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Class II methanol masers and their environment at high resolution

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Phillips, CJ (1998) Class II methanol masers and their environment at high resolution. PhD thesis, University of Tasmania.

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Abstract

Observations of the 6. 7- and 12.2-GHz masing transitions of methanol by Norris
et al. (1988; 1993) 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 \(H_20\) 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. (1996a; 1998). 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 `H_(II)` (`UCH_(II)`) region. The sources
without `UCH_(II)` 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 μm 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.

Item Type: Thesis (PhD)
Keywords: Astronomical masers
Copyright Information:

Copyright 1998 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1998. Includes bibliographical references

Date Deposited: 03 Feb 2015 03:11
Last Modified: 09 Apr 2017 23:03
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