Sub-mm maser VLBI: How do stellar winds break free from the star's gravity?
Paper in proceedings, 2014
Sub-mm VLBI with ALMA will be a superb tool for imaging masers, as well as its capabilities as the "Event Horizon Telescope". Individual masing clouds will be resolved in multiple velocity channels, providing both beamed and unbeamed sizes. The relationship between beaming and intensity can then be used to distinguish between steady or turbulent and shocked outflows. Sub-au resolution will also confirm which transitions are co-spatial or segregated, compared with maser models predicting the required small-scale variations in density, temperature, velocity structure and radiation field. This will be aided further by micro-arcsec cm-wave maser results which are already starting to appear from RadioAstron. ALMA by itself will resolve the overall structure and differing extents of the various maser species, along with thermal lines, dust and continuum. Science verification data has already provided the first-ever resolved images of sub-mm water masers and continuum around an evolved star (VY CMa), showing that the high excitation lines are surprisingly extended. This requires local heating, maybe shocks, at many tens of stellar radii. These lines straddle the dust formation zone and future higher resolution observations will also resolve dust clumps and measure their acceleration. Such studies are vital to understanding how material ejected from cool stars breaks free from the stellar gravity, and the role of dust in this process.