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

We report the observational results of water (H2O) masers around the Mira variable BX Cam with the East Asian VLBI Network (EAVN), which is the first scientific output of the ESTEMA (EAVN Synthesis of Stellar Maser Animations) project. The data reported here cover the period from May 2018 to June 2021 at 3–4 weeks intervals, except during the EAVN maintenance season (June–August). The result is the astrometric animation, over three stellar pulsation cycles. The absolute positions, proper motions, and annual parallaxes of masers are determined using the dual-beam astrometry with VERA (Japanese VLBI Exploration of Radio Astrometry), which has joined the ESTEMA observations as a sub-array. The derived parallax is consistent with Gaia EDR3 and previous VLBI parallaxes. The stellar position with respect to the H2O maser distribution is registered by a ring of silicon-monoxide (SiO) masers using the source-frequency phase-referencing (SFPR) technique that was applied to the data taken with the multi-frequency receiving system of the sub-array KVN (Korean VLBI Network). The derived stellar position is consistent with that of Gaia EDR3 within 3 mas. The three-dimensional H2O maser kinematics indicates that the circumstellar envelope is expanding at a velocity of 13 ± 4 km/s, while there are asymmetries in both the spatial and velocity distributions of the maser features. The H2O maser animation also directly visualizes the propagation of heat waves in the circumstellar envelope of this star through rapid change in the global maser spatial distribution