Tracing wind motion in the H2O maser shell of VX Sgr

Masers provide accurate velocity information on the movement of matter in the circumstellar shells (CSE) of late-type stars. We show here for the case of the Red Supergiant (RSG) VX Sgr, evidence for the radially outward movement of an H2O maser cloud traversing in 28 years a distance of ~90 au. The maser cloud (VXSgr-H2O-1987/14) was detected in 1987 at a distance of ~140 au from the central star moving with a velocity of v = 14 km/s away from the observer. The cloud was followed for decades during a 22 GHz H2O maser monitoring program at Medicina and Effelsberg until 2015, when the cloud was at a distance of ~230 au and had increased its velocity to v = 19 km/s. VXSgr-H2O-1987/14 was identified in MERLIN maps from 1994 and 1999 (Murakawa et al. 2003, Richards et al. 2012), and its location in the shell suggests an angle of 16+-1 degrees between the line-of sight and the outflow direction. This shows the ability of well-designed H2O maser monitoring programs to map wind motions in CSEs of RSGs at typical radial distances of 100-200 au.