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

TL;DR

This paper confirms the presence of a second white dwarf in the binary system SDSSJ1257+5428 through spectroscopy and ultraviolet data, revealing a complex system with potential supernova progenitor characteristics.

Contribution

It provides the first direct detection and characterization of the second white dwarf in SDSSJ1257+5428, including its temperature, gravity, and possible rapid rotation.

Findings

The system contains a cool, low gravity white dwarf and a hotter, high-mass white dwarf.

The high-mass white dwarf shows unusual broad line profiles possibly due to rapid rotation.

SDSSJ1257+5428 may evolve into a double white dwarf system rather than a supernova progenitor.

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).