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Interstellar masers as evolutionary tracers of high-mass star formation


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Breen, SL (2010) Interstellar masers as evolutionary tracers of high-mass star formation. PhD thesis, University of Tasmania.

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Determining an evolutionary clock for high-mass star formation is a much
sought after result in astrophysics, as it will allow comprehensive quantitative
studies of the formation of high-mass stars to be completed. We have carried
out detailed studies of a large number of sources suspected of undergoing highmass
star formation and have found that common masers transitions can be very
helpful in determining their relative evolutionary stage.
We present new observations of 22 GHz water masers and 12.2 GHz methanol
masers, completed with the Australia Telescope Compact Array and the Parkes
radio telescope, respectively. Hundreds of maser sources are reported here for the
first time. From the large samples, we are able to confidently report detection
statistics, as well as characteristics of the source populations. Analysis of the
maser velocities, flux densities, luminosities and variability are presented.
We have investigated the relative evolutionary phases of high-mass star formation
associated with the presence or absence of combinations of the 22 GHz
water and 12.2 GHz methanol masers that we detect, along with the locations of
6.7 GHz methanol and main-line OH masers. The locations of the different maser
species have been compared with the positions of 1.2 mm dust clumps, molecular
data, radio continuum, GLIMPSE point sources and Extended Green Objects.
Comparison between the characteristics of coincident sources has revealed strong
evidence for an evolutionary sequence for the different maser species in high-mass
star formation regions. We present our proposed sequence for the presence of the
common maser species associated with young high-mass stars, along with the
data supporting our arguments.
Insights into the characteristics and physical conditions associated with the
different relative evolutionary stages are presented. We find a general trend
whereby both the water and methanol masers increase in luminosity as they
evolve. Further to this, we find evidence that the methanol and water masers
increase in velocity range as they evolve, implying that the gas volume conducive
to the maser emission also increases with evolution. The rate of increase of the
12.2 GHz methanol maser luminosity and velocity range is lower than that of their
6.7 GHz counterparts. We find evidence to suggest that water maser sources also
increase in luminosity less rapidly than 6.7 GHz methanol maser sources. We
additionally find evidence to suggest that the increase in luminosity and velocity
range of water maser sources 'turns over' towards the later stages of evolution,
presumably decreasing until the maser emission ceases. We find no evidence for
the methanol masers to show a similar decrease in either luminosity or velocity
range, implying that they cease emitting much more abruptly.
Comparisons with mid-infrared data show that the masers themselves are
more sensitive to evolutionary changes than the associated mid-infrared emission.

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

Copyright 2010 the Author. The author has requested that when citing, please cite the papers where the results have been published.

Additional Information:

Chapter 3 appears to be in part the equivalent of a pre-print or post-print finally published as: Breen, S. L. et al., 2007, A search for 22-GHz water masers within the giant molecular cloud associated with RCW 106, MNRAS, 377(2), 491-506, the chapter also includes the addition of a section from: Bains I., Breen S. L., Burton M. G., Cunningham M. R., Jones P. A., Kawamura A., Lo N., Rowell G. P., Walsh A. (2009) Recent science from Australian large-scale millimetre mapping projects : proceedings from a Swinburne University workshop, Publications of the Astronomical Society of Australia 26, 110–120, © Astronomical Society of Australia 2009

Chapter 4 appears to be in part the equivalent of a pre-print or post-print finally published as: S. L. Breen, S. P. Ellingsen, J. L. Caswell and B. E. Lewis, (2010) 12.2-GHz methanol masers towards 1.2-mm dust clumps: quantifying high-mass star formation evolutionary schemes, MNRAS, 401(4), 2219-2244

Chapter 6 appears to be in part the equivalent of a pre-print or post-print finally published as: S. L. Breen, J. L. Caswell, S. P. Ellingsen, and C. J. Phillips, (2010) Water masers accompanying OH and methanol masers in star formation regions, MNRAS, 406(3), 1487-1532

Date Deposited: 25 Nov 2014 00:58
Last Modified: 11 Mar 2016 05:53
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