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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.