MERLIN observations of the circumstellar envelope of VX Sagittarius

Abstract

MERLIN maps of the H2O and OH maser emission from the circumstellar envelope of the supergiant star VX Sagittarius are presented. The OH masers at 1612 MHz are at a radial distance of 2 × 1016 cm from the star and lie in a ‘thin-shell’ region which has an expansion velocity of ∼19 km s−1. The H2O masers occur nearer the star in an accelerating ‘thick-shell’ region which has a mean radius of ∼ 3 × 1015 cm and a mean expansion velocity of ∼ 9 km s−1. The OH mainline masers have angular and velocity extents similar to those of the H2O masers and are believed to lie in the same thick shell, although absolute position measurements are needed to confirm this. The distribution of masers in this inner region is asymmetric. This may be due to the influence of the stellar magnetic field. The magnetic field in the envelope is determined from Zeeman splitting of the OH mainlines to be ∼ 2 mG. The velocity field of the circumstellar envelope as traced by SiO, H2O and OH maser lines cannot be driven by a 1/R2 radiation pressure law with constant dust opacity. It is suggested that the driving force per unit mass increases outwards to a distance of at least 50 stellar radii.