Detection of a white dwarf companion to the white dwarf SDSSJ125733.63+542850.5

Thomas R. Marsh, B. T. Gänsicke, D. Steeghs, J. Southworth, D. Koester, Vandra Harris, L. Merry

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Abstract

SDSSJ125733.63+542850.5 (hereafter SDSSJ1257+5428) is a compact white dwarf binary from the Sloan Digital Sky Survey that exhibits high-amplitude radial velocity variations on a period of 4.56 hr. While an initial analysis suggested the presence of a neutron star or black hole binary companion, a follow-up study concluded that the spectrum was better understood as a combination of two white dwarfs. Here we present optical spectroscopy and ultraviolet fluxes which directly reveal the presence of the second white dwarf in the system. SDSSJ1257+5428's spectrum is a composite, dominated by the narrow-lined spectrum from a cool, low-gravity white dwarf (Teff ≃ 6300K, log g = 5-6.6) with broad wings from a hotter, high-mass white dwarf companion (11, 000-14, 000K; ≈1 M). The high-mass white dwarf has unusual line profiles which lack the narrow central core to Hα that is usually seen in white dwarfs. This is consistent with rapid rotation with vsin i = 500-1750 km s-1, although other broadening mechanisms such as magnetic fields, pulsations, or a helium-rich atmosphere could also be contributory factors. The cool component is a puzzle since no evolutionary model matches its combination of low gravity and temperature. Within the constraints set by our data, SDSSJ1257+5428 could have a total mass greater than the Chandrasekhar limit and thus be a potential Type Ia supernova progenitor. However, SDSSJ1257+5428's unusually low-mass ratio q ≈ 0.2 suggests that it is more likely that it will evolve into an accreting double white dwarf (AM CVn star).

Original languageEnglish
Article number95
JournalThe Astrophysical Journal
Volume736
Issue number2
DOIs
Publication statusPublished - 1 Aug 2011
Externally publishedYes

Keywords

  • accretion, accretion disks
  • binaries: close
  • supernovae: general
  • white dwarfs

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