22 GHz water and 6.7 GHz methanol masers are usually thought as signposts of
early stages of high-mass star formation but little is known about their
associations and the physical environments they occur in.
The aim was to obtain accurate positions and morphologies of the water maser
emission and relate them to the methanol maser emission recently mapped with
Very Long Baseline Interferometry. A sample of 31 methanol maser sources was
searched for 22 GHz water masers using the VLA and observed in the 6.7 GHz
methanol maser line with the 32 m Torun dish simultaneously. Water maser
clusters were detected towards 27 sites finding 15 new sources. The detection
rate of water maser emission associated with methanol sources was as high as
71%. In a large number of objects (18/21) the structure of water maser is well
aligned with that of the extended emission at 4.5 μm confirming the origin
of water emission from outflows. The sources with methanol emission with
ring-like morphologies, which likely trace a circumstellar disk/torus, either
do not show associated water masers or the distribution of water maser spots is
orthogonal to the major axis of the ring. The two maser species are generally
powered by the same high-mass young stellar object but probe different parts of
its environment. The morphology of water and methanol maser emission in a
minority of sources is consistent with a scenario that 6.7 GHz methanol masers
trace a disc/torus around a protostar while the associated 22 GHz water masers
arise in outflows. The majority of sources in which methanol maser emission is
associated with the water maser appears to trace outflows. The two types of
associations might be related to different evolutionary phases.Comment: accepted by Astronomy & Astrophysic
