DISCOVERY OF PHOTOSPHERIC HELIUM IN THE ULTRAMASSIVE DA WHITE DWARF GD 50

摘要

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 solar mass) 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)(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 n_(He I) = 6 x 1016 and n_(H I) = 9 x 10~(17) 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.