White dwarfs in wide binaries: the strong effects of stellar evolution and mass loss

TL;DR

This study analyzes Gaia data to understand how stellar evolution and mass loss affect the properties and distribution of wide binary systems containing white dwarfs, revealing complex dependencies on mass and evolutionary timescales.

Contribution

It models the impact of post-main-sequence mass loss on wide binary orbits, explaining eccentricity distributions and binary fractions with new dynamical insights.

Findings

White dwarf binary fraction declines steeply with mass above 0.6Msun.

White dwarf binaries have lower eccentricities than main-sequence binaries at similar separations.

Recoil velocities of 0.25-4 km/s are needed to match observed separation distributions.

Abstract

We examine the statistics of main-sequence / main-sequence, main-sequence / white-dwarf and white-dwarf / white-dwarf wide binaries at 10^2.5-10^4 AU separations in Gaia data. For binaries containing a white dwarf, we find a complex dependence of the wide binary fraction on the white dwarf mass, including a steep decline as a function of mass at >0.6Msun. Furthermore, we find that wide binaries containing white dwarfs have significantly lower eccentricities than main-sequence binaries at the same separations. To model these observations, we compute the effects of post-main-sequence mass loss on the orbital parameters of wide binaries in all regimes of timescales, from secular to impulsive, and incorporate this dynamics in a population synthesis model. We find that adiabatic expansion of the orbits in binaries with slow enough evolutionary processes is the most likely explanation for the puzzling eccentricity distribution of white dwarf wide binaries. The steeply declining white dwarf binary fraction as a function of mass requires that the timescale for mass loss must be significantly shorter for high-mass stars (10^3-10^4 years) than for the low-mass ones. We confirm previous studies that suggested that recoil in the range 0.25-4 km/s is required to explain the observed distribution of separations of white dwarf wide binaries. Finally, for low-mass white dwarfs (<0.5Msun), we see interesting signatures of their formation due to close binary evolution in their wide binary statistics. Our observations and modeling provide a novel dynamical constraint on the mass-loss stages of stellar evolution that are difficult to probe with direct observations.