The diversity of methanol maser morphologies from VLBI observations

Abstract

<i>Context. <i/>The 6.7 GHz methanol maser marks an early stage of high-mass star formation, but the origin of this maser is currently a matter of debate. In particular it is unclear whether the maser emission arises in discs, outflows or behind shocks running into rotating molecular clouds.<i>Aims. <i/>We investigated which structures the methanol masers trace in the environment of high-mass protostar candidates by observing a homogenous sample of methanol masers selected from Torun surveys. We also probed their origins by looking for associated H II regions and IR emission.<i>Methods. <i/>We selected 30 methanol sources with improved position accuracies achieved using MERLIN and another 3 from the literature. We imaged 31 of these using the European VLBI Network's expanded array of telescopes with 5-cm (6-GHz) receivers. We used the VLA to search for 8.4 GHz radio continuum counterparts and inspected <i>Spitzer<i/> GLIMPSE data at 3.6–8 <i>μ<i/>m from the archive.<i>Results. <i/>High angular resolution images allowed us to analyze the morphology and kinematics of the methanol masers in great detail and verify their association with radio continuum and mid-infrared emission. A new class of “ring-like” methanol masers in star-forming regions appeared to be suprisingly common, 29% of the sample.<i>Conclusions. <i/>The new morphology strongly suggests that methanol masers originate in the disc or torus around a proto- or a young massive star. However, the maser kinematics indicate the strong influence of outflow or infall. This suggests that they form at the interface between the disc/torus and a flow. This is also strongly supported by <i>Spitzer<i/> results because the majority of the masers coincide with 4.5 <i>μ<i/>m emission to within less than 1´´. Only four masers are associated with the central parts of UC H II regions. This implies that 6.7 GHz methanol maser emission occurs before H II region observable at cm wavelengths is formed.