Signposts of massive star formation

Accurate positions (to sub-arcsecond accuracy) of 25 GHz class I methanol maser emission were obtained for 28 sources in active star forming regions in the Southern Hemisphere. Eight transitions (J=2 to J=9) of the series were observed at 0.006 kms^-1 resolution on the CABB correlator on the ATCA. Of the 15 sources with maser emission; each source has at least 1 maser emission line. Two sources have emission in six of the observed transitions. The upper limits are between 0.23 - 7.19 Jy for non-detected lines. The fluxes for the primary maser components range from 0.2 to 14.3 Jy. Most FWHMs fall within 0.1 to 0.5 kms^-1, and appear broader for the J=7 line in most sources. 10 sources have quasi-thermal line emission. I also report the possible detection of broad thermal emission in 6 spectral components - all with peak fluxes of less than 1 Jy. The computed rotational temperatures for all 25 GHz masers range from 25 - 301 K. The upper limit on the J=2 line is much weaker than the value expected from the rotational diagrams for seven of the sources.

The observed 25 GHz masers fit two common models. Nine sources favour Model 1. Six sources follow Model 2. Neither model is a good predictor of the flux densities measured at J=2. The majority of 44 GHz masers lie within 10" of the 25 GHz masers. The 84 GHz masers lie closer to the 25 GHz masers if they fall into Model 1. Infra-red dark clouds, OH masers at 1720 MHz, and CH₃OH masers at 19.9 and 12 GHz all lie within 15" of 25 GHz masers. In contrast, HII regions do not appear closer than 10". The 25 GHz maser transition series trace all stages of massive young stellar evolution - from the early formation of infra-red dark clouds to the bright ionized HII regions. This survey reports the most accurate positions (to 3.dp. in RA and DEC) of these 25 GHz masers to date..