2002A&A...390..289B

We present a comparison of Class ii CH₃OH (6.7GHz) and H₂O (22.2GHz) masers at high spatial resolution in a sample of 29 massive star-forming regions. Absolute positions of both maser types are compared with mm dust continuum, cm continuum and mid-infrared sources. All maser features - regardless of the species - are associated with massive mm cores, but only 3 out of 18 CH₃OH masers and 6 out of 22 H₂O masers are associated with cm emission likely indicating the presence of a recently ignited massive star. These observations of a homogenous sample of massive, young star-forming regions confirm earlier results, obtained for each maser species separately, that both maser types are signposts of high-mass star formation in very early evolutionary stages. The data are consistent with models that explain CH₃OH maser emission by radiative pumping in moderately hot cores, requiring the absence, or only weak, free-free cm continuum radiation due to recently ignited stars. Mid-infrared sources are associated with both maser types in approximately 60% of the observed fields. Thus, mid-infrared objects may power maser sites, but the detection of strong mid-infrared emission is not strictly necessary because it might be heavily extincted. A comparison of the spatial separations between the different observed quantities and other properties of the star-forming regions does not reveal any correlation. Our data suggest that CH₃OH and H₂O masers need a similar environment (dense and warm molecular gas), but that, due to the different excitation processes (radiative pumping for CH₃OH and collisional pumping for H₂O), no spatial correlations exist. Spatial associations are probably coincidences due to insufficient angular resolution and projection effects. The kinematic structures we find in the different maser species show no recognizable pattern, and we cannot draw firm conclusions as to whether the features are produced in disks, outflows or expanding shock waves.