VLBI Astrometry of AGB Variables with VERA --- A Semiregular Variable S Crateris ---
Hirota, Tomoya; Kurayama, Tomoharu; Honma, Mareki; Sasao, Tetsuo; Matsumoto, Naoko; Nakagawa, Akiharu; Imai, Hiroshi; Omodaka, Toshihiro; Nagayama, Takumi; Shibata, Katsunori M.; Ueno, Yuji; Jike, Takaaki; Tamura, Yoshiaki; Kobayashi, Hideyuki; Kim, Mi Kyoung; Oyama, Tomoaki; Matsui, Makoto; Sato, Katsuhisa; Kameno, Seiji; Miyaji, Takeshi; Kamohara, Ryuichi; Bushimata, Takeshi; Choi, Yoon Kyung; Iwadate, Kenzaburo; Kameya, Osamu; Kan-Ya, Yukitoshi; Kawaguchi, Noriyuki; Kijima, Masachika; Kuji, Seisuke; Manabe, Seiji; Oh, Chung Sik; Sakai, Satoshi; Sato, Mayumi; Yamashita, Kazuyoshi; Tsushima, Miyuki; Ando, Kazuma; Maeda, Toshihisa; Maruyama, Kenta; Nakamura, Kayoko; Nyu, Daisuke; Pradel, Nicolas; Suda, Hiroshi; Ueda, Kousuke
Japan, South Korea
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 34.7km s-1, for which we measured an annual parallax of 2.33±0.13 mas, corresponding to a distance of 430+25-23pc. 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.