Tidal heating in detached double white dwarf binaries

TL;DR

This paper models tidal heating effects in detached double white dwarf binaries, predicting significant temperature increases and larger radii at mass transfer onset, which impacts their evolution and related transient phenomena.

Contribution

It introduces a combined model of tidal heating and temperature-dependent radius for helium white dwarfs in binaries, predicting their thermal and orbital evolution.

Findings

White dwarfs can be up to 40% hotter before mass transfer.

Mass transfer may begin at orbital periods up to 17 minutes.

Donor white dwarfs are generally hot and large at mass transfer onset.

Abstract

Short--period ($P<$1 hr orbits) detached double white dwarf binary (DWDB) components identified with transient surveys (e.g. SDSS, ZTF) have hot surface temperatures ($>$10,000 K) and observed radii a factor two larger than completely degenerate white dwarfs. We formulate tidal heating in helium composition extremely low mass white dwarf (ELM WD) components of detached DWDBs which reach mass transfer within a Hubble time. We combine a mass radius relation which varies with surface temperature and the equilibrium tidal friction model of Hut 1981, where the additional orbital energy loss from tidal friction is accounted for by increases in the primary surface temperature, and hence increasing radius. Applying this heating model to the current sample of binaries with ZTF, we predict temperature increases from the present day of up to $\sim$40\% before the onset of mass transfer. We find that helium white dwarfs are generically hot and large at the onset of mass transfer, even for the oldest DWDBs whose components can cool to be degenerate by the present day. In the population of Galactic DWDBs, we find that the onset of mass transfer should occur at orbital periods as long as 1000s (17 minutes), or binary gravitational wave frequency of 2 mHz. This is over three times longer than periods expected for degenerate WD (5 minutes). Since mass transferring DWDBs are progenitors for a variety of transients and stellar populations e.g. RCrB stars, AM CVn binaries, so-called Type .Ia supernova, the finite temperature of donor white dwarfs should be taken into account.