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arXiv · 1002.4677

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

T. R. Marsh·B. T. Gaensicke·D. Steeghs·J. Southworth·D. Koester·V. 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 hours. 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 to 6.6) with broad wings from a hotter, high-mass white dwarf companion (11,000 to 14,000K; ~ 1Msun). The high-mass white dwarf has unusual line profiles which lack the narrow central core to Halpha that is usually seen in white dwarfs. This is consistent with rapid rotation with v sin i = 500 to 1750km/s, 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).

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T. R. Marsh, B. T. Gaensicke, D. Steeghs, J. Southworth, D. Koester, V. Harris, L. Merry. 2011-05-16. Detection of a white dwarf companion to the white dwarf SDSSJ125733.63+542850.5. https://doi.org/10.1088/0004-637x%2F736%2F2%2F95

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