Circumstellar material around rapidly rotating B stars. II - On the nature of ultraviolet shell lines in the spectra of Be and shell stars

Abstract

Using high-resolution (FWHM approximately 0.05 A) ultraviolet spectral data obtained with Copernicus, the nature of shell lines in Be and shell stars. Scans were obtained for various excited fine-structure lines of Fe III, Si II, and O I. It is found that Be stars, in general, do not display shell lines in the ultraviolet, contrary to the findings of previous work at lower resolution. The only Be stars that display strong UV shell lines are the 'classical' shell stars, i.e., stars that also display shell lines in the optical region. Furthermore, it is found that the shell lines, when present, have essentially zero velocity shift with respect to the star. The resonance shell lines seen in the shell stars are generally asymmetric on the short-wavelength side, indicating that the lines are formed in a low-velocity outflowing region. The results presented here argue strongly in favor of a flattened disk structure for the shell line forming region, and against some recent scenarios involving spherical symmetry.