The Astrometric Animation of Water Masers toward the Mira Variable BX Cam

Abstract

We report very long baseline interferometry (VLBI) monitoring observations of the 22 GHz water (H2O) masers around the Mira variable BX Cam, which were carried out as a part of the EAVN Synthesis of Stellar Maser Animations project. Data of 37 epochs in total were obtained from 2018 May to 2021 June with a time interval of 3–4 weeks, spanning approximately three stellar pulsation periods (P = ∼440 days). In particular, the dual-beam system equipped on the VERA stations was used to measure the kinematics and parallaxes of the H2O maser features. The measured parallax, π = 1.79 ± 0.08 mas, is consistent with Gaia EDR3 and previously measured VLBI parallaxes within a 1σ error level. The position of the central star was estimated, based on both the Gaia EDR3 data and the center position of the ring-like 43 GHz silicon-monoxide (SiO) maser distribution imaged with the Korean VLBI Network. The three-dimensional H2O maser kinematics indicates that the circumstellar envelope is expanding at a velocity of 13 ± 4 km s−1, while there are asymmetries in both the spatial and velocity distributions of the maser features. Furthermore, the H2O maser animation achieved by our dense monitoring program manifests the propagation of shock waves in the circumstellar envelope of BX Cam.