Planar infall of CH3OH gas around Cepheus A HW2
  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 CH3OH
maser spots, which are projected within 1000au of the HW2 object, with an
accuracy of the order of 0.1km/s. Methods: We made use of the European VLBI
Network (EVN) to image the 6.7GHz CH3OH maser emission towards Cepheus A HW2
with 4.5 milli-arcsecond resolution (3au). We observed at three epochs spaced
by one year between 2013 and 2015. During the last epoch, on mid-march 2015, we
benefited from the new deployed Sardinia Radio Telescope. Results: We show that
the CH3OH velocity vectors lie on a preferential plane for the gas motion with
only small deviations of 12+/-9 degrees away from the plane. This plane is
oriented at a position angle of 134 degrees east of north, and inclined by 26
degrees with the line-of-sight, closely matching the orientation of the
disk-like structure previously reported by Patel et al.(2005). Knowing the
orientation of the equatorial plane, we can reconstruct a face-on view of the
CH3OH gas kinematics onto the plane. CH3OH maser emission is detected within a
radius of 900au from HW2, and down to a radius of about 300au, the latter
coincident with the extent of the dust emission at 0.9mm. The velocity field is
dominated by an infall component of about 2km/s down to a radius of 300au,
where a rotational component of 4km/s becomes dominant. We discuss the nature
of this velocity field and the implications for the enclosed mass. Conclusions:
These findings bring direct support to the interpretation that the high-density
gas and dust emission, surrounding Cepheus A HW2, trace an accretion disk.
