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

TL;DR

This paper reports a precise distance measurement to the semiregular variable S Crateris using VLBI astrometry, revealing its outflow structure and stellar properties with unprecedented accuracy.

Contribution

First VLBI parallax measurement of S Crateris providing highly accurate distance and insights into its outflow and stellar characteristics.

Findings

Measured parallax of 2.33±0.13 mas, distance of 430+25−23 pc

Detected bipolar outflow with northeast-southwest axis

Estimated stellar radius and compared with Mira variables

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$0.13 mas corresponding to a distance of 430$^{+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 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.