Gas Content in the Debris Disks of Nearby A-Type Stars

Since the IRAS discovery of Vega’s large thermal infrared excess over the expected photospheric flux, dust disks have been found around a large fraction of main‐sequence A‐type stars. While dust in debris disks has been studied extensively with Spitzer, relatively little is known about their gas content. We have carried out detailed studies of the circumstellar environments of all nearby A‐type stars. We have studied both thermal emission from their circumstellar dust and absorption from circumstellar gas. With high‐resolution and high signal‐to‐noise visible spectra, we identified about a dozen main‐sequence A‐type stars with circumstellar gas through a volume‐limited survey. Although these spectra confirm the existence of the circumstellar gas and serve as a probe of the gas dynamics, they do not allow us to determine the gas density and temperature. Fortunately, there are many absorption lines in the UV and FUV range that are sensitive to the gas density and temperature. Through our FUSE, IUE, HST, and ground‐based observations, we found that about half of these stars with variable circumstellar gas, similar to that seen in spectra of Beta Pic. We observed 3 of these stars with FUSE. This unique set of stars allows us to study the evolution of both gas and dust in the circumstellar environments of A‐type stars.