Journal
ASTROPHYSICAL JOURNAL
Volume 736, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/736/2/95
Keywords
accretion, accretion disks; binaries: close; supernovae: general; white dwarfs
Categories
Funding
- UK's STFC
- Science and Technology Facilities Council [ST/G004269/1, ST/G002355/1, PP/D005914/1, ST/J000035/1, ST/I001719/1, PP/D000955/1, PP/F000057/1] Funding Source: researchfish
- STFC [ST/J000035/1, ST/G002355/1, ST/G004269/1, PP/D005914/1, PP/D000955/1, PP/F000057/1, ST/I001719/1] Funding Source: UKRI
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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 similar or equal to 6300 K, log g = 5-6.6) with broad wings from a hotter, high-mass white dwarf companion (11,000-14,000 K;similar to 1M(circle dot)). The high-mass white dwarf has unusual line profiles which lack the narrow central core to Ha that is usually seen in white dwarfs. This is consistent with rapid rotation with v sin 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 approximate to 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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