VLBI Monitoring Observations of Water Masers around the Semi-Regular Variable Star R Crateris

Abstract

Abstract We monitored water-vapor masers around the semi-regular variable star R Crateris with the Japanese VLBI Network (J-Net) at the 22 GHz band during four epochs with intervals of one month. The relative proper motions and Doppler-velocity drifts of twelve maser features were measured. Most of them existed for longer than 80 days. The 3-D kinematics of the features indicates a bipolar expanding flow. The major axis of the asymmetric flow was estimated to be at P.A.$= 136^{\circ}$. The existence of a bipolar outflow suggests that a Mira variable star had already formed a bipolar outflow. The water masers are in a region of apparent minimum radii of $1.3 \times 10^{12} \,\mathrm{m}$ and maximum radii of $2.6 \times 10^{12} \,\mathrm{m}$, between which the expansion velocity ranges from 4.3 to $7.4 \,\mathrm{km} \,\mathrm{s}^{-1}$. These values suggest that the water masers are radially accelerated, but still gravitationally bound, in the water-maser region. The most positive and negative velocity-drifting features were found relatively close to the systemic velocity of the star. We found that the blue-shifted features are apparently accelerated and the red-shifted apparently decelerated. The acceleration of only the blue-shifted features seems to be consistent with that of the expanding flow from the star.