2014A&A...562A..82S

Proper motion observations of masers can provide information on dynamic motions on scales of a few milliarcseconds per year (mas/yr) at radii of 100-1000 au scales from central young stellar objects (YSOs). The 6.7GHz methanol masers are one of the best probes for investigations of the dynamics of high-mass YSOs, and in particular for tracing the rotating disk. We have measured the internal proper motions of the 6.7GHz methanol masers associated with Cepheus A (Cep A) HW2 using Very Long Baseline Interferometery (VLBI) observations. We conducted three epochs of VLBI monitoring observations of the 6.7GHz methanol masers in Cep A-HW2 with the Japanese VLBI Network (JVN) over the period 2006-2008. In 2006, we were able to use phase-referencing to measure the absolute coordinates of the maser emission with an accuracy of a few milliarcseconds. We compared the maser distribution with other molecular line observations that trace the rotating disk. We measured the internal proper motions for 29 methanol maser spots, of which 19 were identified at all three epochs and the remaining ten at only two epochs. The magnitude of proper motions ranged from 0.2 to 7.4km/s, with an average of 3.1km/s. Although there are large uncertainties in the observed internal proper motions of the methanol maser spots in Cep A, they are well fitted by a disk that includes both rotation and infall velocity components. The derived rotation and infall velocities at the disk radius of 680 au are 0.5±0.7 and 1.8±0.7km/s, respectively. Assuming that the modeled disk motion accurately represents the accretion disk around the Cep A-HW2 high-mass YSO, we estimated the mass infall rate to be 3x10^–4n₈M_☉/yr (n₈ is the gas volume density in units of 10⁸cm^–3). The combination of the estimated mass infall rate and the magnitude of the fitted infall velocity suggests that Cep A-HW2 is at an evolutionary phase of active gas accretion from the disk onto the central high-mass YSO. The infall momentum rate is estimated to be 5x10^–4 n₈M_☉/yr/km/s, which is larger than the estimated stellar radiation pressure of the HW2 object, supporting the hypothesis that this object is in an active gas accretion phase.