. D   ( lcsh )
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Thesis:
Thesis (Ph. D.)--University of Florida, 2000.
Bibliography:
Includes bibliographical references (leaves 188-196).
Statement of Responsibility:
by James Michael De Buizer.
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Printout.
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Vita.

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Full Text







A MID-INFRARED IMAGING SURVEY OF
STAR FORMING REGIONS CONTAINING
METHANOL AND WATER MASER EMISSION














By

JAMES MICHAEL DE BUIZER


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA
2000


I


























Copyright 2000

by

James Michael De Buizer

























To Mom, Dad, and Michele














ACKNOWLEDGMENTS

This dissertation is the fruit of three years of research during which many people

have contributed, through their help, advice, or simply their friendship, to make this

experience fulfilling and enjoyable. To these people I would like to express my gratitude.

First and foremost I would like to thank my advisor, Robert Pifia, for his guidance

and support throughout this whole project. I am most appreciative of his willingness to

assist me no matter how busy he became. His knowledge and work ethic are only

superceded by his enthusiasm, and I thank him for being a good role model as well as a

great educator.

It has also been a pleasure to work with the Infrared Astrophysics Group. To this

group I am eternally grateful, for without their unselfishness and dedicated work, none of

the data presented in this dissertation would have been obtained. I would like to thank

Charles Telesco for his support and knowledge, as well as for having enough confidence

in my ability to make me an integral member of his observing and support staff. I would

like to thank those generous members of the group that gave up personal telescope time

so that I may collect data for my thesis project.

I would like to thank R. Scott Fisher and James "Rathgar" Radomski for their

help in keeping me sane on those long observing runs. I value their friendship, advice,

stimulating conversation, and childishness. It was a pleasure to share an office with them

for three years, travel with them all over the world, and party with them in Hilo and

Santiago.










I thank Jeff Julian and Kevin Hanna for keeping OSCIR purring, the secretarial

staff of Carlon Ann Elton, Glenda Smith, Debra Hunter and Audrey Sims for being my

mothers away from home, and fellow graduate students Veera Boonyasait and Sue

Lederer for their friendship and advice.

Last but not least, I wish to thank my family and my friends outside of astronomy.

I thank my parents and Michele Garnier for their love, support, and encouragement

throughout my long ten year college career. I also thank Tom Kessler, Justin Winn, and

Andy Kellenberger for their warm friendship.



This research was supported by the NASA Florida Space Grant Consortium grant

NGT5-40025 and the University of Florida.















TABLE OF CONTENTS

page

ACKNOWLEDGMENTS .................................................................................... ...iv

LIST O F TABLES ........................................................................................................ x

LIST OF FIGURES ......................................................................................................xi

A B STR A CT ...............................................................................................................xiv

CHAPTERS

1 MASSIVE STAR FORMATION.............................................................................. 1

Why is the Study of Massive Stars Important? .......................................................... 1
Giant Molecular Clouds and Cloud Cores..............................................................2
Low and Intermediate Mass Star Formation ..............................................................3
High Mass Star Formation.............................................................................. ........... 5
The Formation of HII Regions ................................................................................ 8
HII Region Morphologies........................................................................................ 10
Hot Molecular Clumps ........................................................................................... 13
Circum stellar D isks ................................................................................................. 13

2 BACKGROUND CONCEPTS IN PHYSICAL CHEMISTRY AND MASERS......... 15

Introduction to the Maser Phenomena............................. ................................. 15
Hydroxyl Molecular Transitions .............................................................................. 17
The H ydroxyl M aser................................................................................................. 19
Water Molecular Transitions......................................................................................20
The W after M aser..................................................................................................... 23
Methanol Molecular Transitions ..............................................................................23
The Methanol Maser................................................................................................26

3 MOTIVATION AND FORMULATION OF THE DISSERTATION ...................... 27

Summary of Observations of Masers in Massive Star Forming Regions .................. 27
Shock Fronts........................................................................................................ 28
O utflow s .............................................................................................................. 28
Circum stellar D isks .............................................................................................29
Em bedded Sources .............................................................................................. 30








Pumping Considerations....................................................... ............................ 31
M id-Infrared A stronom y ............................................. ........................ ....................32
A Case for the Present Work ................................................................................. 35

4 DATA ACQUISITION AND REDUCTION .......................................... ............. 37

IRTF Water Maser Observations........................................................................37
CTIO Methanol Masers Observations......................................................................40
D ata R education ................................................................................... . ...................42
Airmass Correction.................................................................................................42
Color Corrected Fluxes....................................................... .................................43
Resolved sources .............................................................................................43
Unresolved sources with good S/N ..................................................................44
Unresolved, low S/N sources ............................................................................45
Lum inosities ........................................................................................................46
Sources Observed in Only One Filter............................. .....................................47
Visual Extinction, Bolometric Luminosity, and Spectral Types...............................47
Adopted D istances... ................ ......................................... ........... .............................. 48

5 METHANOL MASER SELECTED SURVEY ....................................................... 53

Individual Sources.............................................. .....................................................54
G 305.21+0.21 ..................................................................................................... 54
G 305.20+0.21 ............................................................................. ............. .....55
G309.92+0.48 (IRAS 13471-6120) ................................... .................... 57
G318.95-0.20 (IRAS 14567-5846)..................... ..........................................60
G323.740-0.263 and G323.741-0.263 (IRAS 15278-5620).................................62
G 328.24-0.55...................................................... ........................................ ..... 64
G 328.25-0.53................ ................................................ ........ ........................... ..... 65
G328.81+0.63 (IRAS 15520-5234).......................................................... .........67
G331.28-0.19 (IRAS 16076-5134) ......................................... .................. 70
G336.43-0.26 (IRAS 16306-4758).................................. ...................................73
G339.88-1.26 (IRAS 16484-4603).... ..................................................................73
G340.78-0. 10 (IRAS 16465-4437)...................................................................... 76
G 345.01+ 1.79 ............................................................................................77
G345.01+1.80............................................................... ........ .............79
G351.42+0.64 (NGC6334F and NGC6334F-NW) ............................................ ..81
G 351.44+0.66 ........................................................... ............................................. 84
G351.77-0.54 (IRAS 17233-3606)................................................... ...................84
G9.621+0.196 and G9.619+0.193 (IRAS 18032-2032).......................................87
Results and D discussion .............................................. ......................................... ....91
Summary of Mid-Infrared Sources Associated with Linearly Distributed Methanol
Masers and the Circumstellar Disk Candidates.....................................................91
The Nature of Massive Stars Exhibiting Methanol Maser Emission ......................95
The Relationships between Mid-Infrared, IRAS and Radio Observations ........... 101
Alternatives to the Disk Hypothesis ....................................................................... 108



vii








6 W ATER MASER SELECTED SURVEY .......................................... ............... 111

Individual Sources ................................................................................................. 112
G00.38+0.04 (IRAS 17432-2835) ......................................................................... 112
G00.55-0.85 (IRAS 17470-2853)...................................................................... 113
G10.62-0.38 (IRAS 18075-1956)............................................................................ 116
G12.68-0.18 ....................................................................................................... 119
G16.59-0.05 (IRAS 18182-1433) ........................................................................ 121
G19.61-0.23 (IRAS 18248-1158)............................................................................ 122
G28.86+0.07 (IRAS 18411-0338)..................................................................... 125
G34.26+0.15 (IRAS 18507+0110) ........................................................................ 126
G35.20-0.74 (IRAS 18556+0136) ........................................................................ 129
G35.20-1.74 (IRAS 19592+0108)...................................................................... 133
G35.58-0.03 (IRAS 18538+0216)......................................................................... 135
G40.62-0.14 (IRAS 19035+0641)..................................................................... 137
G43.80-0.13 (IRAS 19095+0930).................................................................... 138
G45.07+0.13 .......................................................................................................... 140
G45.47+0.05 .................................................................... ................................. 142
G48.61+0.02 (IRAS 19181+1349)...................................................................... 145
G49.49-0.39...................................................... ........................................... 146
Results and Discussion ................................................................................................ 148
The Search for Embedded Sources and Outflows.............................................. 150
The Nature of Sources with W ater M aser Emission ............................................. 155
The Relationship Between Mid-Infrared IRAS and Radio Observations......... 159

7 CONCLUSIONS .................................................................................................. 163

Conclusions from the Methanol Selected Survey...................................................... 163
Conclusions from the W ater Selected Survey........................................................... 164
General Conclusions.............................................................................................. 166
Suggestions for Future W ork....................................................................................... 167

APPENDICES

A OSCIR .................................................................................................................. 170

Overview ..................................................................................................................... 170
The Journey of a M id-Infrared Photon...................................................................... 171
Extracting the Source Signal ....................................................................................... 175
The Standard Chop-Nod Technique...................................................................... 176
The Detector .......................................................................................................... 179

B DERIVATIONS OF IMPORTANT RELATIONSHIPS ......................................... 181

Color Correction Factor............................................................................................... 181
Resolution Limiting Size ............................................................................................. 183
Radio Flux Density as a Function of Lyman Continuum Photon Rate..................... 184


viii








LIST OF REFERENCES ........................................................................................... 188

BIOGRAPH ICAL SKETCH ..................................................................................... 197














LIST OF TABLES


Title Page

4-1. Positions of H20 maser reference features and mid-infrared detections ......... 39

4-2. Positions of methanol maser reference features and mid-infrared detections.... 41

5-1. Physical parameters derived from the mid-infrared observations .............. 92

5-2. The nature of the sources associated with methanol masers...................... 97

5-3. Total integrated flux density observed in the OSCIR field of view and
corresponding IRAS flux density measurements................................. 102

6-1. Observed and derived parameters for the mid-infrared sources................... 149

6-2. Total integrated flux density observed in the OSCIR field of view and
corresponding IRAS flux density measurements................................... 160

6-3. Radio continuum flux and derived spectral types.................................. 161















LIST OF FIGURES

Title Page

1-1. The four stages of intermediate mass star formation............................... 4

1-2. The evolutionary scheme of an expanding HII region....................... .... 8

1-3. Ionizing photon rate vs. diameter for compact and ultracompact HII regions..... 11

1-4. A schematic of the basic UCHII region morphologies ............................. 12

2-1. The OH molecule........................................................................ 18

2-2. The H20 molecule..................................................................... .... 21

2-3. The CH3OH molecule................................................................... 25

3-1. Infrared radiation and dust............................................................. 34

5-1. A three-panel plot for G305.20+0.21................................................ 56

5-2. A four-panel plot of G309.92+0.48................................................ 58

5-3. A possible mode of G309.92+0.48................................................... 59

5-4. G318.95-0.20......................................................................... 61

5-5. G323.74-0.26......................................................................... 63

5-6. G328.25-0.53......................................................................... 66

5-7. G328.81+0.63........................................................................ 68

5-8. G 331.28-0.19.......................................................................... .... 71

5-9. G339.88-1.26................................................................. ....... 75

5-10. G345.01+1.79........... ........................................................... 78

5-11. G345.01+1.80 ................... ......... ................. ...................... 80










5-12. G351.42+0.64 ...................................................................... 82

5-13. G351.77-0.54.............................................. ........................ 85

5-14. G9.62+0.19................................................................................... 88

5-15. Spectral energy distributions for six sources....................................... 100

5-16. Methanol maser distribution size versus physical IHW 18 extent ............ 105

5-17. Methanol maser distribution size for sources with and without UCHII
regions............................................... ........................................ 107

6-1. G00.38+0.04.............................................................................. 113

6-2. G00.55-0.85............................................................................... 114

6-3. G10.62-0.38............................................................................... 117

6-4. G12.68-0.18.............................................................................. .. 120

6-5. G16.59-0.05............................................................................... 122

6-6. G19.61-0.23........................................................................... 124

6-7. G28.86+0.07......................................................................... 126

6-8. G34.26+0.15............................................................................. .. 128

6-9. G35.20-0.74............................................................................... 132

6-10. G 35.20-1.74.......................................................................... .. 135

6-11. G 35.58-0.03......................................................................... .. 136

6-12. G40.62-0.14............................................................................. 138

6-13. G43.80-0.13............................................................................ .. 140

6-14. G45.07+0.13............................................................................ 142

6-15. G45.47+0.05............................................................................. 144

6-16. G48.61+0.02............................................................................. 146








6-17. G49.49-0.39 .................................................... 148

6-18. Separations between masers and mid-infrared and UCHII region source
peaks........................... ...... ..................................... 158

A-1. Atmospheric transmission ............................................................. 172

A-2. A cut-away view of OSCIR.......................................................... 173

A-3. OSCIR broadband filters.............................................................. 175

A-4. A schematic of the chop-nod technique........................................... 177

A-5. Image of OSCIR detector under uniform N-band illumination................... 180














Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

A MID-INFRARED IMAGING SURVEY OF
STAR FORMING REGIONS CONTAINING
METHANOL AND WATER MASER EMISSION

By

James Michael De Buizer

August 2000


Chairman: Robert Pifia
Major Department: Astronomy

This thesis presents the first mid-infrared imaging surveys towards massive star

forming regions. The first of the two surveys presented here is of 21 sites of massive star

formation associated with methanol masers. Recent radio observations of young massive

stars revealed that methanol masers tend to exist in linear arrangements. It has been

argued that these methanol masers exist in, and delineate, edge-on circumstellar disks. In

the mid-infrared survey conducted here, three sources were observed that are elongated at

the same position angle as their linear methanol maser distributions. It is believed that

these elongated mid-infrared objects are indeed circumstellar disks. Furthermore, for the

first time direct evidence has been found showing methanol masers arise inside the mid-

infrared emitting regions of young stellar objects, indicating that they may be pumped by

mid-infrared photons.








The second mid-infrared imaging survey presented here is directed towards 21

star formation regions associated with water maser emission. Water masers are generally

believed to be associated with shocks in outflow from young massive stars, however new

ammonia images have revealed the presence of 'molecular clumps' directly coincident

with water maser emission. It is believed that these clumps are extremely young

embedded stellar sources. The mid-infrared water maser survey presented here reveals

the detection of 5 possible embedded sources associated with water maser emission.

Though it is generally believed that masers are associated with massive stars,

radio studies have shown many sites of maser emission have no UCHII regions. The most

popular explanation for this is that the associated stars are too young to have ionized their

surroundings. For the stellar sources associated with methanol masers it was found that

this is most likely because they are in general lower mass, non-ionizing stars, rather than

being young. The detection of embedded sources in the water maser survey indicates, in

the case of water masers, stellar youth plays a role. Furthermore, a better coincidence was

found between water masers and mid-infrared sources, than radio continuum or near-

infrared sources.

Combined, these two surveys provide new insight into the roles masers play in the

development of massive stars, as well as provide much needed data concerning the

formation of massive stars in general.














CHAPTER 1
MASSIVE STAR FORMATION


Why is the Study of Massive Stars Important?

Massive stars are extremely important astronomical objects on both the smallest

and largest scales. From the moment they form photospheres, massive stars are violently

changing their environment. They produce copious amounts of UV photons which ionize

the interstellar gas around them. They can heat material out to large distances in the

clouds from which they formed, and in the process enrich the interstellar medium with

molecules through evaporation of dust grain mantles. They may have powerful outflows

which shock and churn the interstellar medium. Massive stars live fast and furious, and

end their lives just as spectacularly. They explode as supernovae, releasing as much

energy in the process as they had produced over their entire lives. Supernovae are

invaluable alchemists. They produce the heavy elements that are otherwise impossible to

create by normal stellar fusion processes. The force of their explosions spread these

heavy elements throughout large distances in a galaxy, enriching the interstellar medium

and thereby affecting later generations of stellar chemistry. These supernovae generate

disruptive shock waves, which may in turn spark the formation of the next generation of

stars. In this way, massive stars are responsible for the creation and distribution of

elements in a galaxy, and are therefore ultimately responsible for the chemical building

blocks necessary for the creation of other stars, planets, and life as we know it.








Giant Molecular Clouds and Cloud Cores

Star formation is known to exist in association with giant molecular clouds in our

galactic plane. Observations of giant molecular clouds (GMCs) have been performed,

mostly by tracing their CO radio emission (Blitz 1991). Comparisons of observed CO

emission of Dame et al. (1987) and FIR-selected potentially embedded O stars from

Wood and Churchwell (1989b) show that there are no locations of star formation in the

Galaxy where CO emission is absent (Churchwell 1991). It has been therefore plausibly

argued that stars must form out of condensations of interstellar matter within giant

molecular clouds.

The process by which the material in GMCs collapse into star forming regions is

largely thought to be governed by gravitation. A density enhancement in the GMC,

caused perhaps by the passage of a spiral density wave, gravitationally attracts

surrounding material. The gravitational attraction does not take place without resistance.

Shearing forces due to the rotation of the galactic disk about the center of the Galaxy try

to rip the cloud asunder. Thermal pressure in the gas and centrifugal forces due to

rotation of the coalescing material push outward. Magnetic field strength of the material

amplifies during collapse, and could become strong enough to halt contraction as well.

Ambipolar diffusion, a process by which neutral material slips past field lines in a lightly

ionized medium, has been widely recognized as a way to alleviate built up magnetic

stresses. While this may help in the earliest stages of collapse, it is not known exactly

how a highly magnetized gas will behave as stellar densities are approached. Even though

it is not exactly known how this build up is halted, it is believed that this and all of the

other competing internal pressure forces are not strong enough to combat the

gravitational attraction of the infalling material if the cloud has a sufficiently large mass.











Low and Intermediate Mass Star Formation

A scenario by which stars form can now be pieced together. Observations have

been well matched to the 4-stage low and intermediate star formation paradigm

developed by Shu et al. (1987). In this scenario, the first stage involves the formation of a

slowly rotating molecular clump from the molecular cloud in which it resides, as

described above (Shu et al. 1987; Figure 1-la). Since the inner regions of the clump must

support the weight of the outer layers, thermal (and magnetic) pressure increases towards

the center. The density therefore increases in the center as well. Once this clump becomes

sufficiently centrally concentrated, the thermal support in the dense interior eventually

fails. A consequence of this density configuration is that this interior region of the cloud

begins to collapse before the outer regions. It is therefore said that stars are formed via an

'inside-out' collapse (Shu 1977). The collapse occurs at the free-fall speed, and within

the boundary of an expanding rarefaction wave, which spreads outward at the local sound

speed. Initially, the material within this rarefaction wave has very little angular

momentum, and so the material falls directly onto the growing core. However, as this

boundary moves outward, material with much higher angular momentum from the

rotating cloud's equatorial region begins to fall towards the core. This material has

enough angular momentum that it misses the core, and instead reaches the core by

assuming a spiraling orbit. As more high angular momentum material becomes involved,

more material enters in an orbit around the growing protostellar core, creating a

circumstellar accretion disk. This is the second stage (Figure 1-lb) in the scenario of Shu

et al. (1987). This stage is characterized by the formation of the protostellar object at the













.* : ,' .

* ~ ~ ~ .. .. '".i.;.^"
.



a'

a


/ r \.'- d


C


Figure 1-1: The four stage of low and intermediate mass star formation (from Shu et al
1987). (a) Cores form within a molecular cloud. (b) A protostar with a surrounding
accretion disk forms at the center of a cloud core that has undergone inside-out collapse.
(c) A stellar wind breaks out along the rotational axis of the system, creating a bipolar
outflow. (d) The infall terminates, revealing a newly formed star with a circumstellar
disk.


center with a circumstellar disk, both deeply embedded in an infalling envelope of dust

and gas.

The accretion disk continues to feed the protostar causing it to increase in mass

as well as luminosity. Accretion continues until a stage is reached where the protostar

begins to develop a stellar wind. The stellar wind cannot break out in all direction from

the protostellar surface because of the pressure from material infalling directly onto the





5


protostar suppress this breakout. However, once material begins to preferentially infall to

the disk, rather than directly on the protostellar surface, stellar winds can break out at the

rotational poles. This creates what is called a 'bipolar outflow', which signals the third

stage of stellar evolution in the low-mass star formation paradigm (Figure 1-lc). The

material accreting from the disk prevents the stellar wind from breaking out in the

protostar's equatorial regions. Furthermore, this accretion disk causes the stellar wind to

be well collimated initially. Eventually, the angular extent of the outflow increases, and

as a consequence of clearing away the material nearby, the extinction to the central

source decreases. The final stage is now at hand, as the infall terminates and the central

protostar and its circumstellar disk are exposed (Figure 1-ld). This is referred to as the T-

Tauri phase of stellar evolution. From this point, the protostar proceeds along its pre-

main sequence (PMS) track in the Hertzsprung-Russell diagram toward the zero age main

sequence (ZAMS).




High Mass Star Formation

The formation of massive stars is thought to be similar to that for low-mass stars

as described by the scenario of Shu et al. (1987). If massive stars form via accretion, as

described in the last section, then it is a requirement that the accreting molecular material

be in rotation. Once a protostar reaches a mass >10 Mun, it would generate such a large

radiation pressure, that it would halt the collapse and reverse the infall (Garay and Lizano

1999). The only way to circumvent this is if the infalling material at this stage is in an

accretion disk so that this pressure may be released through the poles. Furthermore, the

accretion rate must be very high, with dM/dt >10-3 Msunyr, about 100 times higher than








that for low-mass stars. Only at these large accretion rates can the material create enough

ram pressure to overcome the radiative pressure on the dust and allow continuous

accretion onto the protostar. Hence if the scenario for low-mass star formation is to hold

in the high mass case, the massive protostars must have circumstellar disks.

Until recently, evidence for circumstellar disks around massive stars did not exist.

The evidence to date is still somewhat inconclusive. Lienert (1986) detected a -100 AU

elongated dust feature at near-infrared wavelengths around the massive star MWC 349A.

Molecular disks have been found by Zhang et al. (1998) around the high mass star IRAS

20126+4104, and G35.2N by Brebner et al. (1987). G35.2N is thought to be one of the

clearest examples of a bipolar molecular outflow surrounded by a molecular circumstellar

disk'. There are a few other candidates as well, but the main point is that evidence is still

weak, and there is, as yet, no conclusive observational evidence that massive stars form

circumstellar disks.

This lack of observational evidence is mostly caused by another facet of massive

star formation that is different from low-mass star formation. In the low mass star

formation paradigm, the infall terminates and the star can be seen in its T-Tauri phase as

it moves through the pre-main sequence (PMS) stage of its life. Massive stars, however,

spend their entire PMS lifetime, which is very short, still embedded in their circumstellar

dust and gas. It is very difficult to observe massive stars at this stage of formation

because of this. They enter the main sequence as actively accreting protostars (Yorke

1993).




SAs will be seen for this source, which is in the survey presented here, the molecular disk
may not be a disk at all (see page 132).








It is also possible that massive stars may not form via accretion, or at least not via

accretion alone. If the molecular cloud is composed of many molecular clumps, a clump

may gain mass through coagulation with other clumps (Larson 1992). It is believed that

massive stars form in close proximity with other massive stars, and in regions of

exceptionally high density at the centers of giant molecular clouds, along with less

massive stars. Interactions may exist between these dense pre-stellar clumps and

accreting protostars. Massive stars may also form via the merging of two already formed

less massive stars (Bonnell, Bate, and Zinnecker 1998; Stahler et al. 2000). In this case, it

is not known if circumstellar disks would survive or reform around the massive stellar

product. This is in stark contrast to the accretion paradigm of Shu et al. (1987) which, as

was stated, would require the existence of a circumstellar disk.

There could also be a two-stage process to massive star formation. A first stage

whereby dense cores or accreting protostars coalesce through mergers, and a second

whereby the massive stellar product accretes the material from its surroundings to reach

its final mass (Garay and Lizano 1999). In this two-stage process one would again expect

a circumstellar accretion disk to form.

During the pre-main sequence phase of massive stellar evolution, massive stars,

whether they formed via accretion, mergers or both, are surrounded by compact regions

of ionized hydrogen. This is the case for two reasons. The first is because massive stars

produce copious amounts of Lyman continuum (UV) photons. The second is that high

mass stars form in dense regions in molecular clouds that provide plenty of molecular

hydrogen that they can ionize. This is another unique feature of the high mass star









HI, no m-3 Expanding
HI, no mI shock HI, no m-'




*



Stationary
I front
Expanding
I fronl H[ n,> no Expanding
Sb I front
a b c

Figure 1-2: The evolutionary scheme of an expanding HII region (from Dyson and
Williams 1980). (a) The initial stages of the expansion of the ionization front. This
stage proceeds rapidly and thus the density of the ionized material, ni is about the same
as that in the surrounding ambient medium, no. (b) The ionization front continues to
expand, however the hot ionized gas begins to expand as well, creating a shock front
that moves out through the ambient medium. There is density enhancement in the
shocked region, i.e. ns >no. (c) The final stage where there is pressure equilibrium
between the ionized region and the ambient medium.



formation process that is not found in the low mass star formation paradigm of Shu et al.

(1987).




The Formation of HII Regions

When hydrogen burning begins in the core of a massive star, energetic photons

are created which ionize the molecular gas surrounding the star. These ionized regions

are also known as HII regions. The classical analysis of HII regions can be found in

Spitzer (1978) and Dyson and Williams (1980). In these analyses, the surrounding

molecular gas is assumed to be uniform in density and temperature, and one neglects the








effects of stellar winds. The energetic UV photons emitted by the star create an ionization

front that expands supersonically through the surrounding ambient medium, ionizing the

medium, but otherwise leaving it undisturbed. The expansion of the ionization front ends

when the total number of photoionizations in the ionized region equals the total number

of recombinations. At this point, the HII region has reached what is called the 'initial'

Strogren radius, Rs

1 2
N 1 c 3
R S=0.032.-- parsec
10 s- no

as given by Stromgren (1939), where Nuy is the rate of ionizing photons emitted by the

star, and no is the initial density of the ionized gas.

The heated gas in this region will begin to expand and form a shock front which

moves out through the neutral gas. Spitzer (1978) show that the rate of expansion of the

HII region is largely governed by the interaction between the shock and ionization fronts.

The radius, RI, of the HII region increases as

4
f( 7 c .t)
RI=Rs. 7cI'


where cl is the speed of sound in the ionized region, and t is time. This expansion ends

when the thermal pressure of the region reaches an equilibrium with the surrounding cool

ambient medium. This is given by the final radius of the HII region, RF

2

R Te
RF= 2e -RS
T 0








where Te in the kinetic temperature of the ionized gas and To is the temperature of the

ambient gas (Dyson and Williams 1980, Figure 1-2).




HII Region Morphologies

HII regions come in a variety of shapes and sizes. They are grouped into

classifications based upon their angular sizes (Habing and Isreal 1979). 'Ultracompact'

HII (or UCHII) regions are the smallest class, with typical sizes of about 0.1 pc or less,

and are characterized by very high electron densities on the order of 104-105 cm -3. The

next class is the 'compact' HII region, which has typical sizes ranging from 0.1 to a few

pc, with electron densities around 103 cm3. Though UCHII regions are thought of as the

earliest stages of the HII region, and in some cases they most certainly are, on average the

stars that excite UCHII regions are lower luminosity than those that excite compact HII

regions (Garay and Lizano 1999; Figure 1-3). In other words, just because a HII region is

small does not necessarily mean that it is extremely young. Finally, some HII regions are

very extended (many pc in size) and do denote a mature stage of HII evolution. This class

is referred to as 'extended' or 'classical' HII regions.

Compact and ultracompact HII regions do not always expand as perfect

Stromgren spheres. Wood and Churchwell (1989a) performed a radio continuum survey

of 75 regions of UC and compact HII regions and found that 20% of them have cometary

shapes, 16% have a core-halo morphology, 4% have a shell structure, 17% are irregular

or have multiple peaks, and 43% are spherical or unresolved (Figure 1-4). As can be see

in Figure 1-4 taken from Wood and Churchwell (1989a), often the term 'UCHII region'










1050





104"





1048











1048





1045





11 44 -


o o
0
a


o
o


A A)



0 0 08
ooo ) o a 01* ,





.0 1* o
U A O .,n 00




G 0
il i o"


K"


0 o 9 0

a0 M








it
U I

n


0.001 0.01 0.1 1 10
Diameter (pc)

Figure 1-3: Ionizing photon rate vs. diameter for compact and ultracompact HI regions
(from Garay et al. 1999). On the right axis is the number of ionizing photons emitted by
ZAMS stars with spectral types from B2 to 04. There is a general trend showing that
more compact radio sources come from less luminous stars.




and 'compact HII region' are simply referred to collectively as UCHII regions. This


nomenclature will be adopted throughout this work.


The morphology which has gained the most attention is the 'cometary' UCHII


region, which is identified as having a bright compact head and a diffuse extended tail.

There are two major hypotheses that are considered to explain the cometary morphology


O0








Ultracompact HII Region Morphologies

Cometary 20% Core-Halo 16% Shell 4%












Irregular or Spherical or
Multiply Peaked 17% Unresolved 43%


Figure 1-4: A schematic of the basic UCHII region morphologies (as seen in high
resolution VLA observations of Wood Churchwell 1989a). The spatial resolution of
the survey was 0.4 arcseconds.


of a UCHII region: champagne flow and bow-shock. Champagne flow or 'blister' models

assume the HII regions evolve in a media with a strong density gradient (Tenorio-Tagle

1979; Bodenhemer, Tenorio-Tagle, and Yorke 1979; Tenorio-Tagle, Yorke, and

Bodenhemer 1979; Bedijn and Tenorio-Tagle 1981; and Yorke, Tenorio-Tagle, and

Bodenheimer 1983). In this scenario the HII region expands preferentially towards the

lower density regions, and is ionization bounded on the high-density side and density

bounded on the low-density side, giving a cometary appearance. The bow-shock model is

one in which the star that excites the HII region is moving supersonically through the

molecular cloud in which it was formed. In this way the head of the cometary HII region

appears compressed due to the motion through the surrounding medium, and the tail is








diffuse and trails behind. Neither scenario has been able to explain all cometary HII

regions, but there are cases where one or the other scenario best fit the observations.



Hot Molecular Clumps

As previously discussed, massive stars are thought to be gregarious by nature, and

are often observed to form in associations. It has also been observed (e.g. Blaauw 1991)

that sites of massive star formation contain sources in various stages of development. For

instance, Cesaroni et al. (1994) observed four sites of UCHII regions in molecular

transitions of NH3. They found small structures (-0.1 pc), with kinetic temperatures

greater than 50 and up to 200 K, densities approximating 107 cm"3, and masses of a

couple hundred solar masses. These hot molecular clumps are believed to be the

precursors to massive stars. They lie within larger and less dense structures of molecular

material, and are observed via molecular line transitions. While they are found near

UCHII regions, not all UCHII regions have associated molecular clumps.




Circumstellar Disks

There are two types of phenomena most often associated with the name

'circumstellar disk'. The first can be lumped into the category 'debris disks'. These

objects include the disks around Fomalhaut, Vega, Beta Pictoris, Epsilon Eridani, and

HR4796A. These debris disks are around main sequence (A type) stars and have

comparable extents of 500 AU in diameter as seen in the submillimeter. The typical

dust mass in these disks is a few times the mass of the Moon (~1023 kg, Chandler and

Richer 1999). Another property of debris disks is that they are depleted of gas.








The other familiar circumstellar disk type are those found around pre-main

sequence, solar mass (so called 'T Tauri') stars and around low-mass protostars. These

types of disks can be grouped into a category called 'accretion disks'. These disks have

typical diameters around a few hundred AU as seen in the submillimeter (Chandler and

Richer 1999). Typical masses for these disks are on the order of a few x 0.01 Msun (-1028

kg, e.g. Beckwith et al. 1990, Osterloh and Beckwith 1995).

The disks that are of concern to this work are of the accretion disk variety.

However, if massive stars have circumstellar accretion disks, there would be major

differences between the disks around massive stars and those around solar mass stars.

First, dust disk sizes will be (as we will see with the results of this work) much larger

than their T-Tauri counterparts (- few thousand AU in diameter). This is a consequence

of the large increase in mass and luminosity between, say, a T Tauri star of spectral type

K and the early B type stars in this work. Rotating molecular disks have been found

around massive protostars stars with sizes ranging up to 10,000 AU in diameter (Cesaroni

et al. 1999, Zhang et al. 1998).














CHAPTER 2
BACKGROUND CONCEPTS IN PHYSICAL CHEMISTRY AND MASERS




Introduction to the Maser Phenomena

The word 'maser' is an acronym for microwave amplification by stimulated

emission of radiation. The predecessor to the laser, the first laboratory maser device was

constructed by Charles Townes in the 1954 (Gordon, Zeiger and Townes 1955). As the

name suggests, amplification of microwave radiation can be caused by stimulated

emission from excited molecules. Terrestrially, however, most molecules exist in their

ground state, and radiation striking them is usually absorbed, not amplified. The

fundamental feature of the maser is that in order to achieve this amplification, a majority

of the population of molecules involved must be in an excited state. This is what is

referred to as a population inversion. In more physical terms, the absorption coefficient is

expressed as

v = (Nh vBi/p)(l g/gjN/Ni)

and masering occurs if this is negative (more precisely if gi/gjN/Ni > 1). Negative

absorption means negative optical depth and a negative source function since the

emission coefficient is always positive. In cases of positive absorption coefficients,

strong spectral lines become saturated. In this case, the negative absorption coefficients

lead to strong lines which grow in strength rapidly as optical d epth increases. This leads








to very high surface brightnesses at the line frequencies and large luminosities from small

volumes.

Population inversions are definitely not a situation where local thermodynamic

equilibrium (LTE) is involved. But as is commonly observed, non-LTE situations are the

norm in interstellar clouds. Because the low densities and large dimensions in the

interstellar environment, line thermalization is rarely observed. However, to produce a

population inversion, some kind of mechanism is needed which has the net effect of

transferring molecules from their lower energy state to an upper metastable energy state.

These mechanisms are usually referred to as pumps.

Strong amplification through stimulated emission of radiation occurs naturally in

space as radiation propagates through an inverted medium. Maser gain is directly related

to column density. In other words, to get large maser gains, large column densities are

required. However, as was just discussed, interstellar volume densities are small. The

way in which this dilemma is reconciled is to have small density but large path length.

This creates another small problem. Because amplification is achieved by induced

emission, maser photons will only interact with molecules whose transition frequency has

not been Doppler shifted outside the linewidth. As observed through the large linewidths

of molecular lines, interstellar clouds exhibit quite a bit of internal motion. Therefore,

maser photons must seek a path through the interstellar cloud such that there is good line-

of-sight velocity coherence in order to achieve large gains.

Basic properties of all interstellar masers are: 1) maser lines are much narrower

and stronger than ordinary thermal lines, 2) interferometric observations show that maser

sources are comprised of many individual maser spots, each with its own well-defined








velocity, and 3) dimensions of maser spots are of the order of -1013 cm for masers in star-

forming regions.




Hydroxyl Molecular Transitions

The hydroxyl radical was the first molecule found in space in 1963 by Weinreb et

al. It also happens to be the first observed astronomical maser (Weaver et al. 1965).

Hydroxyl masers are frequently found with methanol and water masers. Almost every

source in this work has associated hydroxyl masers, so it is worthwhile to embark on a

discussion of the molecule in this work. It also is also a simpler molecule than water or

methanol, so it will be discussed first.

Even though hydroxyl is a simple diatomic molecule (Figure 2-1), it has an

unfilled electron shell and various internal spins, resulting in a complex energy level

structure. For background, we will depart into a discussion of the rotational spectra of

diatomic molecules. A diatomic molecule is symmetric around the inter-nuclei axis.

Because quantities are conserved around this axis, it has been given the convention of

being the z-axis. For instance, projections of the total orbital angular momentum (L) onto

this axis are conserved, and these projections are designated by Lz or A. The molecule has

defined states given by the quantized value of the projected angular momentum onto this

axis. If A = 0, the state is referred to as a E-state. If A = 1 it is a H-state, and if A = 2 it is

called a A-state. The ground electronic state of OH has a Lz = 1, so is therefore a n-state.

The total angular momentum J of any molecule excluding nuclear spin, is

comprised of the molecule end-over-end rotation K, the orbital angular momentum L, and

the electronic spin S. For a diatomic molecule, rotation is about the inter-nuclei axis and








a b
+


1720 1612
1665 1667


S" 2 3/2 (J = 3/2)


Figure 2-1: The OH molecule. (a) Model of the diatomic OH radical. The axis of
symmetry is shown (z-axis) by the dashed line. (b) Ground-state rotation levels for
OH. Shown are the four transitions of ground state OH that exhibit making. The + and
- show the two levels comprising the A doublet. Each doublet is split into two
hyperfine levels as well.


therefore one is not concerned with the end-over-end rotation, meaning that Kz = 0. The

value of the projected orbital angular momentum in the ground state is A = 1, so therefore

Lz = 1. Finally, the electronic spin for OH has a value of Sz = V2 where the signs can

refer to an "up" or "down" spin. Therefore, the total angular momentum excluding

nuclear spin for OH projected onto the z-axis is Jz = 1 /. The -sign therefore allows

there to be two rotation ladders in the ground electronic state: 2n1/ and 2H32. The

subscripts denote the value of Jz and the superscript denotes the two possible electronic

spin orientations.

The projection of L onto the z-axis can have a positive or negative sign for a

given A if A > 0. These two projections have nearly the same energy (to a first

approximation), so each A state is doubly degenerate. Quantum mechanically there are

two states, -Jz and +Jz, which represent the symmetric and anti-symmetric combinations

of the two projections. Interactions between the orbital angular momentum L and the

rotation of the nuclei N in the molecule leads to an energy difference between the -J, and


F=2
F=1


F=2
F= 1








+Jz states, thus splitting each energy level in two. This is referred to as A-doubling. The

upper and lower levels of the doublet are distinguished by their different parities, either +

or -. Because of the parity difference, radiative transitions between the A-doubled

sublevels of a given J level are permitted.

The picture is further complicated by the effects of the nuclear spin I, which

creates a hyperfine energy level structure. The nuclear spin I adds vectorally with the

total angular momentum without nuclear spin J to give the final overall angular

momentum F (F=J + I). A very weak splitting of levels is produced due to magnetic

effects caused by the nuclear spin. The nuclear spin on a diatomic molecule is given by Iz

= + /2, so therefore Fz = Jz+ Iz, or Fz = (1 /2) V2. Therefore each rotation state of OH is

split intofour levels.

The allowed transitions are that AF = 0, 1. For the 2 3/2 state there are 4 spectral

lines: two "main" lines with AF = 0, and two "satellite" lines with AF= 1 (Figure 2-1).

However F = 0 to F = 0 is forbidden. These four transitions are the most extensively

observed astronomical maser transitions of the OH radical at around the 18 cm

wavelength. The main line frequencies are 1667 MHz (F = 2 (upper) to F = 2(lower))

and 1665 MHz (F=l (upper) to F=l (lower)). The satellite lines are at 1720 MHz (F = 2

(upper) to F = 1 (lower)) and 1612 MHz (F = 1 (upper) to F = 2 (lower)).




The Hydroxvl Maser

The maser transition most observed in this survey is the 1665 MHz (X = 18 cm)

main line transition. The inversion occurs between the upper and lower levels of the A-

doublet. Unfortunately, the exact pumping mechanism of OH masers in star-forming








regions is not clear. Explanations through the use of both radiative and collisional

pumping mechanisms have proved to be problematic. For radiatively pumped sources,

the pump photon emission rate must exceed the maser photon emission rate.

Observations in the ultraviolet by Holtz (1968) and near-IR by Wynn-Williams et al.

(1972) proved that the central stars studied emitted far too few photons of these

wavelengths to explain the pumping. Far-IR photon emission rates, as derived from IRAS

(Infrared Astronomical Satellite), show that there are enough far-IR photons, but they are

from the cool dust that lies far away from the maser and central star. The region of far-IR

emission is very large and the resolution of IRAS so poor, that it is likely that the far-IR

photon rate at the maser region itself is smaller than the OH photon emission rate.

Collisional pumping is also problematic because in order to achieve the strong maser

emission observed by OH masers, densities would have to be so large that thermalization

of the levels of OH would occur, quenching the maser inversion. Pumping due to mid-IR

radiation has not been investigated fully. To date there is no final answer.

The most widely held belief is that the OH masers are situated in the enhanced

density environment around an HII region. This environment is located between the

shock and ionization fronts forming a compressed shell. This compressed shell is

expanding outward allowing for systemic motions of the OH molecules, and producing

the coherence needed for making.





Water Molecular Transitions

Discovered in interstellar space by Cheung et al. (1969), water is a planar

molecule with an axis of symmetry (Figure 2-2). However, it has three different moments









a U I
41-
07
252
J=7

41--
16





05
23 --
30 J=5


I 14
21 ---
J=4

212

J=3

12
J=2
10
01
J=1

Figure 2-2: The H20 molecule. (a) Model of the asymmetric top water (H20)
molecule. The axis of symmetry is shown (z-axis) by the dashed line. (b) H20 energy
level J ladders. Levels are marked with K+K- The "backbone" and its transitions are
heavily marked. The 22 GHz maser transition at 616-+523 is shown by the dark arrow.
This ladder pertains to ortho-H20 only (from Emerson, 1996).



of inertia for any set of axes chosen, and is therefore an asymmetric top rotor with a

complex level structure.

For background, a symmetric top rotor has the same moment of inertia about two

principal axes. The three moments of inertia about these three principal axes are labeled

la, Ib and Ic, with the convention that Ic > Ib > la. A symmetric top can have the two larger








moments of inertia equal (I, = Ib > Ia), which is called a prolate rotor, or it can have the

two smallest moments of inertia equal (Ic > Ib =Ia), which is called an oblate rotor.

Molecules with three different moments of inertia, like water, are asymmetric top

rotors. Nonetheless, it is quite often that the moments of inertia about two axes are close

in value and can be assumed to be a symmetric top rotor to a first order, with its levels

split to remove the remaining degeneracy. Each level of the rotational quantum number J

of an asymmetric rotor are split into 2J+l levels which are specified by the quantum

numbers K- and K+. The value K- represents the projection of the angular momentum

on the symmetry axis if the molecule was a prolate symmetric top rotor. The value K+

represents the projection of the angular momentum on the symmetry axis if the molecule

was a oblate symmetric top rotor. In this way, the levels of an asymmetric rotor are

labeled as JK-K+. The radiative selection rules for these molecules require Al = 0 or 1,

and that K- and K+ must change their evenness.

To further complicate things in the case of the water molecule, the two hydrogen

atoms both carry a nuclear spin which can combine to give a total nuclear spin of I = 0 or

I = 1. This leads to two distinct species of water molecules: ortho-H20 for I = 1 and (K-,

K+) = (odd, even) or (even, odd), and para-H20 for I = 0 and (K- K+) = (odd, odd) or

(even, even). These are treated as two distinctly separate species since they are

unconnected by radiative transitions.

The water masers observed in the survey presented in this work are the ortho-H20

616-- 523 rotational transition, which is the most commonly observed transition and

whose frequency is 22 GHz (X = 1.35 cm). This happens to be the transition first

discovered by Cheung et al. (1969).











The Water Maser

In the case of ortho-H20, transitions are permitted between adjacent levels within

a ladder and neighboring ladders. In some cases, the energy levels of neighboring ladders

are nearly equal in energy, and selection rules concerning a change of evenness in the K

quantum numbers can be obeyed, so a transition can occur. This is how the ortho-H20 616

-4 523 transition is believed to work.

The inversion process of this transition was first identified by de Jong in 1973. He

concluded that the bottom levels of each rotational ladder (lo0, 212, 303, 414, 505, 606, etc.)

formed what he called a "backbone" (Figure 2-2), which were connected through strong

radiative transitions which will therefore tend to be optically thick. In a single rotation

ladder, there will tend to be cascades down to the backbone level. In this way a backbone

level like the 616 level tends to have a higher population than the off-backbone level 523,

which is actually slightly lower in energy, thus producing the population inversion.

Like OH masers, the H20 maser pump mechanism is just as elusive. The most

widely-held belief is that H20 masers are created and pumped by shocks. One such

popular scenario is that water masers could be locally created and collisionally pumped

by the interaction of highly supersonic outflows from a central young star with clumps or

inhomogeneities in the surrounding cloud (Elitzur 1992a and Elitzur et al. 1989).




Methanol Molecular Transitions

Methanol was first detected in interstellar space during observations of the

Galactic center by Ball et al. in 1970. Over 200 spectral lines have been discovered to








date, lying in the frequency range between 834 MHz and 350 GHz. Of these, over twenty

are known to display maser emission and have been observed in regions of star

formation. The transitions of methanol are quite a bit more complex than OH or water.

It is an asymmetric top rotor, where the tetrahedron defined by the CH3 group can

rotate relative to the OH bond (Figure 2-3). However, this internal rotation is hindered by

mutual interactions between the CH3 group and the OH bond, thus creating a torsional

oscillation. This torsional oscillation produces an angular momentum about the internal

rotation axis defined by the CH3 tetrahedron. This angular momentum becomes strongly

coupled to the over-all angular momentum, and results in two torsional symmetry states.

These are labeled A and E.

One can consider A- and E-type methanol as two distinct molecular species since

selection rules require that transitions from one A-level can only be to another A-level,

and transitions from an E-level can only be to another E-level. Maser emission can occur

in either species. Rotation states in each of the two types are characterized by JK. Here

again J is the total angular momentum of the molecule, but in this case K is the

component of the angular momentum on the molecular symmetry axis. For the E-type

methanol, one uses K = ()k to designate the levels, where the quantum number k can

carry a positive or negative value. This is because the E-levels of methanol are doubly

degenerate. For A-type methanol, one uses K = Iki to designate the levels. This is because

the +K and -K A-levels are torsionally degenerate, splitting the K levels (except K = 0)

into degenerate pairs labeled A+ and A-.









Class I / E-Type Masers
37 GHz
7-


6-


5-


6-
4-

3- 5-


6-


5-


A-


5-

4-


3-
19 GHz
4- 3__2-
12 GHz 1-
3A2
1-


J = 1-

-2 -1 0 1
k


Class I / A-Type Masers


7- 6.
6+

6.7 GHz 5
j -5


0 1 2 3
k


Figure 2-3: The CH30H molecule. (a) Model of the asymmetric top methanol
(CH30H) molecule. Axis of symmetry is shown by the dashed line. This axis is defined
by the symmetry of the CH3 group. The CO bond is offset from this symmetry axis. The
axis of the hindered internal rotation by the OH group is shown with the double-headed
arrow. (b) Energy level diagrams for E-type (left) and A-type (right) methanol. Arrows
indicate the more important maser transitions of the Class II masers. The A-type
methanol levels are split into doublets too small to be shown here. Instead + and -
symbols are used to show the doublet.




Strong methanol maser emission was first discovered toward the Orion-KL region

by Barrett et al. (1971) in the series of Jk=2 Jk=l E transitions (J = 2,3,4,...) near 25

GHz. This work mostly concerns the 12 GHz 2o -- 3-1 E transitions of methanol which

were first discovered by Batrla et al. (1987) toward W3(OH) and several other regions, as


0e


I

iQl








well as the 6.7 GHz 51 -- 60 A' transition first discovered by Menten (1991) toward

several star-forming regions.




The Methanol Maser


There are certain characteristics peculiar to methanol masers which led to the

division of them into two distinct classes. The classification scheme was first put forth by

Batrla et al. (1987). It was observed that sites with maser emission from E transitions at

25, 36 and 84 GHz and A transitions at 44 and 95 GHz showed enhanced absorption at 12

GHz. These are now referred to as Class I sources. In contrast, other sites with maser

action from E transitions at 19 and 37 GHz and A transitions at 6.7 and 23 GHz showed

intense maser emission at 12 GHz. These are called Class II sources. Another

fundamental difference between Class I and Class II methanol masers is that the latter

ones are coincident with ultracompact HII/OH star-forming regions, while Class I masers

seem to be located further away at distances of the order of 1 pc from the UCHII region.

The methanol maser emission of interest in this work is obviously Class II. Methanol

masers, though frequently correlated with OH masers, are suspected to originate within

disks of material surrounding young stars (Norris et al. 1993).

Exactly how Class I masers are pumped and inverted is thought to be understood,

however Class II masers pumping and inversion remains a mystery. Whatever

mechanisms are devised must work in a way that suppresses Class I maser emission since

Class I and Class II maser are never found toward the same location.















CHAPTER 3
MOTIVATION AND FORMULATION OF THE DISSERTATION




Summary of Observations of Masers in Massive Star Forming Regions

As was discussed in the last chapter, maser emission is closely related to massive

star forming regions. In fact, it is commonly assumed that masers trace young massive

stars. However, advances in higher resolution imaging and accurate astrometry in recent

years has led to observations that show masers are often not directly coincident with

massive stars. So the question remains: If masers do not trace massive stars, what do they

trace?

The last chapter reviewed the current ideas on how each maser species was

thought to be excited, and gave the most popular scenario for each. The situation is not

that simple however. For each maser species, OH, water, or methanol, there are

observations that do not fit the scenarios given in the last chapter. In fact, recent

observations (and the work to be presented here) are teaching us that masers of each

species can be excited in a variety of ways.

The following subsections will review the objects and processes believed to be

associated with maser emission, and will give examples of each from the literature.








Shock Fronts

As discussed in Chapter 1, compact HII regions grow via ionization and shock

fronts. It has been suggested (e.g. Elitzur 1992b) that the cool, dense layer of gas between

the ionization and shock fronts provide a habitable zone for masers (OH masers, in

particular). Velocity coherence for the making material would be provided by the bulk

motion of the expansion. Elitzur and de Jong (1978) modelled the chemistry that would

occur in the regions between ionization and shock fronts. They found that water exists in

large quantities there, and that photodissociation of water molecules closer to the central

star would release oxygen which, in turn, would be efficiently channeled into OH.

The idea that masers can exist in a compressed shell around UCHII regions was

confirmed by observations showing a close association of OH masers with the outer

edges of several compact HII regions (Ho et al. 1983; Baart and Cohen 1985; Garay,

Reid, and Moran 1985; and Gaume and Mutel 1987). One source that Elitzur (1992b)

presents as an example of this situation is the object W3(OH). Furthermore, observations

by Menten et al. (1988) showed that methanol and OH masers are found occupying the

same outlying regions of W3(OH).

It seems reasonable that masers can indeed exist in shock fronts. They may

display linear or arc-like arrangements if this is the case.



Outflows

The bulk motion and relatively high density of molecular material caught up in a

well-collimated bipolar outflow from a young star may, in principle, be a good location

for maser emission. The masers taking part in the outflow will be distributed in linear

distributions pointing radially away from the parent star or UCHII region. Furthermore,








well-collimated outflows are generally high-velocity outflows. Therefore the masers

would have velocities similar to those found for other material taking part in outflows

(-100 km/sec). Such high velocities have been observed in water masers. For instance,

the detailed study of W51M by Genzel et al. (1981) shows water masers with velocities

as high as 157 km/sec.

Even if masers are not taking part in the outflow from a young stellar object, the

shock created by an outflow as it impinges on the ambient medium (or on knots of

material in the immediate vicinity of an outflowing star) also seems to be perfect

locations for masers (water masers, in particular). The high temperatures in shocks trigger

chemical reactions that produce large quantities of molecules like water (Elitzur 1992b).

Shock fronts are sheet-like, so in the shock plane one would expect coherent velocities

suitable for making. The arrangement of masers in this scenario could be linear, but in

general could be quite varied.



Circumstellar Disks

An exciting hypothesis for the location of masers is in circumstellar disks. In this

scenario, velocity coherence of the maser molecules comes from the rotation of the

material in the disk, and of course the material is sufficiently dense. Masers would be

preferentially observed in disks that are edge-on for two reasons. First there is a larger

column density through the plane of the disk than normal to it. This added column

density creates large maser gains producing easily detectable masers. Second, above and

below the plane of the disk there are most likely HII regions, created by the escaping UV

photons from the poles. These regions are optically thick to the maser emission, and

hence no maser radiation can get out in these directions.








There are several observations where masers seem to be located in circumstellar

disks. For instance, Brebner et al. (1987) observe a linear arrangement of OH masers that

lie in, and are at the same position angle as, a rotating molecular disk seen in ammonia

(NH3). Circumstellar disks have also been used to explain water maser observations in

several other sources (e.g. Torrelles et al. 1996; Slysh et al. 1999).

Methanol masers also seem to display signs of existing in disks. Radio results by

Norris et al. (1993, 1998) of more than a dozen 12.2 GHz (X = 2.5 cm) and 6.7 GHz (X =

4.5 cm) methanol maser groups (each group of maser spots being associated with a single

star) indicate a very strong preference for individual methanol maser spots to be located

along lines or arcs (size 1000 4000 AU). Furthermore, in some instances the velocities

of these features (determined from the Doppler shift of the maser line) show a gradient

along the distribution, indicative of rotation. Norris et al. (1993) concludes that the most

likely explanation is that the methanol maser spots are located in a circumstellar disks

that are viewed nearly edge-on.



Embedded Sources

The idea that masers are excited by embedded protostellar objects was originally

suggested by Mezger and Robinson (1968) for OH masers. Specifically, these masers

would exist in the accreting envelopes of massive protostars and be excited by the energy

from accretion shocks. This scenario became a legitimate explanation of the H20 maser

phenomenon when Turner and Welch (1984) discovered a massive molecular core in

HCN emission at the position of an isolated H20 maser group that exists 7" away from

the UCHII region W3(OH). While this scenario certainly has promise, observations








searching for hot molecular cores associated with embedded stellar sources have only

been done for a few H20 maser sites (Turner and Welch 1984; Cesaroni et al. 1994), but

with encouraging results.



Pumping Considerations

All of the above cases for maser location satisfy the criteria for velocity coherence

and large column density, but nothing was mentioned about how the masers are pumped

in each scenario. This is because observations are ahead of theory, and definite pump

mechanisms for each molecule in each situation simply have not been worked out. Below

is a qualitative discussion of some of the possible pump mechanisms in each scenario.

The shock front scenario would most likely involve collisional pumping. This

scenario has been perhaps the most theoretically worked out scenario (at least in the case

of water maser emission). The energy to pump the masers can be provided by the

collisional dissipation of the relative kinetic energies of the shocked and unshocked gas.

Quantitative discussion for the case of water masers can be found in Elitzur (1992b) and

references therein. At least for water masers, it appears that pumping by collisional

excitation of rotational levels behind dissociative shocks can provide an adequate

explanation.

In the outflow scenario, masers would most likely be radiatively pumped. The

masers would exist near the origin of the outflow, where material is most collimated and

can be heated by the outflow source. Masers in circumstellar disks would most likely be

radiatively pumped as well. The central source could heat out to large distances in the

disk if the disk is flared (i.e. thickness increases with increasing radius from the center;

see Hartmann 1998). For embedded sources, the pumping of the masers could be via the








collisional excitation from accretion shocks, or from the radiation from the protostar or

accretion disk itself.




Mid-Infrared Astronomy

It is difficult to ascertain what stellar process maser emission is tracing when it is

not known where the locations of the associated massive stars are with respect to the

maser spots. One way to observe the young massive stars associated with masers is to

look for and image the radio continuum from the UCHII regions produced by these stars.

However, surveys have discovered that many sites of maser emission have no detectable

radio continuum emission (e.g. Wood and Churchwell 1989a; Walsh et al. 1998).

Imaging at another wavelength is needed to find the associated young stellar sources.

Optical imaging is not useful because massive stars evolve so quickly that they

stay heavily embedded in their birth clouds, even after they reach the main sequence. In

the visible, dust absorbs and scatters light giving rise to significant extinction. This effect

is much less of a problem in the infrared. For instance, observations towards the Galactic

Center suffer 30 magnitudes of extinction in the visible, which is the equivalent of 1 out

of every 1012 photons reaching the observer. In contrast, in the mid-infrared the

extinction is only 1.2 magnitudes at 10 rim, or 33% of the photons reaching the observer,

and 0.6 magnitudes at 18 gim, or 58% of the photons reaching the observer (assuming the

interstellar extinction law of Mathis 1990). Therefore, because infrared radiation is much

less affected by extinction than visible radiation, infrared images can probe through the

cool obscuring dust enshrouding the massive stellar environment.








As one observes at longer and longer wavelengths in the infrared, one generally

observes objects with lower and lower temperatures. This can be explained in the ideal

case of a blackbody, whose temperature is dependent upon the peak wavelength it emits

radiation at, as given by the Wien displacement law

Tp = 2898

where pak is in mr and T in Kelvin. From this law it can be seen that a blackbody that

has peak emission at 10 am will have a temperature of 290K.

In star forming regions, thermal (heat) emission from dust (and dust

conglomerates such as planetesimals and planets) is the dominant source of radiation at

10 jm. Dust, such as circumstellar dust (a = 0.1 gim to a few gm), is very efficient at

absorbing radiation at short wavelengths (X < a) which in turn heats the dust to

temperatures up to a few hundred of Kelvin. Consequently, dust emits significant thermal

radiation at mid-infrared wavelengths. However, dust absorption can also be seen in the

mid-infrared as well. This occurs when there is a cooler layer of dust between the mid-

infrared source and the observer. These cool dust grains absorb the mid-infrared photons,

which heat the dust to a few tens of Kelvin. These cooler dust grains reemit this heat in

the far-infrared around 100 gm. Hence, in the extreme where there is enough cool dust

between the observer and the mid-infrared emitting dust, the mid-infrared radiation could

be undetectable.

Figure 3-la shows the process of dust emission schematically. Whittet (1988) has

produced a dust emission spectrum from observations near the galactic center where there

is ongoing star formation. This spectrum, shown in Figure 3-lb, has two peaks, one

corresponding to the warm dust around 10 gm, and the other to the cool dust around 100















MIR


,' Warm dust
a few x 1OOK
a MIR emissive


-4


(I
(%IJ
VI

E1E






0
-1


-6


-8


100


Cool dust
- a few x 10K
MIR absorptive


1000

Au(pm)


Figure 3-1: Infrared radiation and dust. (a) A schematic diagram showing the probable
locations of mid-infrared emission and absorption by warm and cool dust grains,
respectively in a massive star-forming region (from Fujiyoshi 1999) (b) The spectral
energy distribution of dust emission from the Galactic disk between longitudes 3 and 30
degrees. It clearly indicates the presence of cool and warm dust grains (from Whittet
1988).


I I I


..


r
~
, r
r r rr
r


UV


FIR
FIR


-5-


-7-








im, confirming the existence of mid and far infrared emitting dust near regions of star

formation.

Mid-infrared imaging presents a viable alternative to radio continuum imaging for

observations of massive star forming regions. Mid-infrared radiation traces the warm dust

close to the stellar sources, allowing one to observe the spatial relationship between

masers and young massive stars.




A Case for the Present Work

One of the fundamental questions in astronomy is How do stars form? Much has

been done both observationally and theoretically to understand the phenomena associated

with this question. However, despite the importance of massive stars, most of these

studies have focused on stars that are similar to our Sun in mass or smaller. It still

remains unclear exactly how massive stars form. Studies have led to a hypothesis for the

formation of low mass stars (Shu et al. 1987). This scenario is predominantly controlled

by accretion, with the end product being a new star surrounded by a circumstellar disk.

Nevertheless, while the scenario of Shu et al. (1987) has worked well to explain

star formation for low mass stars, it is not yet clear if it is applicable to massive star

formation. It is still not known if massive stars form by accretion or by stellar mergers.

Consequently, it is not known if the formation of circumstellar disks are a general

characteristic of massive star formation. Furthermore, young massive stars are often

associated with masers. We are only just now beginning to understand how masers of

different species relate to the physical processes that occur during the formation and

earliest evolutionary stages of massive stars.








The general goal of the work to be presented here is to try to determine the

relationship between masers and massive stars. In the process, it is hoped that more will

be learned about the properties and processes of massive star formation, and how they are

different or similar to their low-mass counterparts. The approach taken in this work was

to survey as many sources of maser emission as possible in the mid-infrared. No high-

resolution (<1") surveys of massive stars have ever been performed in the mid-infrared

until this work. As mentioned in the last section, mid-infrared images (5-25 gim)

penetrate the significant extinction in these regions and can trace the radiation from the

-200 K dust close to the stellar sources associated with the masers. Furthermore, a

resolution will be achieved that is on the threshold of being able to detect and resolve

circumstellar disks around these stars if they exist.

Since this work is the first survey of its kind, many questions may potentially be

answered: Are massive stars surrounded by disks? Do methanol masers trace these disks?

Do massive stars have a bipolar outflow phase, as is thought to be the case for low-mass

stars? Are the different maser species signposts of different evolutionary stages and/or of

different stellar processes? Do massive stars form in a highly clustered way? Are water

masers associated with young embedded stellar sources?

The survey is divided into two sections. The first concerns the methanol maser

selected survey performed in the Southern Hemisphere (Chapter 5). The second is a

discussion of the water maser selected survey that was performed in the Northern

Hemisphere (Chapter 6). A summary and concluding remarks concerning what has been

learned from both surveys will be presented in Chapter 7.














CHAPTER 4
DATA ACQUISITION AND REDUCTION




IRTF Water Maser Observations

All images of the water maser selected sites were obtained at the 3-meter NASA

Infrared Telescope Facility (IRTF). This observatory is situated on the peak of Mauna

Kea, an extinct volcano on the Big Island of Hawaii, at an elevation of 14,000 feet.

Observations were made using the University of Florida mid-infrared camera and

spectrometer OSCIR. This instrument employs a Rockwell 128 x 128 Si:As BIB (blocked

impurity band) detector array, which is optimized for wavelength coverage between 8

and 25 pm. The field of view of the array at IRTF is 29" x 29", for a scale of 0.223

"/pixel. Sky and telescope emission were subtracted out by using the standard chop-nod

technique. For more on OSCIR and mid-infrared observing techniques, refer to the

Appendix A.

There were two nights of observations, with the first occurring on 7 September

1997. That night, 21 maser sites were imaged centered on the H20 maser reference

feature coordinates (i.e. the brightest maser spot) given by Forster and Caswell (1989),

with 30 second chopped integration times (i.e. on source plus off source) taken through a

broad band N filter. This filter has a width of 5.1 p.m. The physical center of the filter is

at 10.8 pLm, but it has an effective central wavelength of 10.46 pm. These values are used

interchangeably throughout this work when the N filter is being referred to. There were








only 14 detections on this first night. The same objects were revisited on the second night

of observations which occurred on 10 September 1997, this time with the exposure times

were increased to 120 seconds. In addition to the N-band filter, images were also taken

using 120 second exposure times with an IHW18 filter (International Halley Watch 18

utm). This filter has a width of 1.7 pm, a physical center at 18.2 pm, but an effective

central wavelength of 18.06 u.m (again, these last two values are used interchangeably

here). Cirrus clouds terminated the second night of the survey after only the first nine

sites were imaged. Even though all the observations were taken at low airmass (<1.5),

cirrus clouds were unfortunately present most of the time on both nights. Cirrus presents

one of the worst terrestrial hazards for mid-infrared observations, because it efficiently

absorbs the mid-infrared photons from the target source, and at the same time emits

copious quantities of mid-infrared radiation itself. It can therefore be assumed that the

lower detection rate on the first night may have been due to low exposure times and/or

poor observing conditions.

The standard star used for the calibration of the sources was Gamma Aquila. The

flux densities for the star were taken to be 77.2 Jy at N and 25.6 Jy at IHW18 (from

stellar atmospheric modeling of Cohen 1999).

Table 4-1 lists the targets in this water maser selected survey. The coordinates

listed are the reference maser spot positions given by Forster and Caswell (1989), which

were obtained with a connected element interferometer and have absolute positional

accuracies of -0.5". Achieving accurate astrometry for infrared observations is not trivial.

Most telescopes employ visual guide cameras that point exactly where the telescope is

pointing. Once the telescope is moved near the position of the target it can be seen on the








Table 4-1: Positions of H20 maser reference features and mid-infrared detections.
Target Name H20 H20 Mid-IR Detection?
R.A. (1950) Dec. (1950) 09-07-97 09-10-97

G00.38+0.04 1743 11.23 -28 3434.0 N marginal
GOO.55-0.85 1747 03.83 -28 53 39.5 Y Y
G10.62-0.38 18 07 30.56 -19 5628.8 Y Y
G12.68-0.18 181059.17 -180242.5 N Y
G16.59-0.05 18 18 18.05 -1433 17.7 N Y
G19.61-0.23 182450.25 -115831.7 Y Y
G28.86+0.07 1841 08.28 -03 38 34.5 Y Y
G32.74-0.07 184847.81 -00 15 43.5 N
G33.13-0.09 184934.25 +000432.2 N
G34.26+0.15 185046.36 +01 11 13.9 Y
G35.03+0.35 185129.12 +01 5730.7 N
G35.20-0.74 185541.05 +013631.1 Y
G35.20-1.74 185913.24 +010913.5 Y
G35.58-0.03 18 53 51.38 +02 1629.4 Y
G40.62-0.14 190335.43 +064157.2 Y
G43.80-0.13 190930.98 +09 3046.8 Y
G45.07+0.13 191100.40 +104543.1 Y
G45.47+0.13 191145.97 +1107 02.9 N
G45.47+0.05 191204.42 +110411.0 Y
G48.61+0.02 1918 12.93 +134944.7 Y
G49.49-0.39 192126.32 +142441.8 N


visual camera, where it is centered in the crosshairs. Thus, the target will appear in the

center of an image taken by an instrument mounted on the telescope. However, because

the mid-infrared sources cannot be seen at visible wavelengths, one cannot center the

targets using the visual camera. Instead, the mid-infrared absolute astrometry is obtained

by offsetting the telescope to the maser reference coordinates from nearby reference stars

(typically about 10 arcminutes away) whose accurate coordinates were obtained from the

Hipparcos Main Catalogue. Using this technique, the error in the absolute astrometry is

given by the pointing accuracy of the telescope. For the IRTF, the absolute pointing

accuracy of the telescope was estimated to be about 1.0", determined by repeatedly








offsetting between reference stars. It was therefore concluded that the relative accuracy of

the astrometry between the maser reference features and the pointing is also 1.0" at the

IRTF.

Five point-spread function (PSF) stars were observed throughout the course of the

second night (no PSF stars were imaged on the first night). Error in the PSF FWHM was

taken to be the standard deviation of the size of the PSF stars imaged throughout the

night. A target object was considered to be resolved if the measured full width at half-

maximum (FWHM) was greater than three standard deviations from its closest PSF

FWHM. Marginally resolved objects are between 2 and 3 standard deviations of the PSF

FWHM.




CTIO Methanol Masers Observations

Observations of the methanol maser selected survey were carried out at Cerro

Tololo Inter-American Observatory (CTIO) on the Victor M. Blanco 4-meter telescope.

This observatory is situated on a mountain in the Chilean Andes at an elevation of 7100

ft. Once again the University of Florida mid-infrared imager/spectrometer, OSCIR, was

used. The field of view of the array at CTIO is 23" x 23", with a scale of 0.183"/pixel.

Sky and telescope emission were removed using the standard chop-nod technique.

The methanol maser sites were observed on the evenings of 1998 July 2 and 3,

under very clear skies with low relative humidity (<20%) and at low airmasses (<1.2).

Images were taken using OSCIR's broad-band N and IHW18 filters with 2-minute

chopped integration times, except for the G339.88-1.26 images, which were taken with 4-








Table 4-2: Positions of methanol maser reference features and mid-infrared detections.
Target Name Methanol Methanol Mid-IR
R.A. (J2000) Dec. (J2000) Dectection?

G305.21+0.21 13 11 13.72 -623441.6 N
G305.20+0.21 13 11 10.47 -623439.0 Y
G309.92+0.48 135041.76 -61 35 10.1 Y
G318.95-0.20 15 0055.40 -58 58 53.0 Y
G323.740-0.263 153145.46 -563050.2 Y
G323.741-0.263 153145.88 -563050.3 Y
G328.24-0.55 155758.38 -535923.1 N
G328.25-0.53 15 57 59.79 -53 58 00.9 Y
G328.81+0.63 155548.61 -524306.2 Y
G331.28-0.19 161126.60 -514156.6 Y
G336.43-0.26 16 34 20.34 -480532.5 N
G339.88-1.26 16 52 04.66 -4608 34.2 Y
G340.78-0.10 16 50 14.86 -4442 26.4 N
G345.01+1.79 165647.56 -40 1426.2 Y
G345.01+1.80 165646.80 -401409.1 Y
NGC 6334F 172053.50 -354701.7 Y
NGC 6334F-NW 172053.31 -354659.7 Y
G351.44+0.66 172054.67 -354508.5 N
G351.77-0.54 17 2642.55 -36 09 17.7 N
G9.621+0.196 180614.78 -203132.1 N
G9.619+0.193 180615.04 -203144.2 Y


minute chopped integration. The primary standard star for these observations was Eta

Sagittarius, for which the flux densities were taken to be 188 Jy at N and 66 Jy at IHW18

(Cohen 1999). However, Eta Sgr is a known variable star, so a secondary standard (Beta

Grus) was also observed, whose calibration correction agreed with that of Eta Sgr to a

few percent.

Table 4-2 lists the targets in this methanol maser selected survey. The coordinates

listed are the reference maser spot positions given by Norris et al. (1993), which were

also obtained with a connected element interferometer and have absolute positional

accuracies less than 0.5". The mid-infrared absolute astrometry was obtained the same








way as for the water maser survey, that is, by offsetting the telescope to the maser

reference coordinates from nearby reference stars (typically about 10' away). For these

reference stars, the coordinates were obtained from the Positions and Proper Motions

South Catalogue (PPM/South). The absolute pointing accuracy of the Blanco 4-m

telescope was estimated to be about 2.5", determined by repeatedly offsetting between

reference stars. It can therefore be concluded that the relative accuracy of the astrometry

between the maser reference features and the center of the mid-infrared field of view is

also 2.5". One further note of caution is that there were also occasionally large non-

reproducible offsets from 3 to 6" when the telescope was slewed from one reference star

to another. This may account for some of the larger offsets observed between the maser

reference positions and mid-infrared sources.

Point-spread function (PSF) stars were imaged near the positions of most of the

targets. As for the water masers, the error in the PSF FWHM was taken to be the standard

deviation of the size of the PSF stars imaged throughout the night. Likewise, a target

object was considered to be resolved if the measured full width at half-maximum

(FWHM) was greater than three standard deviations from its closest PSF FWHM.




Data Reduction

Airmass Correction

Measured fluxes for a source decrease as one observes them closer and closer to

the horizon, so one typically must make 'airmass' corrections. However, no airmass

corrections were made to the flux densities observed in either survey. It was found for

each night of observations that the airmass correction would be negligible, and therefore








none were applied. This is most likely due to the fact that the sources were observed over

a very limited range of airmasses.



Color Corrected Fluxes

Due to the large bandwidth of the filters (especially the N filter), the observed

fluxes for the sources must be 'color' corrected to account for the intrinsic source

spectrum, the filter transmission, and the atmospheric transmission. For the calibration

star, the spectrum was assumed to be a blackbody at its effective stellar temperature.

Color corrected monochromatic flux densities, dust color temperatures, and optical depth

values were obtained in a self-consistent manner by numerically integrating the product

of the Planck function (By), emissivity function (given by 1-e'v, where Ty is given by the

Mathis 1990 extinction law), filter transmission (Tamosphere), solid angle subtended by the

source with a measured size (a), and model atmospheric transmission (Tfiter) through the

filter bandpass (vi to v2). Because this procedure depends on source size, the method

employed for color correction depends on the whether or not the sources involved were

resolved or not.



Resolved sources

The general equation for flux in the manner described above is given by


FV= 2 ( e ) v( (T) T filterT atmosphere dv Eqution 1

where a is the source size. For resolved sources, the source sizes were taken to be the N-

band FWHMs subtracted in quadrature from the PSF FWHM. Since there are two

observed fluxes (N and IHW18), one has a situation of two equations for flux and two








unknowns, T and T, which can be solved for. Once one has a temperature for the program

source, the flux can be multiplied by the color correction, which is given by the factor



B vo(TSm) )-t fite()- dv

Equation 2

(1--e-o)-BV ) atm T filte v)- -dv
(l- Vo -B TP) h


where T' is the effective stellar temperature of the calibration star and TP is the

temperature derived above for the program source (Appendix B). This correction factor

takes into account the difference in spectral slopes between the calibration star and

program source through the filter bandpasses as well. These color-corrected fluxes are

then fed back into Equation 1 and solved for a new T and T, from which the fluxes are

color-corrected again, etc. This procedure is done iteratively until the values for T an T

converge to their final values, and a final color correction can be applied to the fluxes

yielding the final color-corrected fluxes listed in the tables throughout this work for

resolved sources.



Unresolved sources with good S/N

For unresolved sources, calculations of color-corrected fluxes were made using

the resolution limiting sizes and the blackbody limiting sizes, thereby yielding upper and

lower limits on the true fluxes. From the equation (Appendix B)

1

r=2-. ps 1
\Y PSf/










where, 6psf is the median size (FWHM) of the PSF star measurements throughout the

night of observations and Opsf is the standard deviation, one can get the resolution limiting

size 6r. The N-band resolution limiting size was chosen for the calculations. For the

methanol maser observations at the CTIO 4-meter 8r(N) = 0.55", and for the water maser

sources at IRTF 8r(N) = 0.26". These values were used for the source size, and color-

corrected fluxes were found using the same iterative procedure for resolved sources as

defined above.

The equation for the blackbody limiting size (or optically thick lower limit size),

we have the following equation for flux



F = .a b 2 -B (T bb)

Again, since there are two fluxes, there are two equations, so one can solve for the two

unknowns, abb and Tbb. This is merely the limit in which T goes to infinity in Equation 1.

Using the calculated Tbb, we can employ the same color-correction factor as above

(Equation 2) to iteratively solve for the color-corrected fluxes in the blackbody lower-

limit case.



Unresolved, low S/N sources

For extremely low S/N sources, one can not be sure what the sizes of the sources

are. Therefore the calculations were performed in the limits where the sources are

optically thick (blackbody limit) and optically thin. The blackbody limiting size








employed is the same as that defined in the last section. The equation for the optically

thin case is given by

F thin=() (T thin)

In this limit, one can only solve for the UT product and Tthin, where 0 is 7t-(a/2)2. One

again can iteratively solve for color-corrected fluxes using Equation 2.



Luminosities

Mid-infrared luminosities were computed by integrating the Planck function from

1 to 600 mrn at the derived dust color temperature and optical depth for each source, again

using the emissivity function 1-e"', assuming emission into 4n steradians, and using the

estimated distance, D, to the source


2L 4. 16D20*[0 I M ( e V) -B (T )d v


This equation is for resolved sources, and is different for the blackbody and optically thin

limits, since it is a function of source size, a. For the blackbody (optically thick) limit, the

extinction term drops out, and the source size used is abb. For the optically thin limit this

equation becomes

tin 600 (T thin)
Lthin=4-D ( T) Bv Tthin)dv
m1 pm


where i)r is the product found above.








Sources Observed in Only One Filter

The water maser selected survey was incomplete, with only one-third of the

sources being observed with the IHW18 filter. For these and all other sources that were

detected at only one wavelength, one can only report the observed flux densities, and

cannot estimate a color temperature, optical depth, or luminosity.



Visual Extinction, Bolometric Luminosity, and Spectral Types

Visual extinctions associated with the mid-infrared emitting dust were found for

the sources in the survey. These were calculated by using the derived emission optical

depth values at 9.7 jRm and the Mathis (1990) extinction law, which yields the relation Av

= 18.09-'9.7n. For instance, it was found that half of the sources in the methanol maser

selected survey have Av > 2.5 in the emitting regions. Thus, >90% of the visual radiation

from the star is absorbed by the surrounding dust and converted into mid-infrared

radiation, assuming 4n steradian coverage. However, depending on how deeply

embedded the sources are, some reabsorption of the mid-infrared photons by the dust

occurs, which further reprocesses them as far-infrared photons.

Combined with the assumption of 4n steradian coverage, the mid-infrared

luminosities derived from the mid-infrared fluxes can be considered reasonable lower

limits to the bolometric luminosities for these sources. Those estimates of the bolometric

luminosities were used to estimate zero-age main sequence spectral types for the sources

using the tables Doyon (1990), which are based on stellar atmospheric models Kurucz

(1979). However, because the mid-infrared luminosity measurements are lower limits,

the true spectral types of the sources are likely earlier than their calculated spectral types.








For the sources in this survey that have measured radio continuum fluxes, one can

derive radio spectral types to compare with the spectral types derived from the mid-

infrared observations. The Lyman continuum photon rates can be derived from the

standard equation for free-free emission (Appendix B)


14 a 0.e v lyc 1 -2
S =3.24.10 2.-.. mJy
0.99 04. -K \GH a 2 cm"3


where Sv is the flux density at radio wavelength v, Te is the electron temperature which is

taken to be 10,000 K from observations of typical HII regions (Dyson and Williams

1980), D is the distance to the source. The other parameters are a, which is a slowly

varying function of frequency and electron temperature that has values very close to

unity, and a2 is the recombination coefficient ignoring recombinations to the ground level

(Case B recombination), which has a value of 2.6x 10"3 cm3sec-1. This equation is solved

for Niyc, the Lyman continuum photon rate. From there the tables of Doyon (1990) were

used to find the spectral type corresponding to that Lyman photon rate. This spectral type

is a much more accurate estimate of the stellar spectral type because radio emission is not

as effected by dust extinction.


Adopted Distances

Finding distances to the sources in this study pose a few problems. The methods

available are crude and give only a general idea of how far these regions are from the

Sun. All of the distances given in this study are kinematic distances. These distances are

derived from some measurement of the radial velocity of the region in question. For

instance, radio recombination lines from UCHII regions may be used to determine their

radial velocities. Atomic transitions like HI, molecular line transitions like formaldehyde,








and even masers themselves can yield a radial velocity estimate for a region in space.

When this radial velocity information is combined with a model for the rotation of our

Galaxy, distances to sources may be determined. These rotation models for the Galaxy

are usually simple formulas that relate distance from the Galactic Center, R,, with orbital

velocity, E,. The model used in this work is the galactic rotation curve of Wouterloot

and Brand (1989), given by E,= Eo (R,/ Ro)00382, where Oo is the orbital velocity of the

Sun around the center of the Galaxy, which is taken to be 220 km/sec, and Ro is the Sun's

galactocentric distance which is taken to be 8.5 kpc. This particular rotation model was

chosen simply because it was oft quoted in the literature. The obvious errors associated

with this distance determination method are:

1. The distance will be dependent upon the Galactic rotation curve used. Most models

like the one used here are simple power laws, and do not reflect accurately the true

rotation of our Galaxy. From one rotation law to another, one may expect a difference

in the distance estimates to be as high as 1 kpc, in the extreme.

2. The values used for the Galactocentric distance and orbital velocity of the Sun will

affect the results. The present IAU accepted values are used in this work, however,

some researchers still use the older values of Eo = 250 km/sec and Ro = 10 kpc,

which are quite prevalent in the literature.

3. It is not known how accurately the radial velocities derived from atomic and

molecular transitions mimic the holistic velocity of the region. For instance, Forster

and Caswell (1989) used the radial velocities from OH masers to calculate the

distances to the associated regions. However, if the OH masers are participating in an








outflow or are tracing some other dynamic process, the radial velocity measured will

most certainly not be appropriate for determining the distance to the region.

Other uncertainties include small fluctuations due to turbulence, larger variations due to

peculiar velocities and the fact that rotation models do not account for velocity variations

due to galactic latitude and non-circular orbital motions.

Another problem arises when the source or region in question lies within the solar

circle. When this is the case, the distance to the source cannot be simply determined from

its radial velocity. If simple circular orbits are assumed around the Galactic center, a line

of sight will cross an orbit at two points with the same velocity but different distances

from the Sun. This leads to the kinematic distance ambiguity for sources within the solar

circle, as they may lie at either the near or far distance given by a radial velocity. The

only exception is when the source lies at a point in its orbit where it is tangent to the line

of sight. This is where the radial velocity for a source it at its maximum, and there is no

distance ambiguity.

There are some methods for determining the actual distance to a source. For

instance, for nearby stellar sources, one can determine a star's spectral type and UBV

flux. In this way, accurate spectrophotometric distances can be obtained. However, if one

only has a near and far kinematic distance, the distance ambiguity may be resolved in

three ways. First, if a HII region can be seen optically, it is believed to be evidence for it

being at the near distance. However, absence of optical emission does not necessarily

imply the far distance because the regions such as the ones in this survey suffer heavy

optical obscuration. Second, in the case of star forming regions, the galactic latitude can

be a helpful clue. Star-forming regions are mostly located in or near the galactic plane. If








a region has a galactic latitude greater than -0.50, it is most likely at the near distance,

otherwise it would be located too far out of the plane of the Galaxy. Third, absorption

components of radio spectral lines at smaller velocities than that of the radial velocity

determined for the region or source, means the near distance is most likely. For instance,

this method is employed by Kuchar and Bania (1990, 1994) using HI absorption towards

Galactic plane HII regions. They first make the reasonable assumption that the line of

sight to an HII region in the plane of the Galaxy will cross several HI clouds. The HI in

front of the HII region will absorb the thermal continuum from the HII region. The

distance ambiguity can be resolved by measuring the maximum velocity of the HI

absorption. The HI gas at higher radial velocity than the HII region will be behind the HII

region and will not contribute to the absorption spectra. Therefore the absorption

spectrum will only show absorption up to the velocity of the HII region (as determined

from recombination lines or masers). Likewise, absorption components with velocities

greater than the velocity at the tangent point is evidence for it being located at the far

kinematic distance. Forth and finally, one can make an argument based upon maser

luminosities, as outlined in Caswell et al. (1995a). The usual assumption is employed

that the maser emission beamed in our direction is representative of the intensity in other

directions, and can be considered quasi-isotropic. The peak maser luminosity is defined

as Fd2, where F is the peak maser flux density in Jy and d is the distance in kpc. Caswell

et al. (1995a) argues that the maser source in their survey with the highest flux density is

G9.62+0.20 at 5090 Jy. At the well-determined near distance (from absorption

measurements) of 0.7 kpc, its luminosity is 2500 Jy kpc2. The highest luminosity sources

in the survey of Caswell et al (1995a) are around 80,000 Jy kpc2. Some sources in our








survey can be excluded from the far distance because their maser luminosities would be

much larger than 80,000 Jy kpc2. For instance, G323.74-0.26 has a maser luminosity of

either 37,000 or 340,000 Jy kpc2 given the near and far distances, respectively. While

there is no theoretical reason precluding a maser achieving a luminosity as high as

340,000 Jy kpc2, it is certainly far outside the norm, and thus the source is most likely at

the near distance instead.

For the maser sources where the information was available, the HI absorption

observations of Kuchar and Bania (1990, 1994) or the formaldehyde absorption

measurements of Downes et al. (1980) were employed to determine whether to use the

near or far kinematic distance. For sources where this information is unavailable, one of

the other methods was used. The only source in methanol maser survey for which the

distance ambiguity could not be resolved was the source G331.28-0.19. For this source

the near distance is assumed. For the water maser survey, the results presented are not

strictly dependent upon the distances to the sources. For those water maser sources where

there is no data available to resolve the distance ambiguity, the near distance is assumed.

Since the determination of distances to these sources is fraught with error, the values

quoted throughout this work should be considered estimates only.














CHAPTER 5
METHANOL MASER SELECTED SURVEY



This chapter presents a mid-infrared imaging survey of 21 methanol maser sites.

This particular set of sites was chosen because they were observed in the radio methanol

maser survey of Norris et al. (1993). At the time the mid-infrared survey was conducted,

the sources in the Norris et al. (1993) survey were the only sites available with accurate

methanol maser positions (i.e. absolute astrometry better than 1"). Furthermore, OSCIR

was a facility instrument at CTIO at this time, and all of the sources are located in the

Southern Hemisphere.

The main thrust of the observations was to examine the theory of Norris et al.

(1993) that linearly distributed methanol masers exist in and trace circumstellar disks

around massive stars. Initially, this was to be accomplished by determining if the

methanol masers were situated at the exact location of the mid-infrared source peaks.

This is a necessary (but insufficient) condition if the methanol masers are tracing disks

around these embedded stars. It was known from radio continuum measurements that

water and OH masers generally are not coincident with UCHII source peaks, however no

radio continuum survey was performed for these southern sources at the time of these

observations. Only recently have most of these sources been observed for UCHII regions

by Phillips et al. (1998). Unfortunately, the pointing accuracy of the CTIO 4-m, and

hence the absolute astrometry of the mid-infrared measurements, was only accurate to








2.5". This disappointment was soon alleviated with the resolution of three circumstellar

disk candidates, a result that will be discussed in length in this chapter.

This chapter will begin with a discussion of each source observed. The first

section of this chapter will include what is known about each source from the literature,

and will describe briefly the mid-infrared observations of each. The second section

contains a detailed discussion of the analysis and results of the survey.


Individual Sources

This section will review the available information for each maser site and discuss

briefly the mid-infrared images that were obtained. The labeling convention for the mid-

infrared sources is given by the International Astronomical Union and is in the form

Glll.llb.bb:DPT00 #, where DPTOO stands for De Buizer-Pina-Telesco-2000 and the # is

the source number. For instance, the field of G309.92+0.48 has three mid-infrared

sources labeled G309.92+0.48:DPT00 1, G309.92+0.48:DPTOO 2, and

G309.92+0.48:DPT00 3. Often in this work these labels are shorten to "1", "2", and "3".



G305.21+0.21

This site contains both OH and methanol masers (Caswell, Vaile, and Forster

1995), and is located only 22" to the east of G305.20+0.21. One or both of these two sites

lie at the center of a large-scale CO outflow (Zinchenko, Mattila, and Toriseva 1995).

The outflow is elongated predominantly east-west, extending for 2'. No radio continuum

emission was found from this site by Phillips et al. (1998), with an upper limit on the 8.6

GHz peak flux density of 0.5 mJy beam'. Furthermore, Walsh et al. (1999) find no near-

infrared source at this site in any of their passbands (J, H, K and L).








Walsh et al. (1997) believe the site is located at either 3.9 or 5.9 kpc, due to

confusion associated with deriving kinematic distances based on methanol maser

velocities. This disagrees with the distances of Caswell and Haynes (1987), who derive

values of 2.9 and 7.0 kpc based on OH maser velocities. Caswell and Haynes (1987) also

suggest that this object lies at the near kinematic distance (because the region can be seen

optically), and therefore the near distance of Walsh et al. (1997) will be adopted for this

source.

Norris et al. (1993) detected a linear distribution of four methanol masers spots

from this site, with a clear monotonic spatial-velocity gradient. No source was detected at

this site in either the N or IHW18 filters. Table 5-1 (located at the end of this section)

indicates the 95% confidence upper limits for a point source detection.



G305.20+0.21

Figure 5-1 shows a strong 10 and 18 upm detection of a single, unresolved source.

This appears to be the same source observed by Walsh et al. (1999) in the near-infrared.

Norris et al. (1993) describe the distribution of masers here as compact, detecting only

two methanol maser spots at this site, almost coincident with each other. Like

G305.21+0.21, this site contains both OH and methanol masers. Phillips et al. (1998) find

no 8.6 GHz radio continuum coincident with this source with a 0.9 mJy beam-' upper

limit. The distance is assumed to be the same as for G305.21+0.21. This source has an

unusually high emission optical depth of T9.7. =1.0, giving it the highest emission optical

depth for any source in this survey and also the largest optical extinction at a value of

Av=18.1.









10.8 yim



0









10 5 0 -5 -10
Right Ascension offset (orcsec)


10
Right


masers


-0.1 F


18.2 ,m




+


5 0 -5
Ascension offset (orcsec)


0.1 0.0 -0.1
Right Ascension offset (orcsec)


-0.2


Figure 5-1: A three-panel plot for G305.20+0.21. The upper left is a contour plot of the
observed N-band image, the upper right is a contour of the observed IHW18 image, and
the bottom panel is the methanol maser spot distribution of Norris et al. (1993). The
large crosses in the upper panels represent the location of the maser reference feature
given our telescopic pointing. The thicker triangle in the lower panel denotes this maser
reference feature, whose coordinates are given in Table 4-2. The size of the cross
depicts the estimated error in relative astrometry between the reference feature and our
telescopic pointing. In the lower panel, triangles represent 6.7 GHz methanol maser
spots, and squares represent 12.2 GHz spots. The lower panel is on a larger scale for
detail.


" + .. II








G309.92+0.48 (IRAS 13471-6120)

This site contains both OH and methanol masers, as well as radio continuum

emission from a UCHII region. The OH and methanol spots are distributed along an arc

that spans 1.1" (Caswell 1997) and is concave to the southeast. The radio continuum peak

is at the center of the arc. Norris et al. (1993) consider this site to have a well-defined

velocity gradient in the 6.7 GHz methanol masers, and to be a disk candidate. Both

methanol and OH spots show a velocity gradient, with increasingly negative velocities

from north to south along the arc (Caswell 1997).

Walsh et al. (1997) find 9 methanol maser spots at 6.7 GHz (one more than Norris

et al. 1993) lying at a tangent point distance of 5.4 kpc, which is slightly different than

the distance of 4.7 kpc adopted by Norris et al. (1993). Walsh et al. (1997) also conclude

that if the region were powered by a single star, it would have to be an 05.5 star with a

luminosity of 3.1x105 L~un in order to explain the far-infrared (FIR) radiation observed by

IRAS.

Walsh et al. (1998) report methanol maser positions that are offset I" from those

of Norris et al. (1993). They also conclude that the methanol masers are slightly offset

(0.1") from their 8.64 GHz continuum peak. Their radio continuum observations show an

azimuthally symmetric UCHII region. Phillips et al. (1998) observed the same UCHII

region with an integrated flux density of 676 mJy.

The mid-infrared observations (Figure 5-2) show two resolved sources (1 and 2)

at a position angle of 530, both seen at 8.6 GHz by Phillips et al. (1998) and in the near-

infrared by Walsh et al. (1999). A third source that is only seen in the mid-infrared in N-

band to the west, has no associated radio continuum, but is seen in the near-infrared by











18.2 Am


0
-10^ 0
i ...... ... .. . 0 . i ,

10 5 0 -5 -10
Right Ascension offset (orcsec)

masers
0.4 i

0.2

0.0 A

-0.2
A
-0.4 A

-0.6 -

-0.8 I I
0.8 0.6 0.4 0.2 0.0 -0.2 -0.4
Right Ascension offset (orcsec)


10 5 0 -5 -10
Right Ascension offset (orcsec)


6 4 2 0 -2
Right Ascension offset (orcsec)


Figure 5-2: A four-panel plot of G309.92+0.48. The upper two, and lower left panels
are set up the same as they are in Figure 5-1. The lower right panel shows the filled
contours for our IHW18 image with the 8.6 GHz radio continuum image of Phillips et
al. (1998) overlaid. The radio peak was assumed to be coincident with the mid-
infrared peak. In the lower right panel, the methanol masers are shown as filled circles
and the size of the radio Gaussian restoring beam is shown in the upper right of the
panel. Notice that source 1 is elongated and arced in the opposite sense as the masers.



Walsh et al. (1999). At 18.2 mr, the central peak of source 1 is clearly arc-shaped and

concave to the northwest in the opposite sense in comparison with the methanol masers.


Relative astrometry between the radio continuum sources and masers in Phillips


et al. (1998) is stated as being better than 0.3". Assuming radio continuum peaks are


10.8 u/m










Larger Opening
Angle for Maser
/ Beaming


Cloe Warm
Dust




Figure 5-3: A possible model of G309.92+0.48. The black circle represents the massive
star. The disk is flared, so the disk surface is irradiated by the central star (dark gray), and
there is a large opening angle for the maser emission (light gray), increasing the
likelihood of maser emission to be beamed towards the Earth. The bright mid-infrared
emission would come from the close, warm dust near the inner edge of the disk (black)
and the disk surface (dark gray).


coincident with mid-infrared peaks, the mid-infrared images were registered with those of

Phillips et al. (1998) to obtain more accurate relative astrometry between the mid-infrared

source 1 and maser positions (Figure 5-2, lower right panel). This technique yielded the

result that the mid-infrared arc is located just southeast of the maser arc, and that they

have similar extent.

There are two possible explanations for the nature of G309.92+0.48:DPT00 1.

Since the masers are slightly offset from the radio continuum peak and follow the curves

along the contours of constant intensity, it is plausible that they might be tracing a shock

associated with the expanding UCHII region. An alternate explanation is that the arcs of

methanol and thermal emission extending from the northeast to the southwest may be








tracing out a nearly edge-on flared disk (Figure 5-3). The northwest pole of the rotation

axis would have to be tilted slightly away from us. In the mid-infrared, one would view

the warm emission associated with the irradiated disk surface (and perhaps the inner

hole). The maser photons, on the other hand, are coming through the flared edge of the

disk closest to the Earth, where the line of site is unobscured at radio wavelengths. Maser

photons beamed to us from the far side of the disk would not be seen, because they would

have to travel through the HII gas below the disk, and would be absorbed.



G318.95-0.20 (IRAS 14567-5846)

Methanol masers were discovered at this site by Kemball, Gaylard, and Nicolson

(1988) at the 12.2 GHz transition. Norris et al. (1993) show this site to have a linear

distribution of seven methanol masers with a reasonable velocity gradient along them

(Walsh et al. 1997 found 11 spots).

Walsh et al. (1997) find that the source may lie at kinematic distances of either

2.0 kpc or 10.9 kpc. Caswell et al. (1995) believe that the larger distance is more likely

because they find no optical counterpart to this object. However, if this is the case,

G318.95-0.20 would have one of the largest maser luminosities known to date (97,000 Jy

kpc2). Furthermore, given that the sources in this survey are highly embedded, one would

not expect to detect anything in the optical, so the near distance given by Walsh et al.

(1997) of 2.0 kpc is adopted here. Phillips et al. (1998) detect no 8.5 GHz continuum

detected at this site, with an upper limit peak flux density of 0.7 mJy beam'.






61



10.8 ,um 18.2 Lm

10 10
10


a 55
55


I o
C -5 C
o 0
S0 00


c -5- c
-10 -

-10
1 0 . . . . . . . . . .
10 5 0 -5 -10 10 5 0 -5 -10
Right Ascension offset (orcsec) Right Ascension offset (orcsec)


masers
0.3
A
0.2 A
4)
U
0.1 -



0
S-0.1
U


i -0.2 A

-0.3
0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3
Right Ascension offset (orcsec)



Figure 5-4: G318.95-0.20. Symbols and setup are the same as for Figure 5-1. This
source appears elongated in the north-south direction, similar to the maser distribution.
However, the PSF for this source is elongated in the same direction, and therefore the
elongation is not believed to be real.




Ellingsen, Norris, and McCulloch (1996) estimate this to be the site of a star with


a spectral type later than B2, as derived from their upper-limit non-detection of radio


continuum at 8.5 GHz, and using a distance of 2.0 kpc. It was found that a single mid-


infrared source is located here coincident with the near-infrared source seen by Walsh et








al. (1999), which is slightly elongated in the north-south direction, similar to the position

angle of the maser spots (Figure 5-4). However, this source looks similar to the PSF star

imaged near this position, and consequently the elongation in the source may not be real.



G323.740-0.263 and G323.741-0.263 (IRAS 15278-5620)

Like G318.95-0.20, Kemball, Gaylard, and Nicolson (1988) were the first to

discover methanol masers at this location in the 12.2 GHz transition. This site contains

two maser groups separated by 3.5" (Phillips et al. 1998). The western grouping,

G323.740-0.263, is the same observed by Norris et al. (1993), and the eastern maser

grouping, G323.741-0.263, contains four maser spots distributed non-linearly. Due to the

seemingly non-linear distribution of the western group of methanol masers, it is

categorized as complex by Norris et al. (1993). The spots of G323.740-0.263 are not

evenly distributed in a line or arc, and there is no clear velocity gradient. However, there

may be two distinct lines of methanol masers crossing each other. Most points can be fit

by two lines, with one at a position angle of -300, and the other -20.

Walsh et al. (1997) determined the distance to be 3.1 or 10.9 kpc. The near

distance is adopted here based on the fact that at the far distance the peak maser

luminosity would be abnormally large (340,000 Jy kpc2). Using IRAS FIR fluxes, Walsh

et al. (1999) determine that a single star would have a luminosity of 5.0x104 or 6.3x105

Lsun, with a spectral type of 08.5 or 05, respectively, for the two given distances. Phillips

et al. (1998) detect no radio continuum in this area with a 40 upper limit peak flux

density of 0.2 mJy beam'.


































0.3


0.2


0.0


-0.1


-0.2
0.2


10.8 /,m
'd---^,----------,-----r-



0323.740-0.263:
DPTOO 1





G323.741-0.263:
DPTOO 1



10 5 0 -5 -10
Right Ascension offset (orcsec)


masers
0.1
0.0
-O i ,

-0.2 A
A 3.6 3.5 3.4 3.3
A El
A A







0.1 0.0 0.2 0.3 0.4
Right Ascension offset (orcsec)


18.2 ,m


U
Q-

0

0
-0
-2
0
s> *


G323.740-0.263:
DPTOO 1





G323.741-0.263:
DPTOO 1


10 5 0 -5 -10
Right Ascension offset (orcsec)


5 4 3 2 1 0 -1
Right Ascension offset (orcsec)


Figure 5-5: G323.74-0.26. Symbols and setup are the same as for Figure 5-2. This site
contains both G323.740-0.263 and G323.741-0.263. The lower left panel shows the
masers for G323.740-0.263, and the inset shows the masers of G323.741-0.263 which
lie -3.5" to the east of G323.740-0.263. The lower right panel shows a filled contour
plot of the IHW18 image with the masers overlaid as circles. The methanol masers in
the western group are assumed to be coincident with the elongated western mid-infrared
source, G323.740-0.263:DPT00 1. To more clearly show the line of masers associated
with this source, the masers not in the linear distribution are shown as open circles.
These may be tracing an outflow. The masers associated with the western mid-infrared
source may also be tracing an outflow.



The mid-infrared observations of this site reveal a double source (Figure 5-5) that


was also imaged by Walsh et al. (1999) in the near-infrared. The western source, which


Walsh et al. (1999) detected only at L, is resolved and elongated in the mid-infrared at the








same position angle as the line of masers (300) in G323.740-0.263. Note that a majority

(12 of 17) of the 6.7 and 12.2 GHz methanol masers observed by Norris et al. (1993) at

this site lie along the 3000 line. The two distinct sites of methanol masers found by

Phillips et al. (1998) have relative offsets similar to the offsets between the two mid-

infrared sources (Figure 5-5, lower right panel). It can therefore be said with confidence

that the line of masers in G323.740-0.263 is coincident with the elongated western mid-

infrared source, in both location and position angle. The masers along this line may trace

out a circumstellar disk, while the other remaining masers at the position angle of -200

may be associated with an outflow (Phillips et al. 1998). The eastern mid-infrared source

is unresolved. It lies 3" away from the western source, which places the G323.741-0.263

masers seen by Phillips et al. (1998) just outside the mid-infrared contours of the eastern

source. This maser group may exist in an outflow associated with the eastern source.



G328.24-0.55

This site lies approximately 82.5" south and 13.0" west of G328.25-0.53, and is

assumed to be at the same kinematical distance. Walsh et al. (1997) determined the

distance to the source to be 2.3 or 12.1 kpc, which generally agrees with Caswell et al.

(1995) (2.6/11.8 kpc). Again Caswell et al. (1995) favor the larger distance because there

is no optical counterpart, however the site has a galactic latitude greater than 0.5.

Therefore the near distance of 2.3 kpc is adopted here. The methanol spots are not

distributed linearly, and according to Norris this source does not show a well-defined

velocity gradient along the spots. Phillips et al. (1998) detect an 8.6 GHz continuum

source at the location of the methanol masers with an integrated flux density of 27.7 mJy.








Phillips et al. (1998) suggest, among other scenarios, that since the masers are distributed

in two clumps lying on either side of the UCHII region they may be tracing the two edges

of a circumstellar disk, or two sources in a binary. No mid-infrared source was detected

at this location with a 30 upper limit on point source flux densities of 15 mJy at N and

179 mJy at IHW18.



G328.25-0.53

Walsh et al. (1997) find five masers at this site (same as Norris et al. 1993), and

infer from the IRAS FIR flux that this area would contain a star with a luminosity of

4.3x104 or 1.2x106 L,,, depending on the assumed kinematical distance (see G328.24-

0.55). This corresponds to a spectral class of 09 or 04, respectively (Panagia 1973).

Phillips et al. (1998) detect no 8.6 GHz radio continuum at this site with a upper

limit of 0.6 mJy beam-'. The survey presented here, however, yielded the detection of two

elongated low signal-to-noise sources at N, both of which are seen in the near-infrared by

Walsh et al (1999). There is an additional source only seen at IHW18. The IHW18

detections are even lower S/N than the N-band detections (Figure 5-6). Using the mid-

infrared astrometry alone, it is not clear which of the three sources the masers are

associated with even though the pointing places them closest to source 2. However, the

near-infrared imaging of Walsh et al. (1999) seems to confirm the association of the

methanol masers with the source corresponding to the mid-infrared source 2.










10.8 /m





1



2





10 5 0 -5 -10
Right Ascension offset (orcsec)


masers
1.0


0.5

A A
nn -A A


0.5 0.0 -0.5
Right Ascension offset (orcsec)


18.2 /Lm





1



2 3


10 5 0 -5
Right Ascension offset (orcsec)


-1.0


Figure 5-6: G328.25-0.53. Symbols and setup are the same as for Figure 5-1. All three
mid-infrared sources have low S/N. Sources 1 and 2 are resolved and elongated. Source 2
is most likely the mid-infrared source associated with the methanol masers.



The five methanol masers are not distributed in a single line, but are in two tight


groups separated by 0.8", and are not considered linearly distributed by Norris et al


(1993). Source 2 is elongated at a position angle of -30, and interestingly the western


group of three masers are best fit with a regression line at an angle of 27. One possibility

is that the three masers are tracing a disk, and the other two masers could be excited by


-10


' i . . . . . . . . .


-0.5 -


-1.0
1.0








an outflow or other stellar process. However, a maser grouping is not considered to be

linear in this work unless there are at least four points in the linear distribution.

Furthermore, it is unknown where exactly the methanol masers are with respect to source

2, and they may not be exactly coincident with the mid-infrared source.



G328.81+0.63 (IRAS 15520-5234)

This site contains OH and methanol masers, as well as prominent radio continuum

emission (Caswell 1997). Caswell (1997) believes that there are perhaps two separate

sites of maser emission, however they are only separated by 30 mpc, and the velocity

ranges of the masers overlap. This site contains OH masers at the 1612, 1665, and 1667

MHz transitions (Caswell, Haynes, and Goss 1980), as well as the 1720 MHz (MacLeod

et al. 1998) and 6.035 GHz OH maser transitions (Caswell and Vaile 1995). Methanol

masers have been seen at both the 6.7 GHz transition (MacLeod, Gaylard, and Nicolson

1992; Norris et al. 1993) and 12.2 GHz transition (Norris et al. 1987). Though the

methanol masers are distributed in a linear fashion, Norris et al. (1993) point out that

there is no coherent velocity gradient along the spots. The OH and methanol masers at

this site are intermingled spatially. Walsh et al. (1997) and Caswell et al. (1995a)

detected 6 methanol masers from this region (Norris et al. 1993 found eight).

Caswell (1997) and Norris et al. (1993) both believe this source to be located at

2.6 kpc away. MacLeod et al. (1998) made formaldehyde measurements which lead to a

near distance of 2.9 kpc, and a far distance of 11.7 kpc. These values are consistent with

the near and far values given by Walsh et al. (1997) (3.2/13.6 kpc). Caswell et al. (1995)

claim they prefer their larger kinematic distance of 11.9 kpc, because the site lacks an

optical counterpart, but again, this is likely due to large extinction at optical wavelengths.










10.8 /.m


10 5 0 -5 -10
Right Ascension offset (orcsec)


masers







A AA.


Q




-2
to

C,
S-2

0
c
Q,


3 2 1 0
Right Ascension offset (orcsec)


5 0 -5 -10
Right Ascension offset (orcsec)


4 2 0 -2 -4
Right Ascension offset (arcsec)


Figure 5-7: G328.81+0.63. Symbols and setup are the same as for Figure 5-2. By
overlaying the 8.6 GHz radio continuum image of Walsh et al. (1998, lower right panel),
we find that our astrometry as shown in the upper panels was off by almost 5". The radio
map traces our sources 1 and 3 well, however show no signs of source 2. Instead there is
radio emission from an unresolved source to the east, that we do not see in the mid-
infrared. Methanol masers are shown as filled circles. The methanol masers are
distributed linearly over the mid-infrared peak of source 2. The open square is an OH
maser from Caswell, Vaile, and Forster (1995). The size of the Gaussian restoring beam
for the radio continuum is shown in the upper right of the plot.



It would seem that the near distance is probably the correct distance given the argument

of Caswell (1997) who state that the far kinematic distance is unlikely in this case








because the source is greater than 0.5 from the Galactic plane. The near distance of

MacLeod et al. (1998) will be adopted here, based on the fact that formaldehyde is most

likely tracing the systemic motion of the molecular material in which the mid-infrared

sources reside.

Caswell (1997) categorize this as one of the brightest "steep-spectrum" sources in

the IRAS Point Source Catalog. This means that the spectrum of the source increases in

flux rapidly towards longer infrared wavelengths, and it may indicate an early epoch in

the formation of a massive star. They also observed G328.81+0.63 in 6 cm radio

continuum. Using the IRAS FIR fluxes and a distance of 2.9 kpc, they estimate the

spectral type of a single star in this area to be an 06.3, and using the 6 cm data they find a

spectral type of 09.3. Walsh et al. (1997) concluded that the IRAS FIR flux would yield

a 2.69x 105 Lsun star with a spectral type of 06, if there were only one star responsible for

the flux in this region. This is consistent with the findings of Caswell (1997).

Caswell (1997) finds an extended HII region with a flux density of 2 Jy, and an

unresolved companion UCHII region with a flux density of 360 mJy -3" to the east.

Walsh et al. (1998) detects both the compact and extended HII regions observed by

Caswell (1997). Their astrometry places the maser group to the west coincident with the

extended HII region peak, and the smaller maser group to the east coincident with the

unresolved UCHII region. Osterloh, Henning, and Launhardt (1997) present a K' (2.15

pm) image of this area. They determine it to be an embedded cluster of >4 stars sitting

behind a foreground star, which precludes them from assigning the objects accurate K'

fluxes. They also find evidence for CO outflow using the deviation from the Gaussian

profile in the C0(2-- 1) line.








Even contaminated with a foreground star in the K' image, the near-infrared

image of G328.81+0.63 looks similar to the mid-infrared images. However, with the

observations in this survey having a factor of 3 better resolution than the K' image and no

foreground star, one can distinguish six separate objects in the IHW18 image (Figure 5-

7). All six sources are not as prevalent in the N-band image (consistent with the IRAS

steep-spectrum). The two brightest sources (1 and 2 in Figure 5-7) are close to each other

but resolved (and perhaps a binary), and are coincident with the confused source at K' of

Osterloh, Henning, and Launhardt (1997). Once again the technique of registering the

mid-infrared images with radio maps was used to achieve better relative astrometry

between the mid-infrared sources and maser positions In this case, the radio maps of

Walsh et al. (1998) were used, and it was concluded that the western group of masers

overlap the mid-infrared peak of source 2 (the mid-infrared pointing was poor, as seen in

Figure 5-7), and are distributed at a similar position angle to that of the sources 1 and 2

(Figure 5-7, lower right panel). Sources 1 and 3 match the radio contours of the extended

HII region seen by Walsh et al (1998). However, there was no detection of any mid-

infrared object at the location of the compact HII region to the east, and it seems there is

either a suppression or no radio continuum emission whatsoever associated with the mi-

infrared source 2.



G331.28-0.19 (IRAS 16076-5134)

This site, according to Caswell et al. (1995b), is one of the few maser sites where

the strength of the 12 GHz methanol masers rival that of the 6.7 GHz. Norris et al. (1993)

find no coherent velocity gradient along the maser spots, though they are linearly









10.8 4.m














10 5 0 -5 -10
Right Ascension offset (orcsec)


0.0 F


-0.1 -


-0.211 I
0.2 0.1 0.0
Right Ascension offset (orcsec)


18.2 um


10 5 0 -5
Right Ascension offset (orcsec)


masers


Figure 5-8: G331.28-0.19. Symbols and setup are the same as for Figure 5-1. The
source appears elongated in the north-south direction, similar to the maser distribution.
However, the PSF for this source is elongated in the same direction, and therefore the
source elongation is assumed to not be real.




distributed. Whereas Norris et al. (1993) found nine 6.7 GHz methanol maser sources,

Walsh et al. (1997) has found twelve.

Walsh et al. (1997) find that the sources lie at either 4.8 or 10.1 kpc. Norris et al.

(1993) use the shorter distance, though they determined this distance to be 4.1 kpc. If this


__0


A
A








site lies at the distance of 4.8 kpc, Walsh et al. (1997) derive a luminosity of 1.9x105 Lun

for a single star, which would be a spectral class of 06.5. At the larger distance, a single

star would have a luminosity of 8.5x105 Lsun and have a spectral type of 05. The distance

of 4.8 kpc is adopted here.

Henning, Chan, and Assendorp (1996) find this to be a site of an interesting 21

im emission feature, as determined from IRAS LRS (Low Resolution Spectrometer)

spectra. It is not known what the carrier of the 21 [mn feature is, although the most

accepted theories at present are that they are due to PAHs (polyaeromatic hydrocarbons)

or PAH clusters (Hrivnak, Kwok, and Geballe 1994; Omont et al. 1995) or the inorganic

substance SiS2 (Goebel 1993). However, it should be noted that the LRS confusion limit

is 6', and these spectra may have very little to do with the source directly associated with

the methanol masers.

Phillips et al. (1998) find an amorphous shaped 8.6 GHz continuum source

(poorly imaged due to lack of short baselines for this observation) at the location of the

masers with an integrated flux density of 4.1 mJy. The mid-infrared images (Figure 5-8)

only show one source which is very slightly elongated north-south at a similar position

angle as the methanol masers. However, the PSF for this object appears similarly

elongated and comparable in extent. It was concluded that this mid-infrared source is

unresolved. It is also not clear if the masers or the radio continuum source are directly

associated with this mid-infrared source, given the fact that they are over 4" west of the

mid-infrared pointing center. Walsh et al. (1999) also fails to detect a source in the near-

infrared coincident with the masers, but finds a elongated source -2" to the west. This








elongated near-infrared source may be associated with the source seen in the mid-

infrared.



G336.43-0.26 (IRAS 16306-4758)

Norris et al. (1993) find this site to contain five 6.7 GHz methanol masers

arranged in an arc and displaying an strong systematic velocity gradient. However,

Caswell et al. (1995a) and Walsh et al. (1997) find eight methanol maser spots in all.

Caswell et al. (1995a) also did not find any evidence for OH emission from this area.

Furthermore, Phillips et al. (1998) detect no 8.6 GHz continuum here at a flux density

limit of 0.3 mJy beam'.

Walsh et al. (1997) determined the distances to this site to be either 5.9 or 9.9 kpc,

and so the near distance is adopted here. They give no estimate of the luminosity or

spectral types for a single star at these distances because the FIR fluxes from IRAS are

only given as upper limits. Walsh et al (1999) does not detect any near-infrared source

coincident with the masers. Likewise, no mid-infrared sources were detected at this

location, with upper limit flux densities of 14 mJy for N-band and 177 mJy for IHW18

with a confidence level of 95%.



G339.88-1.26 (IRAS 16484-4603)

Ellingsen, Norris, and McCulloch (1996) produced the first radio continuum

image of this site at 8.5 GHz with an integrated flux density of 14 mJy. Their image is

mostly unresolved but does show some extension in the northeast direction, which they

suggest may be a UCHII region with a cometary morphology. In contrast, Walsh et al.

(1998) present 8.6 GHz radio maps of the region showing a just unresolved, almost








circular source. However, there are slight elongations in the northeast and northwest

directions.

Ellingsen, Norris, and McCulloch (1996) consider this to be one of the strongest

sites of methanol maser emission at both 6.7 and 12.2 GHz. The maser spots of Norris et

al. (1993) lie across the radio source almost perpendicular to the position angle of the

continuum extension. Two OH masers are known to straddle the methanol masers in both

the north-south and east-west directions (Caswell, Vaile, and Forster 1995). An H20

maser is also known to exist -1" south of the methanol masers Forster and Caswell

(1989).

Caswell et al. (1995a) and Walsh et al. (1997) find there to be twelve 6.7 GHz

methanol masers here, whereas Norris et al. (1993) finds only 8. Norris et al. (1993) show

this source to have a velocity gradient (except for their maser spot 'a') in the 6.7 GHz

masers, whereas at 12.2 GHz there is no clear velocity gradient. Walsh et al. (1997)

determine the distance to this site to be either 3.1 or 12.9 kpc, and since the galactic

latitude is greater than 0.5, the near distance is adopted here. If this site were to contain

only one star, IRAS FIR fluxes would lead to a luminosity of 8.0x104 Lun or 1.4x106

Lun, with spectral types of 07.5 or earlier than 04, depending on distance.

This site was first imaged at 10 pm by the TIMMI instrument at the ESO 3.6-m

telescope in March of 1998 by Stecklum and Kaufl (1998). They found a 10 pm source

and assume that this is a circumstellar disk due to the elongation they observe, combined

with the apparent coincidence of the position angles of the elongation and the methanol

maser linear distribution. This elongation is also observed by Walsh et al. (1999) in the

L-band. The mid-infrared observations show this site to contain at least two sources in






75




10.8 .m 18.2 Am



5 0 -10
ao 2 2


0 -10
o 0
C C
o-1 -10
0o
c 2

-15

10 5 0 -5 -10 10 5 0 -5 -10
Right Ascension offset (orcsec) Right Ascension offset (orcsec)

masers
0.8 ''

0.6

0.4 A 2

0.2 A
C AA C
0 D
0.0 O
n S

o -0.2 -2

-0.4
0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 4 2 0 -2
Right Ascension offset (orcsec) Right Ascension offset (orcsec)

Figure 5-9: G339.88-1.26 Symbols and setup are the same as for Figure 5-2. Source 1
is clearly elongated at N, however there may be another source seen at IHW18 just to
the west. The lower right panel is a filled contour plot of source 1 at N with the 8.5 GHz
radio continuum image of Ellingsen et al. (1996) overlaid. Methanol masers are shown
as filled circles and are distributed at the same position angle as the mid-infrared
elongation of source 1. The radio continuum seems to be extended perpendicular to the
mid-infrared source elongation, and may be tracing an outflow. The OH masers of
Caswell, Vaile, and Forster (1995) are shown as open squares, and the water maser of
Forster and Caswell (1989) is marked by the open circle. The size of the Gaussian
restoring beam for the radio continuum is shown in the upper right of the plot.




the mid-infrared (Figure 5-9, sources 1 and 2). The more northerly source, 1, is the source

imaged by TIMMI and by Walsh et al. (1999), but actually may be a double source,








appearing clearly elongated in the northwest direction at N, and appearing as two

components at IHW18. The lower right panel of Figure 5-9 shows the radio continuum of

Ellingsen, Norris, and McCulloch (1996) and the methanol masers overlaying the mid-

infrared filled contour map of source 1 at N. It was assumed here that the radio peak

coincides with the peak at N. It is clearly seen that the elongation in the infrared is similar

to the position angle of the distribution of methanol masers. Furthermore, the ionized gas

is elongated in a direction almost perpendicular to the mid-infrared elongation, perhaps

tracing an outflow from the disk.



G340.78-0.10 (IRAS 16465-4437)

This is a site of OH masers, as well as methanol, and also contains a weak (9 mJy)

UCHII region at 6.0 GHz detected for the first time by Caswell (1997). The OH and

methanol masers found here overlap spatially and in velocity as well. Caswell (1997)

observed this site over a larger velocity range than Norris et al. (1993), and observed

more methanol maser spots. Caswell (1997) found this location to contain the largest

velocity spread (-112 to -86 km/sec) of any known maser site. Later observations by

Phillips et al. (1998) confirm this velocity range, and resolve 19 maser spots. They also

detect the weak UCHII region seen by Caswell (1997). It is argued by Caswell (1997)

that the kinematic distance to this site is unambiguously 9 kpc, because the systemic

velocity of the region indicates a location near the tangent point, so this distance is

adopted here.

The methanol maser spot morphology is very complex. It appears that the masers

lie in two distinct lines: one north-south and one east-west all contained within 1 square

arcsecond. Caswell (1997) suggests that this might be the site of two stars surrounded by








two toroids. Features in the north-south line within the velocity range of -92 and -86

km/sec have a velocity field more indicative of outflow, rather than rotation. Norris et al.

(1993) do not find any significant velocity gradient in either of the two linear maser

structures.

Like the near-infrared survey of by Walsh et al. (1999), the mid-infrared

observations yielded no detection of any sources. An upper limit on the point source flux

density of an N-band source is 15 mJy and 177 mJy for IHW18 with a 95% level of

confidence.



G345.01+1.79

This site has methanol and OH masers, and is coincident with a UCHII region

(Caswell 1997; Caswell et al. 1995a). Walsh et al. (1997) determine the distance to this

site to be either 2.1 or 14.5 kpc. This is in fair agreement with Caswell (1997) who

derives distances of 2.6 and 13.9 kpc. Since this site has a galactic latitude greater than

0.50, the near value of Walsh et al. (1997) of 2.1 kpc seems reasonable. Using the

distances of Walsh et al. (1997), this would yield a single star of 8.5x 04 or 4.0x 106 Lsn,

and a corresponding spectral type of 07.5 or < 04, depending on which distance was

chosen.

The methanol masers here are distributed in a linear fashion and show a

moderately systematic velocity gradient among the spots Norris et al. (1993). They seem

to be offset from the peak of the radio continuum by approximately 0.5" to the west. The

UCHII region has an integrated flux density of 260 mJy (Caswell 1997). Walsh et al.

(1998) confirm the displacement of the masers with their 8.6 GHz radio map of the area,






78


10.8 /um 18.2 / m

1010

(U U



0
o a 5


0o 0


C -5 C
U -5

-10
-10
10 5 0 -5 -10 10 5 0 -5 -10
Right Ascension offset (arcsec) Right Ascension offset (orcsec)

masers
0.3 2

0.2 -

o 0.1 0

Z: 0.0 0
0.0 0
o C
-0.1 I

0 -0.2 )

-0.3 I I I -2
0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 2 1 0 -1 -2
Right Ascension offset (arcsec) Right Ascension offset (arcsec)

Figure 5-10: G345.01+1.79. Symbols and setup are the same as for Figure 5-2. The
source appears elongated at IHW18. However, the PSF for this source is elongated in
the north-south direction, and therefore the source elongation may not real. The lower
right panel is a filled contour plot of the IHW18 image with the 8.6 GHz radio
continuum image of Walsh et al. (1998) overlaid. Methanol masers are shown as
filled circles and seem to be distributed radially to the radio continuum, and
perpendicularly to the mid-infrared elongation. If the elongation is real, the methanol
masers may be tracing an outflow. The OH masers of Caswell, Vaile, and Forster
(1995) are shown as open squares. The size of the Gaussian restoring beam for the
radio continuum is shown in the upper right of the plot.



and conclude that the masers extend radially with respect to the circular UCHII region.

These masers could be indicative of outflow. The two OH masers lie above and below the


methanol maser distribution.








Walsh et al. (1999) observed this site in the near-infrared and find a single source

visible only at L. A single bright mid-infrared source (Figure 5-10) was found here

coincident with the near-infrared source, that may be marginally resolved at IHW18. It is

appears slightly elongated in the northeast direction, at a position angle that differs from

the methanol maser position angle by about 500. However, the two OH masers lie at a

position angle close to that of the mid-infrared elongation. The PSF image taken at this

location is elongated north-south, so it is uncertain if the source is truly elongated. If the

source elongation is real, this is a large discrepancy in position angle between the

methanol maser distribution and mid-infrared elongation angle. Given the fact that these

methanol masers are offset from the center of the UCHII region by 0.5" and distributed

radially, they may be tracing an outflow, as suggested by Walsh et al. (1998), and the OH

masers may be coming from a disk in the perpendicular direction whose elongation may

be marginally resolved in the mid-infrared image.



G345.01+1.80

This site lies only 19" away from G345.01+1.79. Caswell (1997) finds no UCHII

region at 6 GHz, and neither does Phillips et al. (1998) with an upper limit on the peak

8.5 GHz continuum emission of 0.7 mJy beamr'. Walsh et al. (1999) also failed to detect

a source at this location in the near-infrared. Interestingly, this site has also been found to

contain no OH maser emission. Caswell (1997) confirm the Norris et al. (1993)

observations of the existence of a linear distribution of four methanol maser sources,

spread over 0.22" with three of the four spots showing a velocity gradient. The same

kinematic distance to this region is adopted as for G345.01+1.79.










10.8 Am
















10 5 0 -5 -10
Right Ascension offset (orcsec)


0.3

0.2

0.1

0.0

-0.1

-0.2

-n i


18.2 fm


10 5 0 -5 -10
Right Ascension offset (orcsec)


masers


0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3
Right Ascension offset (orcsec)


Figure 5-11: G345.01+1.80. Symbols and setup are the same as for Figure 5-1. The
source has very low signal-to-noise.




The mid-infrared images reveal the discovery of a weak mid-infrared source at a

low S/N at both N (S/N 4) and IHW18. Because of the low S/N, nothing can be said

about the morphology of the source other than it looks at this detection level to be point-


like (Figure 5-11).


A








G351.42+0.64 (NGC6334F and NGC6334F-NW)

G351.42+0.64 is the location of a well-known cometary-shaped UCHII region

named NGC6334F (Rodriguez, Canto, and Moran 1982). The methanol masers are

known to exist here in two sites separated by about 6" (Norris et al. 1998).

The southern group of methanol masers is associated with NGC6334F and are

accompanied by OH masers, which run in a north-south line (Caswell 1997).

Approximately 4" north of the OH masers, water masers are known to exist in a linear

pattern, pointing radially north away from the UCHII region peak (Carral et al. 1997).

The distribution of methanol masers in NGC6334F does not appear to be linear, but does

have a similar velocity range to that of the OH masers (-13 to -6 km/sec; Caswell 1997,

Caswell et al. 1995a), and does not display a systematic velocity gradient (Norris et al.

1993).

Caswell (1997) find no 6 GHz radio continuum (same as Ellingsen, Norris, and

McCulloch 1996) at the northern methanol maser site, NGC6334F-NW. Also,

NGC6334F-NW does not have any associated OH or water masers. Again, there is no

velocity gradient along the spots in the northern group.

Walsh et al. (1997) determined a distance to this site of either 1.9 or 12.9 kpc.

This near distance is slightly different than the accepted value of 1.7 kpc, photometrically

found by Neckel (1978). At the distance of 1.9 kpc, Walsh et al. (1997) determine this

site would contain an 07 star with a luminosity of 1.1x105 Lsun, if it contained one star.

The more accurate photometrically-derived distance of 1.7 kpc for this source will be

used here.









10.8 am


10 5 0 -5 -10
Right Ascension offset (orcsec)

masers

NGC6334F-NW








&A NGC6334F






1 0 -1 -2 -
Right Ascension offset (orcsec)


10





0
cr


0
0


-5
c
Q


18.2


10 5 0 -5
Right Ascension offset (orcsec)


10 5 0 -5
Right Ascension offset (orcsec)


Figure 5-12: G351.42+0.64. Symbols and setup are the same as for Figure 5-2. Our
source 1 is coincident with the UCHII region NGC 6334F, which is also known as IRS1
(Harvey and Gatley 1983). Source 3 is known as NGC 6334F-NW, or IRS2. Source 4 is
also known as IRS3. The lower right panel is a filled contour plot of the IHW18 image
with the 6.7 GHz radio continuum image of Caswell (1997) overlaid. Methanol masers
are shown as filled circles. The OH masers of Gaume and Mutel (1987) are shown as
open circles, whereas the OH masers of Forster and Caswell (1989) are shown as
crosses. The OH masers and methanol masers seem mixed both spatially and in
velocity, and most likely trace the shock associated with the sharp western boundary of
the expanding UCHII region. The methanol masers near source 3, are not coincident
with the mid-infrared source, and may be tracing an outflow. The water masers of
Forster and Caswell (1989) are also plotted here as triangles, and may be tracing an
outflow from source 1 as well.


(D 2
U,
0
U,
o
c 0
0
0
c
1, -1
n








Harvey and Gatley (1983) observed 20 gLm infrared sources at the locations of

both NGC6334F-NW (to within I", named IRS2) and NGC6334F (IRS1). The mid-

infrared images reveal at least four sources in this region (Figure 5-12). The UCHII

region NGC6334F is very prominent, and labelled source 1 in Figure 5-12. The infrared

source associated with NGC6334F-NW (source 3) can best be seen at IHW18. Just to the

east of the UCHII region is an elongated mid-infrared object (source 2).

Another source (or sources) is seen about 14" east of the NGC6334F (source 4).

This is designated IRS3 by Harvey and Gatley (1983). IRS3 appears to be a double

source in the IHW18 images positioned in the north-south direction. It might also be an

edge on circumstellar disk, as pointed out by Kraemer et al. (1999), who base this

assumption upon morphology only.

Once more the technique of registering the mid-infrared images with radio maps

to achieve better relative astrometry between the mid-infrared sources and maser

positions was performed. In this case the radio maps of Caswell (1997) were used, and it

can be concluded that the southern methanol masers seem to be located at the sharp

western boundary of the mid-infrared contours of the UCHII region (source 1, see Figure

5-12, lower right panel). These masers may therefore be associated with a shock front,

rather than a circumstellar disk. This astrometry also places the northerly group of

methanol masers about 1.5" below source 3. These masers may also be tracing an outflow

or shock region rather than a disk.

This area is also the site of a large-scale CO outflow, with a lobe redshifted in the

northeast direction and another blueshifted in the southwest direction. Some authors

speculate that NGC6334F-NW may have a disk and be the source of the outflow,








however the thermal images show this source to be resolved and elongated in the east-

west direction, and the position angle of the source elongation is not parallel to that of the

methanol maser distribution nor perpendicular to the outflow axis. Other authors

speculate that NGC6334F is the center of the outflow, but water masers are seen

emanating to the northwest in a linear fashion and are thought to be tracing an outflow or

jet (Figure 5-12, lower right panel). If this is the case, NGC6334F would most likely not

be the source of the outflow to the northeast as well. The elongated source 2 is near the

center of the CO outflow and is perpendicular to the outflow axis. This could mean that

source 2 may be a disk, and the outflow may originate there. Source 3 (NGC6334F-NW)

may be a disk, but the methanol masers may instead trace a southerly outflow since they

are offset from the mid-infrared peak and distributed almost perpendicular to the mid-

infrared elongation.



G351.44+0.66

Although included in their survey, Norris et al. (1993) do not present any data on

this source, and there was no detection any objects at this site in the mid-infrared. There

is also a dearth of information in the literature on this particular site as well. Therefore

this object will not be considered any further in this work, but only to mention it here for

completeness.



G351.77-0.54 (IRAS 17233-3606)

This site contains the largest peak intensity of any known OH maser, at 1000 Jy

(Caswell et al. 1995). It is also found to contain a highly variable OH and methanol maser

(Fix et al. 1982; Caswell et al. 1995a). Norris et al. (1993) only found four methanol









18.2 /m

I~ . .- i . . . .



IIP


10 5 0 -5 -10
Right Ascension offset (orcsec)


0.3


-0.3 F


-0.5 L
0.5


10 5 0 -5 -10
Right Ascension offset (orcsec)


masers


0.3 0.0 -0.3 -0.5
Right Ascension offset (orcsec)


Figure 5-13: G351.77-0.54. Symbols and setup are the same as for Figure 5-1. It is not
known if source 2 is real. Both sources are low signal-to-noise detections.



masers here coincident with the location of these OH masers, and they display a well-

defined velocity gradient along the spots. Forster and Caswell (1989) detected a water

maser about 3" west of the OH and methanol maser location.

Walsh et al. (1997) found four methanol sources here, like Norris et al. (1993),

but could not determine a useful distance to the site because of large errors associated

with the observations of this location. However, Caswell (1997) determined the distance


1


A


10.8 /Am