2000A&A...362..723C

We present the results of a statistical survey of the 6.7GHz CH₃OH and 22.2GHz H₂O emissions towards a large homogeneous sample of ultracompact (UC) HII regions, performed to investigate the nature of the methanol emission: does it point out ionized regions or does it trace earlier stages of the star forming process? The present detection rates are almost identical: 23% for CH₃OH and 22% for H₂O. Even considered the uncertainties of the survey, the large number of non-detections is consistent with the scenario where methanol masers disappear during the UC phase, like water masers do. Moreover, the probability to have the CH₃OH counterpart of a H₂O source is at least 40%, indicating that these maser emissions are emphasizing two at least partly overlapping evolutionary phases. The comparison between the velocity ranges suggests that methanol and water masers form in different gas components related to the star forming process.