The diversity of methanol maser morphologies from VLBI observations
  We investigate which structures the 6.7 GHz 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. 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 Spitzer GLIMPSE
data at 3.6-8 um from the archive. 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. 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 Spitzer results
because the majority of the masers coincide with 4.5 um emission to within less
than 1 arcsec. 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.
