Inferences from the kinematic properties of 6.7 GHz methanol masers

Abstract

Context. It is now well established that the strong and widespread 6.7 GHz methanol masers are associated with young high mass stars. A still unsolved question is where in the circumstellar environment the masers arise. Aims. We address this question by considering an ensemble of rest frame maser velocities of 337 maser features. Methods. The CS(2-1) spectra of 63 methanol maser sources were used to derive systemic velocities and velocity dispersion of the thermal gas. Using the systemic velocities and the velocities of the 337 maser features in the 63 sources, a single distribution of rest frame maser velocities was constructed. This distribution as well as other kinematic information about the masers are used to evaluate four proposed scenarios for where the masers might arise in the circumstellar environment. Results. It is shown that kinematically the masers are not associated with hot cores. We also argue that the scenario in which the masers are associated with an external generated planar shock that propagates into a rotating core cannot explain the observed kinematic properties of the masers. It was found that a simple Keplerian-like disk model is consistent with the observed distribution of rest frame maser velocities. Although outflows have the potential to explain the data, it was not possible to fully test this possibility due to the diverse nature of outflows. Conclusions. © ESO 2007.