Detection of a White Dwarf Companion to the White Dwarf SDSSJ125733.63+542850.5
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 (T eff ~= 6300 K, log g = 5-6.6) with broad wings from a hotter, high-mass white dwarf companion (11, 000-14, 000 K ~1 M sun). 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).
Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2011
- DOI:
- 10.1088/0004-637X/736/2/95
- arXiv:
- arXiv:1002.4677
- Bibcode:
- 2011ApJ...736...95M
- Keywords:
-
- accretion;
- accretion disks;
- binaries: close;
- supernovae: general;
- white dwarfs;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 10 figures, 1 table, ApJ in press. Substantially revised and expanded from the original version to understand the uncertainties of fitting the spectra and in response to the referee's report