A global view of post-interaction white dwarf-main sequence binaries

Abstract

Common-envelope evolution (CEE) is among the most uncertain phases in binary evolution. To empirically constrain CEE, we construct a uniformly selected sample of eclipsing post--common-envelope binaries (PCEBs). Starting from an unresolved white dwarf-main-sequence (WDMS) candidate sample within 200 pc selected from the Gaia color-magnitude diagram, we identify 39 detached eclipsing WDMS binaries using ZTF light curves. The binaries contain cool M dwarfs orbiting warm white dwarfs with orbital periods ($P_{\mathrm{orb}}$) of 0.1-2 d. The sample's simple selection function allows us to model observational incompleteness and infer intrinsic properties of the PCEB population. We find an orbital-period distribution consistent with being log-uniform over 0.1-2 d, contrary to recent reports of a bimodal distribution. The companion-mass distribution peaks around $0.25{\mathrm{M}}_{\odot }$ and declines steeply toward larger masses. The estimated local space density is $7.2\times 10^{-5}\mathrm{p}{\mathrm{c}}^{-3}$, corresponding to a Galaxy-wide birth rate of 0.01 per year. Combining our results with recent Gaia-based constraints on wider WDMS binaries, we construct an empirical period distribution of post-interaction WDMS binaries spanning 0.1-1000 d. The emerging period distribution is roughly log-flat (d$N/\mathrm{d}\log P_{\mathrm{orb}}\propto P_{\mathrm{orb}}^0$) at $P_{\mathrm{orb}}<2$ d and log-increasing (d$N/\mathrm{d}\log P_{\mathrm{orb}}\propto P_{\mathrm{orb}}^1$) at $P_{\mathrm{orb}}=100-1000$ d. The 10-100 d regime remains poorly constrained, but a few nearby systems suggest it is also well-populated. Short-period PCEBs ($P_{\mathrm{orb}}<2$ d) with M dwarf companions are roughly 2-3 times more common than wide ($P_{\mathrm{orb}}=100-1000$ d) WDMS binaries with FGK companions, which likely formed through stable mass transfer. These results provide direct observational constraints on CEE and an empirical benchmark for binary-population models.

accepted for publication in PASP