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

Abstract

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$-$0724 located at an angular separation of 1.23D. In observations spanning nearly two years, we detected the maser spot at an LSR velocity of 34.7kms$^{-1}$, for which we measured an annual parallax of 2.33$\pm$0.13mas, corresponding to a distance of 430$^{+25}_{-23}$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 the northeast-southwest direction. Using the distance and photospheric temperature, we estimated the stellar radius of S Crt and compared it with those of Mira variables.