2022ApJ...941..105X

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.