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Class II methanol masers in star formation regions

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Ellingsen, Simon Peter (1996) Class II methanol masers in star formation regions. PhD thesis, University of Tasmania.

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Abstract

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_ 1E) transition discovered four years previously.
This thesis contains the results of a detailed study of 6.7-GHz CH3 OH 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 3 OH 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 3 OH masers were detected during
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
CH3 OH masers. The sample of 6.7-GHz CH3 OH masers has been used to critically
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 3 OH masers.
In addition, a search for 6.7-GHz CH3OH megamasers was carried out toward
10 Extragalactic sources, nearly all of which are known OH or H 2 0 megamasers.
No CH3 OH megamasers were detected with a peak flux comparable to the OH or
H2 0 megamasers in the galaxies searched. Single dish spectra of 6.7- and 12.2-GHz CH 3 OH masers are often complicated,
with many spectral features spread over a velocity range of 10 km s -1or 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 H2 0 masers the
high resolution spatial morphology typically shows little or no simple structure.
Conversely, high resolution observations of 6.7- and 12.2-GHz CH3 OH 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 3 OH 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 CH3OH maser emission associated with two star formation regions. The
milli-arcsecond resolution images detected many new maser spots, but all of these follow the general morphology revealed by lower resolution observations. Comparison
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 (≈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.

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

Copyright 1996 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, 1996. Includes bibliographical references (leaves 155-161)

Date Deposited: 09 Dec 2014 00:01
Last Modified: 15 Aug 2016 03:58
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