The Hα Spectroscopy of Classical B-emission Stars

Classical B-emission (Be) stars are rapidly-rotating, massive stars that possess a dense, equatorial, gaseous disk. The presence of a disk was first inferred from the Balmer series emission that these stars exhibit, and Hα emission lines remain both a hallmark observational feature and one of the key diagnostics in determining the physical conditions within the disk. In the first chapter of this thesis, we investigate the possible role of line-driven winds in disk formation. To test if line-driven winds could supply enough material to account for the equatorial disk, we check for the presence of Hα emission in the models that result from different combinations of line-force parameters. We find that certain combinations of line-force parameters can indeed produce significant Hα emission, and that line-driven winds may be an important mechanism in removing material from the central star. The next chapter of this thesis employs the code Beray to perform a detailed study of the Hα emission profiles of Be shell stars. Modelling the peak height and separation of these profiles gives an indication of the average density structure of the disk, although in some cases we found that more than one density structure could adequately reproduce these features. This finding indicates that, ideally, other observables should be simultaneously considered in order to constrain the models. It was found, however, that the absorption depth of the synthetic Hα profiles was relatively insensitive to the choice of density structure, depending instead largely on the inclination angle of the model. Thus, the inclination angles of these stars are determined to within a few degrees in our models, and we find that Be shell stars may not be oriented as close to 90° as originally thought. The final chapter of this thesis investigates the density structure of an asymmetric disk surrounding the well-known Be shell star 48 Librae. We begin our study with a thorough investigation of the central star’s spectral type (as it is not well-determined and has a significant impact on all other aspects of the model) by modelling the star’s spectral energy distribution (SED). Next, the SED and polarization levels are used to determine the inclination angle of the system. We then employ the HDUST code to model an observed Hα emission profile. We consider each peak separately, and then average the initial densities required to reproduce each peak to obtain the average initial disk density. We find that 48 Librae is best represented by a B3V central star surrounded by a very dense disk whose average initial density is 1.1 × 10−10 g cm−3 , and that the system is oriented at 85°