Physical and chemical conditions in methanol maser selected hot cores and UCH II regions

Abstract

We present the results of a targeted 3-mm spectral line survey towards the eighty-three 6.67GHz methanol maser selected star-forming clumps observed by Purcell. In addition to the previously reported measurements of HCO+(1-0), H13CO+(1-0) and CH3CN(5-4) and (6-5), we used the Mopra antenna to detect emission lines of N2H+(1-0), HCN(1-0) and HNC(1-0) towards 82/83 clumps (99 per cent), and CH3OH(2-1) towards 78/83 clumps (94 per cent). The molecular line data have been used to derive virial and local thermodynamic equilibrium masses, rotational temperatures and chemical abundances in the clumps, and these properties have been compared between subsamples associated with different indicators of evolution. The greatest differences are found between clumps associated with 8.6GHz radio emission, indicating the presence of an Ultra-Compact HII (UCHII) region, and `isolated'masers (without associated radio emission), and between clumps exhibiting CH3CN emission and those without. In particular, thermal CH3OH is found to be brighter and more abundant in UCHII regions and in sources with detected CH3CN, and may constitute a crude molecular clock in single dish observations. Clumps associated with 8.6GHz radio emission tend to be more massive andmore luminous than clumps without radio emission. This is likely because the most massive clumps evolve so rapidly that a Hyper-Compact HII or UCHII region is the first visible tracer of star formation.The gas mass to submm/infrared luminosity relation for the combined sample was found to be L ~ M0.68, considerably shallower than expected for massive main-sequence stars. This implies that the mass of the clumps is comparable to, or greater than, the mass of the stellar content.We also find that the mass of the hot core is correlated with the mass of the clump in which it is embedded.