Physics
Scientific paper
Nov 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt........34p&link_type=abstract
PhD Thesis, Department of Physics, University of Tasmania, Australia, 1998.
Physics
8
Scientific paper
Observations of the 6.7- and 12.2-GHz masing transitions of methanol by Norris et al. (1998, Nature, 335, 149; 1993, ApJ, 412, 222) using the Australia Telescope Compact Array (ATCA) and the Parkes-Tidbinbilla Interferometer (PTI) found that for ten of the seventeen sources imaged the individual maser components had a linear spatial distribution. Many of the linear sources also had a velocity gradient along the source. This is in marked contrast to similar observations of OH and water masers. Norris et al. interpreted this to be evidence of an edge-on circumstellar disc surrounding a young massive star. The evolution of young massive stars is poorly understood, because the stars are surrounded by a cocoon of dust and dense molecular gas. Thus the direct detection of a disc around these stars would be a major step in understanding the dynamics of the formation of massive stars. In this thesis I have taken a sample of methanol sources and observed them using a variety of instruments and techniques to improve our understanding of these interesting objects and find further evidence to investigate the circumstellar disc model of linear methanol maser sources. The sources imaged by Norris et al. were some of the brightest known methanol masers. In this thesis, I have used the ATCA to imaged a further ten methanol sources which have a more modest flux. All the methanol sources originally observed by Norris et al. have been reobserved either in this thesis or by Ellingsen et al. (1996, MNRAS, 279, 101). As many of the maser sources have separate sites of emission separated by a few arcseconds to a few arcminutes, there are 45 maser sources which have now been observed at high resolution using the ATCA. Seventeen of these sources have a linear morphology. No difference in the properties of the linear and non-linear sources could be found. Simultaneous observations of the 8.6 GHz continuum towards the methanol sources have been made for all the sources in the sample. Half of the methanol sources are associated with an ultra-compact HII (UCHII) region. The sources without UCHII regions may either be pre-main sequence, or are associated with a less massive embedded star. The Anglo-Australian Telescope has been used to observed the 11.5 micron continuum emission associated with 35 of the maser sources. Nine of the sources were found to have detectable mid-infrared (MIR) emission. No correlation has been found between the the methanol and MIR luminosities. The observations were sensitive enough to detect embedded stars of spectral type B3 and above. That so few sources were found to have a MIR counterpart indicates that there are large variations in the circumstellar environment of the methanol sources. A VLBI proper motion study at 6.7 and 12.2 GHz of a subsample of eight of the linear sources has been started using the Australian VLBI network. If the lines do represent edge-on circumstellar discs then it should be possible to detect Keplerian rotation in 3 to 5 years. The ATCA has been used to search for extragalactic 6.7-GHz methanol ``megamasers'' towards 87 galaxies selected on their IRAS colours and known OH megamaser galaxies. No methanol emission was found, with a typically detection limit of 15-40 mJy. This is surprising given the close association between Galactic OH and methanol masers. http://www.nfra.nl/~phillips/thesis.html
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