Imprints of white dwarf recoil in the separation distribution of Gaia wide binaries

TL;DR

This study uses Gaia DR2 data to analyze the separation distribution of wide binaries with white dwarfs, revealing that white dwarf formation involves kicks that influence binary stability and separation.

Contribution

It provides the first extensive Gaia-based catalog of wide binaries with white dwarfs and models their separation distributions, highlighting the impact of white dwarf kicks on binary evolution.

Findings

White dwarf binaries show distinct separation distribution breaks at ~1500-3000 AU.

White dwarf formation involves a kick velocity of about 0.75 km/s.

Most wide binaries with separations over a few thousand AU become unbound after white dwarf formation.

Abstract

We construct from Gaia DR2 an extensive and very pure ($\lesssim 0.2\%$ contamination) catalog of wide binaries containing main-sequence (MS) and white dwarf (WD) components within 200 pc of the Sun. The public catalog contains, after removal of clusters and resolved higher-order multiples, $>$50,000 MS/MS, $>$3,000 WD/MS, and nearly 400 WD/WD binaries with projected separations of $50 \lesssim s/{\rm AU} < 50,000$. Accounting for incompleteness and selection effects, we model the separation distribution of each class of binaries as a broken power-law, revealing marked differences between the three populations. The separation distribution of MS/MS systems is nearly consistent with a single power-law of slope $-1.6$ over at least $500 < s/{\rm AU} < 50,000$, with marginal steepening at $s > 10,000$ AU. In contrast, the separation distributions of WD/MS and WD/WD binaries show distinct breaks at $\sim$ 3,000 AU and $\sim$1,500 AU, respectively: they are flatter than the MS/MS distribution at small separations and steeper at large separations. Using binary population synthesis models, we show that these breaks are unlikely to be caused by external factors but can be explained if the WDs incur a kick of $\sim$ 0.75 km s$^{-1}$ during their formation, presumably due to asymmetric mass loss. The data rule out typical kick velocities above 2km s$^{-1}$. Our results imply that most wide binaries with separations exceeding a few thousand AU become unbound during post-MS evolution.