Astronomy and Astrophysics – Astrophysics – Solar and Stellar Astrophysics
Scientific paper
2010-02-25
Astronomy and Astrophysics
Astrophysics
Solar and Stellar Astrophysics
10 figures, 1 table, ApJ in press. Substantially revised and expanded from the original version to understand the uncertaintie
Scientific paper
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).
Gaensicke Boris T.
Harris Vincent
Koester Detlev
Marsh Thomas R.
Merry L.
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