H-Alpha Emission in Hot Degenerates and OB Subdwarfs

Abstract

Based on observations obtained using the 60-inch telescope at Palomar Observatory which is jointly owned by the California Institute of Technology and the Carnegie Institution of Washington and on observations obtained with the 200-inch Hale telescope which is owned by the California Institute of Technology.The intial observations of white dwarf stars and their immediate precursors, the hot subdwarfs, suggested that these stars possess the simplest (and, aesthetically, the most pleasing) spectra of any astronomical object. The high gravity leads to the spectrum being dominated, in the most stars, by broad lines of either hydrogen or helium, depending on the composition of the photospheric layers, with a few stars exhibiting lines from both species. However, the more detailed observations of recent years have revealed a higher degree of complexity. In particular, absorption lines of high excitation species (N V, C IV, etc.) have been detected in the ultraviolet spectra of several hot white dwarfs (Bruhweiler & Kondo, 1982) and, most recently, high resolution optical spectra have shown that one of the latter stars, the hottest known DA, G191-B2B, exhibits significant emission in the core of the H-alpha absorption line (Reid &: Wegner, 1988). Following up the latter observation, we have obtained high resolution spectra of a number of hot subdwarfs, with temperatures ranging from ~ 20,000K to more than 60,000 K. Most of these stars also exhibit Baimer emission, at Hβ as well as Hα in at least one case. We suggest the temperature reversal in the stellar atmosphere may be a function of the He/H ratio at the level of the photosphere.