DISTANCE AND KINEMATICS OF THE RED HYPERGIANT VY CMa: VERY LONG BASELINE ARRAY AND VERY LARGE ARRAY ASTROMETRY

Abstract

We report astrometric results of phase-referencing very long baseline interferometry observations of 43 GHz SiO maser emission toward the red hypergiant VY Canis Majoris (VY CMa) using the Very Long Baseline Array (VLBA). We measured a trigonometric parallax of 0.83 +/- 0.08 mas, corresponding to a distance of 1.20(-0.10)(+0.13) kpc. Compared to previous studies, the spatial distribution of SiO masers has changed dramatically, while its total extent remains similar. The internal motions of the maser spots are up to 1.4 mas yr(-1), corresponding to 8 km s(-1), and show a tendency for expansion. After modeling the expansion of maser spots, we derived an absolute proper motion for the central star of mu(x) =-2.8 +/- 0.2 and mu(y) = 2.6 +/- 0.2 mas yr(-1) eastward and northward, respectively. Based on the maser distribution from the VLBA observations, and the relative position between the radio photosphere and the SiO maser emission at 43 GHz from the complementary Very Large Array observations, we estimate the absolute position of VY CMa at mean epoch 2006.53 to be alpha(J2000) = 07(h)22(m)58.(s)3259 +/- 0.(s)0007, delta(J2000) =-25 degrees 46'03.''063 +/- 0.''010. The position and proper motion of VY CMa from the VLBA observations differ significantly with values measured by the Hipparcos satellite. These discrepancies are most likely associated with inhomogeneities and dust scattering the optical light in the circumstellar envelope. The absolute proper motion measured with VLBA suggests that VY CMa may be drifting out of the giant molecular cloud to the east of it.