Population demographics of post-interaction WDMS binaries: From common envelope evolution to stable mass transfer

Abstract

Close white dwarf (WD) + main-sequence (MS) binaries are end products of mass transfer (MT) that occurred when the WD progenitor was a giant, making their population demographics a powerful probe of binary evolution. Several recent works have constructed samples of WD+MS binaries with well-understood selection functions using data from wide-field surveys. These include (a) au-scale astrometric binaries from Gaia that can be shown to contain a WD on dynamical grounds, (b) au-scale astrometric binaries in which a hot WD is detected through a GALEX UV excess, and (c) close binaries discovered through eclipses. Together, these samples probe outcomes of both stable MT and common-envelope evolution, and interactions on both the red giant branch (RGB) and asymptotic giant branch (AGB). We forward-model the three observed samples simultaneously. This approach produces robust constraints on uncertain binary evolution parameters because binaries removed from one population are predicted to appear in another. Our modeling includes a realistic initial binary population and treatments of the selection effects affecting all samples. We find that MT from AGB donors requires a critical accretor-to-donor mass ratio of $\sim0.4$ and is more stable than MT from RGB donors, for which we find a critical ratio of $\gtrsim0.65$. A common-envelope efficiency of $αλ\sim0.3$ matches the relative numbers of close and wide systems and the period distribution of close systems. Most stable-MT products in the sample, including those with RGB donors, retain nonzero eccentricities ($\simeq0.1$). The model does not fully reproduce the mass distribution of main-sequence stars in post-common-envelope binaries, which exhibits a cliff well below the fully convective limit, pointing to missing physics that should be explored in future work.