The Atmospheres of White Dwarfs -- III: The Line Spectra

Model atmospheres with temperatures in the range |${10\,000}^{\circ}\,\leqslant\,{T}_\text{eff}\,\leqslant\,{25\,000}^{\circ}\,\text{K}$| and with gravities |$6\,\leqslant\,\text{log}\,(g)\,\leqslant\,9$| have been constructed for various helium abundances with a view to understanding the spectra of the hotter white dwarfs. The roles of accretion, convection, circulation, gravitational diffusion and magnetic fields are also examined. It is shown that the DB stars are confined to the temperature range |${15\,000}^{\circ}\,\leqslant\,{T}_\text{eff}\,\leqslant\,{18\,000}^{\circ}$| in which convection also becomes important in the outer layers of a helium rich star. The DA stars seem to avoid this temperature range. The hydrogen and metal abundances in DB atmospheres are shown to be reduced by factors 105 and 103 respectively compared to solar values. Possible explanations of DC spectra are also discussed.