A COMPREHENSIVE SPECTROSCOPIC ANALYSIS OF DB WHITE DWARFS

Abstract

We present a detailed analysis of 108 helium-line (DB) white dwarfs based on\nmodel atmosphere fits to high signal-to-noise optical spectroscopy. We derive a\nmean mass of 0.67 Mo for our sample, with a dispersion of only 0.09 Mo. White\ndwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample,\nand their mass distribution appears similar to that of DB stars. As in our\nprevious investigation, we find no evidence for the existence of low-mass (M <\n0.5 Mo) DB white dwarfs. We derive a luminosity function based on a subset of\nDB white dwarfs identified in the Palomar-Green survey. We show that 20% of all\nwhite dwarfs in the temperature range of interest are DB stars, although the\nfraction drops to half this value above Teff ~ 20,000 K. We also show that the\npersistence of DB stars with no hydrogen features at low temperatures is\ndifficult to reconcile with a scenario involving accretion from the\ninterstellar medium, often invoked to account for the observed hydrogen\nabundances in DBA stars. We present evidence for the existence of two different\nevolutionary channels that produce DB white dwarfs: the standard model where DA\nstars are transformed into DB stars through the convective dilution of a thin\nhydrogen layer, and a second channel where DB stars retain a helium-atmosphere\nthroughout their evolution. We finally demonstrate that the instability strip\nof pulsating V777 Her white dwarfs contains no nonvariables, if the hydrogen\ncontent of these stars is properly accounted for.\n