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