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1997MNRAS.291..261W
1997MNRAS.301..640W

Studies of ultracompact HII regions - I. Methanol maser survey of IRAS-selected sources

Walsh A.J.; Hyland, A.R.; Rorinson G.; Burton M.G.

MNRAS 291, pp. 261-278 (1997)

Abstract

A survey of ultracompact (UC) Hii regions has been carried out by searching for 6.669-GHz methanol maser emission from a sample of 535 IRAS-selected candidates. A total of 201 candidates exhibit methanol emission. These sources have been used, in conjunction with previously identified UC Hii regions, to provide a base for further studies of such regions. Estimates of distances have indicated that the identified UC Hii regions tend to have some Galactic structure but it is not clear whether they lie in or between the spiral arms of the Galaxy. The regions are tightly constrained to the plane of the Galaxy. Comparison of identified regions and IRAS sources selected by Wood & Churchwell indicates that there there is some degree of contamination, which could be due to an older phase in the life on an UC Hii region where methanol maser emission is not apparent. Luminosities and spectral types have been derived for many of the regions. The maximum number of maser spots observed seems to increase with increasing peak maser luminosity, which indicates that the maser emission is more dependent on the abundance of methanol than the availability of far-infrared radiation.
1997MNRAS.301..640W

Studies of ultracompact HII regions - II. High-resolution radio continuum and methanol maser survey

A. J. Walsh, M. G. Burton, A. R. Hyland G. Robinson

Department of Astrophysics and Optics, School of Physics, University of New South Wales, NSW 2052, Australia Southern Cross University, Lismore, NSW 2480, Australia School of Physics, University College, University of New South Wales, Canberra, ACT 2600, Australia

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.