White Dwarfs in Wide Binaries: The Strong Effects of Stellar Evolution and Mass Loss

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 >0.6 M ⊙ . 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 these 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 yr) than for the low-mass ones. We confirm previous studies that suggested that recoil in the range 0.25–4 km s −1 is required to explain the observed distribution of separations of white-dwarf wide binaries. Finally, for low-mass white dwarfs (<0.5 M ⊙ ), 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.