The fate of Gaia's wide binaries: Interplay of white-dwarf recoil and tidal capture

Abstract

White dwarfs (WDs) receive natal velocity boosts of $\sim 1 \, \mathrm{km \, s^{-1}}$ due to recoil from asymmetric mass loss during the late asymptotic giant branch (AGB) stage. In a wide binary, the acceleration of a proto-WD exerts a torque, modifying the orbital eccentricity. Potential signatures of this effect have been detected in Gaia's astrometric binary sample. At the same time, an AGB star's puffy envelope facilitates strong tidal interactions in binaries with periapsis separations of a few AU, capturing the companion into a tighter orbit and potentially driving the system towards a common-envelope phase. Using an analytical model for wide binary evolution under asymmetric AGB mass loss, we find that recoil can induce tidal interactions in up to $30\%$ of initially wide binaries on the AGB or post-AGB for initial separations of $\sim 100 \mbox{--} 1000$ AU. We relate these interactions to three recent observational puzzles: (i) The dearth of wide WD+MS and WD+WD binaries in Gaia DR3 with eccentricities $\gtrsim 0.9$. (ii) The formation of moderately eccentric WD+MS and WD+WD binaries with orbital periods of $\sim 100 \mbox{--} 1000$ days, which may happen via a high-eccentricity common-envelope phase. (iii) The origin of low-luminosity, long-timescale, dust-obscured transients towards AGB progenitors, such as the ongoing event WNTR23bzdiq in M31. Our findings have potential implications for the survival and dynamical evolution of planetary systems around WD progenitors, to be investigated in future works.