HS 0209+0832 : a DAB white dawrf with a temperature fittinginto the DB gap.

Abstract

The white dwarf H50209+0832 (B ≈13^m.9), belonging to the sparse class of spectral type DAB, was discovered by follow-up spectroscopy of candidates selected from objective- prism plates of the Hamburg Schmidt sky survey. An analysis of the optical spectrum, clearly showing the He I λ4471 line besides the Balmer lines, yields a He/H ratio of 2% and an effective temperature of 36000 K, which could be confirmed by lUE observations. Among the hybrid DAB stars, key objects for an understanding of the DA/non DA sequences, H50209+0832 is particularly interesting since its temperature lies well in the "DB gap" between 28000 and 45000 K. Compared to a homogeneously mixed H+He atmosphere a stratified atmosphere with a hydrogen layer mass of about 5 × 10^-16 M_sun in diffusion equilibrium with the underlying helium envelope does not give an equally satisfying fit to the observed spectrum. So we conclude that diffusion equilibrium is not yet reached, possibly because meridional streams prevent gravitational separation or accretion from the interstellar medium has occurred only recently. This may also explain the occurrence of the C IV line at 1549Å. The alternative interpretation of H50209+0832 as a binary DA+DB appears less likely but cannot be ruled out completely.