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

TL;DR

This study presents a detailed VLBI monitoring of water masers around Mira variable BX Cam, revealing its parallax, stellar position, envelope expansion, and shock wave propagation over three pulsation periods.

Contribution

It provides the first dense, multi-epoch VLBI maser animation of BX Cam, measuring its parallax, stellar position, and revealing shock wave dynamics in its circumstellar envelope.

Findings

Parallax of 1.79±0.08 mas consistent with Gaia data.

Circumstellar envelope expanding at 13±4 km/s.

Detected shock wave propagation in the maser features.

Abstract

We report VLBI monitoring observations of the 22 GHz water (H$_{2}$O) masers around the Mira variable BX Cam, which were carried out as a part of the EAVN Synthesis of Stellar Maser Animations (ESTEMA) 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= \sim$440 d). In particular, the dual-beam system equipped on the VERA stations was used to measure the kinematics and parallaxes of the H$_{2}$O maser features. The measured parallax, $\pi=1.79\pm 0.08$ mas, is consistent with $Gaia$ EDR3 and previously measured VLBI parallaxes within a 1-$\sigma$ 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 KVN. The three-dimensional H$_{2}$O maser kinematics indicates that the circumstellar envelope is expanding at a velocity of $13\pm4$ km s$^{-1}$, while there are asymmetries in both the spatial and velocity distributions of the maser features. Furthermore, the H$_{2}$O maser animation achieved by our dense monitoring program manifests the propagation of shock waves in the circumstellar envelope of BX Cam.