Astrometry of AGB Variables with VERA: Annual Parallax and the Orbit of SY Sculptoris in the Galaxy

Abstract

Abstract We report on the results of astrometric observations of H$_{2}$O masers toward a Mira variable, SY Sculptoris (SY Scl), with the VLBI Exploration of Radio Astrometry (VERA) from 2006 October to 2008 October. We determined that SY Scl's annual parallax is 0.75$\ \pm\ $0.03 mas, corresponding to a distance of 1.33$\ \pm\ $0.05 kpc. SY Scl is 1.3 kpc south of the Galactic plane and moves approximately south from the plane with a velocity of 63$\ \pm\ $2 km s$^{-1}$ with respect to the LSR. From the present position and motion, we calculated SY Scl's orbit for the last 1 Gyr based on a parametrized model of the Galactic gravitational potential. SY Scl has an orbit as a member of the Galactic thick disk stars. Subtracting the averaged absolute proper motions of masers, we derived an internal motion of H$_2$O masers in SY Scl. The kinematics of the masers is consistent with motion in a single plane. The masers around SY Scl were found to be distributed in an area of 70 AU $\times$ 80 AU, corresponding to 15 times as large as the stellar photosphere diameter of 5 AU. Based on the measured distance and apparent magnitude of SY Scl, we find the absolute magnitude of SY Scl to be $M_K$$=$$-$8.07$\ \pm\ $0.08 mag. This is approximately consistent with the value of $-$8.09$\ \pm\ $0.07 mag, which was derived from the Galactic Mira period–luminosity relation (2008, MNRAS, 386, 313).