VLBI Astrometry of AGB Variables with VERA -- A Semiregular Variable S Crateris --

Abstract

We present a distance measurement for the semiregular variable S Crateris (S Crt) based on its annual parallax. With the unique dual beam system of the VLBI Exploration for Radio Astrometry (VERA) telescopes, we measured the absolute proper motion of a water maser spot associated with S Crt, referred to the quasar J1147-0724 located at an angular separation of 1.23${}^{\circ }$. In observations spanning nearly two years, we have detected the maser spot at the LSR velocity of 34.7 km s${}^{-1}$, for which we measured the annual parallax of 2.33$\pm$0.13 mas corresponding to a distance of 430${}_{-23}^{+25}$ pc. This measurement has an accuracy one order of magnitude better than the parallax measurements of HIPPARCOS. The angular distribution and three-dimensional velocity field of maser spots indicate a bipolar outflow with the flow axis along northeast-southwest direction. Using the distance and photospheric temperature, we estimate the stellar radius of S Crt and compare it with those of Mira variables.

9 pages, 4 figures, accepted for publication in PASJ (Vol.60, No.5, October 25, VERA special issue)