METHANOL MASERS AND THEIR ENVIRONMENT AT HIGH-RESOLUTION

We have used the Australia Telescope Compact Array (ATCA) to make high -resolution images of the 6.7-GHz 5(1) --> 6(0)A(+) maser transition o f methanol towards 33 sources in the Galactic plane. Including the res ults from 12 methanol sources in the literature, we find that 17 out o f 45 sources have curved or Linear morphology. Most of the 17 have a v elocity gradient along the line, which is consistent with masers lying in an edge-on circumstellar disc surrounding a massive star. We also made simultaneous continuum observations of the sources at 8.6 GHz, in order to image any associated Hn region. 25 of the sources are associ ated with an ultracompact I-I II region, with a detection limit of sim ilar to 0.5 mJy beam(-1). We argue that the methanol sources without a n associated H II region represent less massive embedded stars, not an earlier stage in the lifetime of the star, as previously suggested.