STUDIES OF ULTRACOMPACT H-II REGIONS .1. METHANOL MASER SURVEY OF IRAS-SELECTED SOURCES

A survey of ultracompact (UC) Hn regions has been carried out by searc hing for 6.669-GHz methanol maser emission from a sample of 535 IRAS-s elected candidates. A total of 201 candidates exhibit methanol emissio n. These sources have been used, in conjunction with previously identi fied UC H II regions, to provide a base for further studies of such re gions. Estimates of distances have indicated that the identified UC H II regions tend to have some Galactic structure but it is not clear wh ether they lie in or between the spiral arms of the Galaxy. The region s are tightly constrained to the plane of the Galaxy. Comparison of id entified regions and IRAS sources selected by Wood & Churchwell indica tes that there there is some degree of contamination, which could be d ue to an older phase in the life on an UC H II region where methanol m aser emission is not apparent. Luminosities and spectral types have be en derived for many of the regions. The maximum number of maser spots observed seems to increase with increasing peak maser luminosity, whic h indicates that the maser emission is more dependent on the abundance of methanol than the availability of far-infrared radiation.