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 si tuated in edge-on circumstellar, or protoplanetary, disks. We present VLBI observations of the methanol maser source G309.92+0.48, in the 12.2 GHz (2( 0)-3(-1) E) transition, which confirm previous observations that the masers in this source lie along a line. We show that such sources are not only li near in space but, in many cases, also have a linear velocity gradient. We then model these and other data in both the 6.7 GHz (5(1)-6(0)A(+)) and the 12.2 GHz (2(0)-3(-1) E) transition from a number of star formation regions and show that the observed spatial and velocity distribution of methanol m asers, and the derived Keplerian masses, are consistent with a circumstella r disk rotating around an OB star. We consider this and other hypotheses an d conclude that about half of these methanol masers are probably located in edge-on circumstellar disks around young stars. This is of particular sign ificance for studies of circumstellar disks because of the detailed velocit y information available from the masers.