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Class II Methanol Masers in Star Formation Regions

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Class II Methanol Masers in Star Formation Regions Class II Methanol Masers in Star Formation Regions by Simon Peter Ellingsen, B.Sc.(Hons.) Submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy UNIVERSITY OF TASMANIA HOBART January 1996 Declaration This thesis contains no material which has been accepted for the award of any other higher degree or graduate diploma in any tertiary institution. To the best of my knowledge and belief, this thesis contains no material previously published or written by another person, except where due reference has been made in the text of the thesis. Simon Peter Ellingsen 11 Authority of access This thesis may be made available for loan and limited copying in accordance with the Copyright Act 1968. Simon Peter Ellingsen 111 Thesis Summary In 1991 maser emission from the 51-60 A+ transition of CH3OH at a frequency of 6.7 GHz was discovered by Menten (1991a). This transition is more common and stronger than the 12.2-GHz (2 0-3_ 1 E) transition discovered four years previously. This thesis contains the results of a detailed study of 6.7-GHz CH 3OH maser emission over a wide range of angular resolutions. The University of Tasmania 26-m radio telescope has been used to perform a sensitive search for 6.7-GHz CH 3OH masers in a 28.5 square-degree region of the Galactic Plane. The search is complete, within a well defined velocity and flux density range. One hundred and eight 6.7-GHz CH 3OH masers were detected dur- ing the course of the survey, 57 of these being new detections. These new 6.7-GHz CH3OH masers are generally weaker than those already known, but otherwise their spectral appearance is similar to those detected towards OH and 12.2-GHz CH3OH masers. The sample of 6.7-GHz CH3OH masers has been used to crit- ically evaluate several IRA S- based search techniques and we find that all these techniques fail to detect a large fraction of the masers. Two targeted searches of the Large and Small Magellanic Clouds have been performed, resulting in the detection of three 6.7-GHz CH 3OH masers. In addition, a search for 6.7-GHz CH 3OH megamasers was carried out toward 10 Extragalactic sources, nearly all of which are known OH or H 20 megamasers. No CH3OH megamasers were detected with a peak flux comparable to the OH or H20 megamasers in the galaxies searched. Single dish spectra of 6.7- and 12.2-GHz CH 3OH masers are often complicated, with many spectral features spread over a velocity range of 10 km s -1 or more. High resolution observations of these maser sources show that each of the spectral features arise from a different region in the gas cloud. For OH and H 20 masers the high resolution spatial morphology typically shows little or no simple structure. Conversely, high resolution observations of 6.7- and 12.2-GHz CH 3OH masers (Norris et al., 1988; Norris et al., 1993) show that many have a simple curved, or linear morphology. The Australia Telescope Compact Array has been used to observe the radio continuum emission associated with three strong 6.7-GHz CH3 OH masers. It is shown that the position of the CH 3OH masers with respect to the continuum emission is consistent with the masers originating in a circumstellar disci Very Long Baseline Interferometry (VLBI) has been used to image strong class II CH 3OH maser emission associated with two star formation regions. The milli-arcsecond resolution images detected many new maser spots, but all of these iv follow the general morphology revealed by lower resolution observations. Com- parison of the 6.7- and 12.2-GHz images for the CH 3OH masers associated with NGC 6334F shows that five of the spots are coincident to within the positional errors of the observations (c-:-., 4 milli-arcseconds). VLBI observations were also used to measure the size of the 6.7- and 12.2-GHz CH3OH maser spots. These show that the maser spots contain structure on two different scales, one of the order of tens of astronomical units, the other between a few and ten astronomical units. These findings are supported by the imaging data and the VLBI observations of Menten et al. (1988; 1992). The sizes of the 6.7- and 12.2-GHz spots toward the same sources are similar, which suggests that they are not broadened by interstellar scattering. v Acknowledgments First and foremost I would like to thank three people, without whom this work would not have been possible. Dr. Ray Norris has been a constant source of advice and encouragement and his good humour and friendship have picked me up, and got me going again when things didn't go as planned. He has also patiently tutored me in the concepts and analysis of spectral line VLBI. Prof. Peter McCulloch has contributed enormously to the work described in this thesis. His efforts (in conjunction with Prof. Pip Hamilton) in establishing the Mt Pleasant observatory, and in building the autocorrelation spectrometer and cryogenically cooled receivers were the foundation upon which the 6.7-GHz CH 3OH maser survey rests. My partner Kristi Baker was the third vital person, her support and companionship have kept me focused and sane throughout the course of my studies. I would like to thank Doctors John Whiteoak, Ray Norris, Jim Caswell and Bobbie Vaile for their assistance with the Parkes observations described in Chap- ter 3. John and Ray also assisted with the analysis and interpretation of the data, particularly of the search for CH3OH masers in other galaxies. I would like to also thank Dr Jim Caswell for many useful discussions regarding the implementation of the Mt Pleasant survey and processing of connected element interferometry data. I would like to thank the staff of the Mt Pleasant observatory, Messrs Gordon Gowland, Phil Jenkins and John Smith. They have assisted me above and beyond the call of duty, particularly by changing receivers during their own time and in the most inclement weather. Gordon also contributed greatly to the success of the CH3 OH survey with his ability to quickly determine and fix hardware problems when they arose, and his assistance (along with Phil) during many of the observing sessions. Several other people have also been intimately involved with several stages of the Mt Pleasant maser survey. Messrs Tino Delbourgo and Phil Button played vital roles in the construction and development of the digital autocorrelation spec- trometer and Dr. A A Deshpande assisted me greatly in writing the correlator observing software. Mr Mark von Bibra contributed greatly to the writing of the analysis software and the collection of data in the 330 0 — 335° Galactic longitude region. The VLBI observations presented in Chapter 7 would not have been possible without the assistance of a large number of dedicated observers who supported the observations at various antennas. These were Dr E.King, Mr J.Lovell (Hobart), Dr J.Reynolds (Tidbinbilla), Dr T.Tzioumis, Mr S.Amy (Mopra), Dr R.Norris, Mr R.Ferris, Mr E.Troup (Parkes), Dr G.MacLeod (Hartebeesthoek), Dr R.Gough, Dr M.Wieringa and Ms R.Wark (Culgoora). vi The National Radio Astronomical Observatory kindly provided the facilities I used to correlate the VLBI data presented in Chapter 7. I would like to thank Doctors Phil Diamond and Tony Beasley for instructing me in the use of the now defunct NRAO MK II correlator, and for their assistance with the correlation. The processing of the VLBI data would not have been possible without the work of Phil, who has written many of the specialized AIPS tasks and also guided me in how to use them. Tony and family were very generous in their hospitality during my two visits to Socorro, and provided me with many nights and weekends of company when a long way from home. I would also like to thank the director of the ATNF Dr. Ron Ekers who funded the airfares for my two trips to Socorro. I would like to thank the staff of the Physics Department and fellow postgrad- uate students, Jim, Jenny, Chris, Mark and Edward for their support and the many memorable social occasions. I extend my thanks in particular to Jim, with whom I have had the pleasure of sharing an office for the past 4 and a half years. His ability to retain the most useless trivia and remain cheerful and calm even under extreme provocation continue to amaze me. Finally I would like to thank my family for the kind and supporting environ- ment in which I was raised. The interest in astronomy nurtured by my parents inspired me throughout my high school and undergraduate years. I am grateful to the Australian Government for a Postgraduate Research Award, which provided me with the financial support necessary to undertake the work presented in this thesis vii Contents Thesis Summary iv Acknowledgments vi 1 Introduction 1 1.1 A brief historical overview 1 1.2 The rest frequencies of 6.7- and 12.2-GHz CH 3 OH masers 3 1.3 Outline of the Thesis 3 2 Masers and star formation regions 5 2.1 Introduction 5 2.2 Ultra-compact Hii regions 5 2.2.1 The champagne flow model 6 2.2.2 The infall model 7 2.2.3 The bow shock model 7 2.2.4 The circumstellar disc photoionization model 8 2.2.5 The warm, dense molecular gas hypothesis 8 2.2.6 The far infrared properties of UCHII regions 9 2.3 Masers associated with star formation 10 2.3.1 OH masers in star formation regions 10 2.3.2 H20 masers in star formation regions 11 2.4 CH3 OH masers in star formation regions 12 2.4.1 The difference between class I and class II CH 3 OH masers 13 2.4.2 Pumping schemes 14 2.4.3 The formation of CH3OH in molecular clouds 15 3 Methanol masers in other Galaxies 16 3.1 Introduction 16 3.1.1 OH megamasers 16 3.1.2 Superluminous H20 masers 17
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