Southern class I methanol masers at 36 and 44 GHz

Abstract

The Australia Telescope Compact Array (ATCA) has been used for high angular\nresolution imaging of 71 southern class I methanol maser sources\nquasi-simultaneously at 36 and 44 GHz. The data reveal a high level of\nmorphological and kinematical complexity, and allow us to demonstrate\nassociations, at arcsecond precision, of the class I maser emission with\noutflows, expanding HII regions, dark clouds, shocks traced by the 4.5-micron\nemission and 8.0-micron filaments. More than 700 maser component features were\nfound at each of the two methanol transitions, but with only 23 per cent\nrecognisable at both transitions; the morphology of class I emission is much\nbetter revealed by our survey of both transitions, compared with either one\nalone. We found that the number of masers falls exponentially with the\nprojected linear distance from the associated class II 6.7-GHz methanol maser.\nThis distribution has a scale of 263+/-15 mpc, irrespective of the transition.\nThe class I masers associated with OH masers were found to have a tendency to\nbe more spread out, both spatially and in the velocity domain. This is\nconsistent with the expectation that such sources are more evolved. Apart from\na small number of high-velocity components (which are largely blue-shifted and\npredominantly seen at 36 GHz), the velocity distribution was found to be\nGaussian, peaking near the systemic velocity of the region, which had been\nestimated as the middle of the velocity interval of the associated class II\nmethanol maser at 6.7 GHz. The mean indicated a small, but significant blue\nshift asymmetry of -0.57 km/s (uncertainties are 0.06 and 0.07 km/s for the 36-\nand 44-GHz masers, respectively) with respect to the 6.7-GHz masers. The\nstandard deviation of the velocity distribution was found to be 3.65+/-0.05 and\n3.32+/-0.07 km/s for the 36- and 44-GHz masers, respectively.\n