New Light on Young Stars:
Spitzer's View of Circumstellar Disks

The Circumstellar Environments of B[e] Supergiants in the Magellanic Clouds: Evidence for Dusty Disks

First Author:
Raghvendra Sahai
Email: sahai AT jpl.nasa.gov
JPL/Caltech
Mail Stop 183-900
4800 Oak Grove Drive
Pasadena, CA 91109 USA

Abstract

iThe study of massive stars is central to many areas of astrophysics, including the study of supernovae, nucleosynthesis, galactic evolution, and reionisation of the early universe. One of the more extreme classes of massive stars is that of early-type, emission-line (B[e]) supergiants and hypergiants. These objects represent a late phase in the core-burning lifetimes of very massive stars, and include amongst them the most luminous stars known. As a consequence of the small line-of-sight extinction toward the Magellanic Clouds (MCs), two-thirds of all known B[e] supergiants and hypergiants are found in the LMC and SMC. We present preliminary results from a program of IRS spectroscopy to investigate the circumstellar environments of confirmed and candidate B[e] supergiants in the MCs. More than half of the dozen or so objects observed display near- to far-IR spectral characteristics (relatively flat infrared spectral energy distributions and emission features characteristic of silicate dust and, in a few cases, PAHs) indicating the presence of circumstellar disks. Most of the objects with dusty disks are located near compact HII regions or in the vicinities of massive young star clusters, indicating ages of at most afew Myr. These results suggest that most B[e] supergiants represent scaled-up analogs to intermediate-mass, pre-main sequence (Herbig Ae/Be) star-disk systems. The few candidate objects that do not display disk-like IR spectral energy distributions are all associated with compact HII regions; these objects are probably massive pre-MS or zero-age MS (as opposed to B[e] supergiant) stars. We use models of passive, irradiated disks to fit the observed spectra for specific objects that display disk-like spectra, in order to constrain disk mass and radial density structure.