Methanol Masers as Tracers of Circumstellar Disks

We show that in many methanol maser sources the masers are located in lines,with a velocity gradient along them which suggests that the masers are situatedin edge-on circumstellar, or protoplanetary, disks. We present VLBIobservations of the methanol maser source G309.92+0.48, in the 12.2 GHztransition, which confirm previous observations that the masers in this sourcelie along a line. We show that such sources are not only linear in space but,in many cases, also have a linear velocity gradient. We then model these andother data in both the 6.7 GHz and the 12.2 GHz transition from a number ofstar formation regions, and show that the observed spatial and velocitydistribution of methanol masers, and the derived Keplerian masses, areconsistent with a circumstellar disk rotating around an OB star. We considerthis and other hypotheses, and conclude that about half of these methanolmasers are probably located in edge-on circumstellar disks around young stars.This is of particular significance for studies of circumstellar disks becauseof the detailed velocity information available from the masers.Comment: 38 pages, 13 figures accepted by Ap