The Dusty, Molecular Envelopes of Red Supergiant Stars

Abstract

Red supergiant stars are surrounded by dusty envelopes, which harbor a multitude of molecules. Often, they are also known to show maser emission. We collect a sample of 88 red supergiants associated with Galactic open clusters, and detect SiO maser emission at 86 GHz towards 13 sources with the IRAM 30m telescope. The maser observations allow us to determine the stellar line-of-sight velocity with high accuracy. A comparison of the maser peak flux with previous detections at 43 GHz shows that the 86 GHz maser is stronger in 10 out of the 12 red supergiants detected in both transitions. An analysis of the colors of the observed sources indicates that the detections occur in the reddest sources, supporting a radiative pumping mechanism for the SiO maser.<br /> The hypergiant VY CMa has a famous asymmetric nebula. We apply a two-dimensional radiative transfer code to model the dust continuum spectrum and optical and near-IR images. We find that VY CMa is surrounded by a dusty disk with surface density proportional to r^{-1} which is observed under an inclination angle of 55 degrees. The dust has a total mass of 10^{-3} solar masses and consists of a mixture of metallic iron, alumina, olivine and melilite. The central star is best represented by a black body of 2800 K. The mass of VY CMa is 15 solar masses.<br /> With the Plateau de Bure interferometer, we observe a selection of molecular transition in VY CMaa's nebula: HCN(1-0), HCO+(1-0), SiS(5-4), NaCl(7-6), NaCl(8-7), SO2(3{1,3}-2{0,2}), SO2(16{2,14}-15{3,13}) and SO2(10{1,9}-10{0,10}). The interferometric images show that different molecules trace different parts of the envelope. NaCl is present exclusively in the spherical outflow, while SO2 is found in the bipolar outflow. Others, like HCN and SiS, exist in all components of the envelope. We reproduce the observed emission by constructing a model of the envelope plus outflow. The bipolar outflow has an opening angle of 120 degrees, a constant H2 density and the expansion velocity increases linearly from 15 km/s to 45 km/s.