Planar infall of CH3OH gas around Cepheus?A?HW2

摘要

Aims. In order to test the nature of an (accretion) disk in the vicinity of Cepheus?A?HW2, we measured the three-dimensional velocity field of the CH _(3) OH maser spots, which are projected within 1000?au of the HW2 object, with an accuracy on the order of 0.1?km?s ~(-1) . Methods. We made use of the European VLBI Network (EVN) to image the 6.7?GHz CH _(3) OH maser emission toward Cepheus?A?HW2 with 4.5?milliarcsec resolution (3?au). We observed at three epochs spaced by one year between 2013 and 2015. During the last epoch, in mid-March 2015, we benefited from the newly deployed Sardinia Radio Telescope. Results. We show that the CH _(3) OH velocity vectors lie on a preferential plane for the gas motion with only small deviations of 12° ± 9° away from the plane. This plane is oriented at a position angle of 134° east of north, and inclined by 26° with the line of sight, closely matching the orientation of the previously reported disk-like structure. Knowing the orientation of the equatorial plane, we can reconstruct a face-on view of the CH _(3) OH gas kinematics onto the plane. CH _(3) OH maser emission is detected within a radius of 900?au from HW2, and down to a radius of about 300?au, the latter coincident with the extent of the dust emission at 0.9?mm. The velocity field is dominated by an infall component of about 2?km?s ~(-1) down to a radius of 300?au, where a rotational component of 4?km?s ~(-1) becomes dominant. We discuss the nature of this velocity field and the implications for the enclosed mass. Conclusions. These findings directly support the interpretation that the high-density gas and dust emission that surrounds Cepheus?A?HW2 traces an accretion disk.