SIMBAD references

Among the tracers of the earliest phases in the massive star formation process, methanol masers have gained increasing importance. The phenomenological distinction between Class I and II methanol masers is based on their spatial association with objects such as jets, cores, and ultracompact HII regions, but is also believed to correspond to different pumping mechanisms: radiation for Class II masers, collisions for Class I masers. We surveyed a large sample of massive star-forming regions in Class I and II methanol masers. The sample consists of 296 sources, divided into two groups named high and low according to their [25-12] and [60-12] IRAS colours. Previous studies indicate that the two groups may contain similar sources in different evolutionary stages, with the high sources representing the more evolved stages. Therefore, the sample can be used to assess a sequence for the occurrence of Class I and II methanol masers during the evolution of a massive star-forming region. We observed the 6GHz (Class II) CH₃OH maser with the Effelsberg 100-m telescope, and the 44GHz and 95GHz (Class I) CH₃OH masers with the Nobeyama 45-m telescope. We detected 55 sources in the Class II line (39 high and 16 low, 12 new detections); 27 sources in the 44GHz Class I line (19 high and 8 low, 17 new detections); 11 sources in the 95GHz Class I line (eight high and three low, all except one are new detections). The detection rate of Class II masers decreases with the distance of the source (as expected), whereas that of Class I masers peaks at ∼5kpc. This could be due to the Class I maser spots being spread over a region ≲1pc, comparable to the telescope beam diameter at a distance of ∼5kpc. We also find that the two Class I lines have similar spectral shapes at 44GHz and 95GHz, which confirms that they have the same origin. Our statistical analysis shows that the ratio between the detection rates of Class II and Class I methanol masers is basically the same in high and low sources. Therefore, both maser types seem to be equally associated with each evolutionary phase. In contrast, all maser species (including H₂O) have about three times higher detection rates in high than in low sources. This could indicate that the phenomena from which all masers originate become progressively more active with time during the earliest evolutionary phases of a high-mass star-forming region.