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S. Vennes, S. Bowyer, and J. Dupuis
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_solar) 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 T_eff = 40,300 +/- 100 K and n(He)/n(H) = (2.4 +/- 0.1)E-4 (assuming log g = 9.0). We also constrain the local interstellar medium column densities of neutral helium and hydrogen to n_HeI = 6E+16 and n_HI = 9E+17 cm^(-2). The presence of helium in an isolated, massive DA white dwarf is paradoxical. We examine and rule out a few 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.
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