Studies of ultracompact H II regions - II. High-resolution radio continuumand methanol maser survey

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 cas es towards IRAS-selected regions. For those sources with methanol maser emi ssion, relative positions have been obtained to an accuracy of typically 0. 05 arcsec, with absolute positions accurate to around 1 arcsec, Maps of sel ected sources are provided. The intensity of the maser emission does not se em to depend on the presence of a continuum source. The coincidence of wate r and methanol maser positions in some regions suggests there is overlap in the requirements for methanol and water maser emission to be observable. H owever, there is a striking difference between the general proximity of met hanol 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 met hanol maser is most likely present before an observable UC H II region is f ormed around a massive star and is quickly destroyed as the UC H II 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, howeve r, account for all other maser sites. An alternative model which uses shock s to create die masing spots can more readily reproduce the maser spot dist ributions.