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Astron. Astrophys. 322, 307-310 (1997)

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On the selection of ultra-compact HII regions from the IRAS Point Source Catalogue: What the 6.7-GHz methanol maser surveys tell us

D.J. van der Walt

Space Research Unit, Physics Department, Potchefstroom University for CHE, Private Bag X6001, Potchefstroom, 2520, South Africa

Received 1 October 1996 / Accepted 18 November 1996


The detection rate of 6.7-GHz methanol masers in IRAS based surveys is discussed within the framework of a model for the distribution and far-infrared characteristics of embedded ionizing stars in the Galaxy. It is shown that the fainter IRAS sources selected with the Wood & Churchwell (1989) criteria have a distribution in Galactic latitude that is completely different from what is expected for embedded ionizing stars. It is concluded that a significant fraction of the fainter sources selected with the Wood & Churchwell criteria are most probably not embedded ionizing stars. The model also predicts that a significant fraction of embedded ionizing stars, primarily the less massive ones, are not expected to be exceptionally bright in the far-infrared and are also expected to be located at very small Galactic latitudes.

Key words: masers – Hii regions – ISM: clouds – radio lines: ISM

Send offprint requests to: D.J. van der Walt

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998