Discovery of Photospheric Helium in the Ultra-Massive DA White Dwarf GD 50

Abstract

Extreme-ultraviolet (EUV) spectroscopy of the hot, hydrogen-rich white dwarf GD 50 reveals an unusual photospheric mixture of hydrogen and helium. This hot DA white dwarf is also remarkable for its mass (~1.2 M⊙) near the Chandrasekhar limit. The spectra obtained with the Extreme-Ultraviolet Explorer (EUVE) show a prominent He II resonance line series and constrain its atmospheric parameters to Teff = 40,300 +/- 100 K and n(He)/n(H) = (2.4 +/- 0.1) x 10-4 (assuming log g = 9.0). We also constrain the local interstellar medium column densities of neutral helium and hydrogen to nHe I = 6 x 1016 and nH I = 9 x 1017 cm-2. The presence of helium in an isolated, massive DA white dwarf is paradoxical. We examine and rule out a couple of scenarios (radiative levitation, accretion from the ISM or a hypothetical companion), but we suggest that if massive white dwarfs do result from stellar mergers, large orbital angular momentum may be preserved and result in a large meridional circulation current, possibly dredging up helium from the envelope. Although we find evidence of large rotational velocity in EUV He II line profiles, we propose additional observational tests of the dredge-up model.