Statistical Properties of Stellar HO Masers− Results of Three−Year Single−Dish Observations with the VERA Iriki Telescope

Abstract

Abstract We report on the results of monitoring observations of 242 stellar H$_2$O masers, which have been made with the Iriki 20m telescope of the VLBI Exploration of Radio Astrometry (VERA) from 2003 July to 2006 November. The present paper mainly focuses on 85 stellar H$_2$O masers that have been tightly observed with a time spacing of typically 1−2 months. In particular, 46 masers out of them have been recognized concerning their periodic flux variation and have light−curve data of stellar visual light. Thus, the present paper shows some statistical views of the observed time variability properties of stellar H$_2$O masers. We found a good correlation between a time delay of the variation in the H$_2$O maser flux with respect to that in the stellar visual light and the stellar pulsation period. The corresponding phase lags are mildly scattered, but are mainly concentrated in the range, $0.7\leq\Delta \phi\leq\;$1.5. We also measured line−of−sight velocity drifts of the individual spectral peaks of H$_2$O maser emission, which indicate radial acceleration of mass−loss outflows from the evolved stars. We discuss possible pulsation−driven shock waves that are enhanced near the stellar surface, and are propagating outwards in the circumstellar envelope.