Circumstellar Disks Around Rapidly Rotating Be-Type Stars

Abstract

Be stars are rapidly rotating B-type stars that eject large amounts of material into a circumstellar disk. Evidence of the presence of a disk is found through hydrogen emission lines in their spectra, IR excess flux, and linear intrinsic polarization. In this dissertation, we report the first simultaneous interferometric and spectroscopic observations of circumstellar disks around 24 bright Be stars made using the techniques of long baseline interferometry and moderate resolution spectroscopy in the near infrared. The goal of the project is to characterize the fundamental geometrical and physical properties of the emitting regions that are responsible for the IR flux excesses detected in the K-band in our sample stars. This observational work has been conducted with both the Center for High Angular Resolution Astronomy (CHARA) Array at Mount Wilson Observatory, and the Mimir spectrograph at Lowell Observatory. The visibility measurements were interpreted with different geometrical and physical disk models in order to determine the spatial extension of the disk, the inclination angle, the position angle, and the density profile of the disk. We find that the spatial extension of the circumstellar disk in the K-band is only about a few stellar radii, and that the density structure of the disk is consistent with a radially decreasing function with a density exponent that ranges between 2.5 and 3.5. The resulting disk densities are in a good agreement with those derived from the Infrared Astronomical Satellite (IRAS) measurements, and the resulting disk geometries are consistent with previous polarimetric measurements. We find that the K-band sizes of the emitting regions in the disk are smaller by a factor of two than the Hα sizes, and we show that this is due to the lower opacity of the continuum in the disk. By combining recent measurements of the projected rotational velocities with the disk inclination angles derived from interferometry, we were able to estimate the actual equatorial linear rotational velocities of the Be stars in our sample. The obtained linear rotational velocities indicate that Be stars are rapid rotators with an equatorial velocity that is about 0.7 - 0.9 of their critical velocities