VLBI observations of 6.7 and 12.2 GHz methanol masers toward high mass star-forming regions

Abstract

We compare the absolute positions of methanol maser sites with those of typical tracers of young massive stellar objects, such as ultra-compact Hii regions, outflows and hot molecular cores. We have selected thirteen star-forming regions for which we have produced VLBI maps of 6.7 and 12.2 GHz methanol masers, and for which complementary high resolution images are available in the literature. These sources are namely: IRAS 20126+4104, NGC 7538, S 252, G 31.28+0.06, W 75N, S 255, S 269, Mon R2, G 9.62+0.20, Cep A, W 51, G 59.78+0.06 and G 29.95-0.02. Eight of the methanol maser sources coincide in position within 2000 AU with outflows and/or hot molecular cores, i.e. phenomena occuring at the earliest stage of massive star formation. Only two sources coincide in position with hyper-compact Hii regions. We conclude that 6.7 and 12.2 GHz methanol masers are likely to be one of the first signposts of massive star formation and perhaps the first signpost of a massive protostar.