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We have measured the internal proper motions of the 6.7 GHz methanol masersassociated with Cepheus A (Cep A) HW2 using Very Long Baseline Interferometery(VLBI) observations. We conducted three epochs of VLBI monitoring observationsof the 6.7 GHz methanol masers in Cep A-HW2 with the Japanese VLBI Network(JVN) over the period between 2006-2008. In 2006, we were able to usephase-referencing to measure the absolute coordinates of the maser emissionwith an accuracy of a few milliarcseconds. We compared the maser distributionwith other molecular line observations that trace the rotating disk. Wemeasured the internal proper motions for 29 methanol maser spots, of which 19were identified at all three epochs and the remaining ten at only two epochs.The magnitude of proper motions ranged from 0.2 to 7.4 km/s, with an average of3.1 km/s. Although there are large uncertainties in the observed internalproper motions of the methanol maser spots in Cep A, they are well fitted by adisk that includes both rotation and infall velocity components. The derivedrotation and infall velocities at the disk radius of 680 au are 0.5 +- 0.7 and1.8 +- 0.7 km/s, respectively. Assuming that the modeled disk motion accuratelyrepresents the accretion disk around the Cep A-HW2 high-mass YSO, we estimatedthe mass infall rate to be 3 x 10^{-4} n_8 Msun/yr (n_8 is the gas volumedensity in units of 10^{8} cm^{-3}). The combination of the estimated massinfall rate and the magnitude of the fitted infall velocity suggests that CepA-HW2 is at an evolutionary phase of active gas accretion from the disk ontothe central high-mass YSO. The infall momentum rate is estimated to be 5 x10^{-4} n_8 Msun/yr km/s, which is larger than the estimated stellar radiationpressure of the HW2 object, supporting the hypothesis that this object is in anactive gas accretion phase.

Rotating and infalling motion around the high-mass young stellar object Cepheus A-HW2 observed with the methanol maser at 6.7 GHz