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Title:
Studies of ultracompact HII regions - II. High-resolution radio continuum and methanol maser survey
Authors:
Walsh, A. J.; Burton, M. G.; Hyland, A. R.; Robinson, G.
Affiliation:
AA(Department of Astrophysics and Optics, School of Physics, University of New South Wales, NSW 2052, Australia), AB(Department of Astrophysics and Optics, School of Physics, University of New South Wales, NSW 2052, Australia), AC(Southern Cross University, Lismore, NSW 2480, Australia), AD(School of Physics, University College, University of New South Wales, Canberra, ACT 2600, Australia)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 301, Issue 3, pp. 640-698. (MNRAS Homepage)
Publication Date:
12/1998
Origin:
MNRAS
MNRAS Keywords:
MASERS, STARS: FORMATION, ISM: GENERAL, HII REGIONS, RADIO CONTINUUM: ISM, RADIO LINES: ISM
DOI:
10.1046/j.1365-8711.1998.02014.x
Bibliographic Code:
1998MNRAS.301..640W

Abstract

High spatial resolution radio continuum and 6.67-GHz methanol spectral line data are presented for methanol masers previously detected by Walsh et al. (1997). Methanol maser and/or radio continuum emission is found in 364 cases towards IRAS-selected regions. For those sources with methanol maser emission, relative positions have been obtained to an accuracy of typically 0.05 arcsec, with absolute positions accurate to around 1 arcsec. Maps of selected sources are provided. The intensity of the maser emission does not seem to depend on the presence of a continuum source. The coincidence of water and methanol maser positions in some regions suggests there is overlap in the requirements for methanol and water maser emission to be observable. However, there is a striking difference between the general proximity of methanol and water masers to both cometary and irregularly shaped ultracompact (UC) Hii regions, indicating that, in other cases, there must be differing environments conducive to stimulating their emission. We show that the methanol maser is most likely present before an observable UC Hii region is formed around a massive star and is quickly destroyed as the UC Hii region evolves. There are 36 out of 97 maser sites that are linearly extended. The hypothesis that the maser emission is found in a circumstellar disc is not inconsistent with these 36 maser sites, but is unlikely. It cannot, however, account for all other maser sites. An alternative model which uses shocks to create the masing spots can more readily reproduce the maser spot distributions.

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