(the following information provided by Dr. Pierre Chayer)
Drs. Stephane Vennes, Stuart Bowyer, and Jean Dupuis obtained and analyzed the peculiar EUVE spectrum of the hot, hydrogen-rich white dwarf, GD 50. Extreme ultraviolet (EUV) spectroscopy of this massive white dwarf reveals an unusual photospheric mixture of hydrogen and helium, showing a prominent He II resonance line series. Dr. Vennes and his collaborators point out that the presence of helium in an isolated, massive DA white dwarf is paradoxical, because radiative levitation of helium is clearly undone by the high surface gravity (log g = 9.0 for GD 50).
They propose two possible mechanisms to explain the presence of helium in the photosphere of GD 50. They suggest that helium is accreted from the ISM at the Bondi-Hoyle rate. But, because accretion onto GD 50 would constitute a unique occurrence for a hot white dwarf, they examined an alternative explanation. If massive white dwarfs do result from stellar mergers, large orbital angular momentum may be preserved and result in large meridional circulation current, possibly dredging-up helium from the envelope. Although Dr. Vennes and his team do find evidence of large rotational velocity in EUV He II line profiles, they propose additional observational tests of the dredge-up model.
Figure 1 (98 Kb post-script): EUV spectroscopy of the hot DA white dwarf GD 50