GD 323: New Observations and Analysis of the Prototype DAB White Dwarf

Abstract

In this paper we present new spectroscopic observations of very high quality of the hybrid DAB white dwarf GD 323. GD 323 shows both hydrogen and helium lines and is located at an effective temperature (approximately equals 28,750 K), which places it at the hot end of the DB sequence just below the empirical DB gap. With recent results for the related object G104-7 in mind (Holberg et al. 1990), we first discount the possibility of variability. Repeated spectra obtained with similar instrument parameters reveal little evidence of spectroscopic variability on time scales from hours to a decade. We explore four different scenarios in our attempt to understand the energy distribution from the UV to the red and the optical spectrum: a mixed H/He atmosphere, a binary DA + DB, a simple spot model of hydrogen and helium-rich areas on the surface, and a stratified H/He atmosphere in diffusion equilibrium. None of the models studied allows a completely satisfactory fit to all observations; the scenario that seems most promising is that of a stratified atmosphere. However, with standard input physics, even this cannot explain the detailed line profiles observed.