The association between masers and outflows in massivre star forming regions

Abstract

We report the results of a single-dish survey of molecular outflows towards a homogeneous sample of 136 ultracompact H II regions for which we had previously obtained observations in the methanol 6.7 GHz and water 22.2 GHz maser lines. The line profiles of the 13CO J = 1-0 and 2-1 transitions have been compared to those of the corresponding lines of the C 18O isotopomer to reveal the occurrence of line wings and hence of molecular outflows. We found 53 outflows resulting in an overall detection rate of ∼39%. The probability to have an outflow increases to about 50% in regions with maser emission, whereas it is about 25% in those without masers. If we consider just the outflow sources, the chance to find a maser is very high: 74%, without a significant difference between H2O and CH 3OH. These results strongly confirm from a statistical point of view that both types of masers are closely associated with the evolutionary phase when outflows occur. The temperatures and optical depths of the molecular cloud hosting the ultracompact H II regions and the comparison between the detection rates suggest a tentative evolutionary scheme for massive star forming regions: the earliest phase is associated with maser emission and with an outflow not yet developed enough to be detected with single-dish observations; then maser emission disappears while the outflow is still present; and finally, only the ultracompact HII region without masers or outflows is present.