Abstract
We have undertaken a comprehensive survey to investigate the relationship between methanol maser emission and massive star formation. We have selected 534 southern IRAS sources and have detected methanol maser emission at 6.7 GHz from 201 of them, using the Parkes radiotelescope. We have further surveyed most of the IRAS sources, using the Australia Telescope Compact Array (ATCA). The ATCA allows us to obtain arcsecond resolution of the maser emission, as well as radio continuum emission at both 6.7 and 8.6 GHz. The radio continuum emission is presumed to come from ultracompact (UC) H II regions, the small bubbles of ionised hydrogen that surround a newly formed, embedded massive star.
We have detected 187 maser sites and 177 uchii regions in the ATCA survey. Only 25% of all maser sites have an associated uchii region, contrary to the previous assumption (eg. Menten, 1997) that Class II methanol maser emission is only associated with uchii regions. It may be that the maser emission is associated lower mass (non-ionising, with spectral type > B3) stars. However, we put forward the hypothesis that the maser emission is associated with a stage of massive star formation before the uchii region has had time to develop. Thus, the methanol maser emission may be one of the first easily observable signs of massive star formation. Results that support this argument include:
-- uch ii regions with coincident maser emission are generally smaller (therefore younger) than those with no associated maser emission.
-- The maser sites are tightly confined to the Galactic plane, with a scale height of 0.4 plus or minus 0.1 degrees. Such a tight distribution is expected to come from only the most luminous stars throughout the Galaxy.
-- Methanol maser sites are coincident in a number of cases with hot ammonia cores, detected by Cesaroni et al. (1994), and interpreted as an early evolutionary phase of massive star formation, before
the uch ii phase.
We have conducted a NIR survey of a selection of 30 IRAS sources, to identify the stellar counterpart to the maser sites and uch ii regions. About half the maser sites and uch ii regions have an easily identifiable NIR counterpart. Furthermore, the luminosities of six sources can be derived from their IRAS fluxes and all are consistent with luminosities expected from massive, ionising ( < B3) stars.