Population demographics of white dwarf binaries with intermediate separations: Gaia constraints on post-AGB mass transfer

TL;DR

This study uses Gaia data to analyze the demographics of white dwarf-main sequence binaries with intermediate separations, revealing discrepancies with standard models and constraining their formation and population characteristics.

Contribution

It presents a comprehensive forward-modeling approach incorporating Gaia's selection effects to better understand the formation and demographics of intermediate-separation WD+MS binaries, challenging existing binary evolution models.

Findings

The period distribution is nearly flat, proportional to P^{0.12}.

The WD mass distribution peaks sharply at 0.6 solar masses.

Approximately 0.4% of solar-type stars have WD companions with 100-1000 day periods.

Abstract

Astrometry from the Gaia mission has revealed a large population of white dwarf (WD) + main sequence (MS) binaries with periods of $100 - 1000\,$d. These systems have separations intermediate to predictions from standard binary evolution scenarios, challenging models of binary interaction and mass transfer. Because the selection function of Gaia astrometric catalogs is complex, the underlying population demographics of WD+MS binaries remain imperfectly understood. We present a forward-model of the AU-scale WD+MS binary population probed by Gaia that begins with a realistic binary population and incorporates a full model of Gaia mock observations and astrometric model fitting, as well as cuts employed in producing the Gaia astrometric catalog and selecting WD+MS binary candidates. We model the formation of AU-scale WD+MS binaries as the result of interaction when the WD progenitor is an AGB star. We test several models for the binaries' formation, including stable mass transfer with theoretically predicted stability criteria and two different formalisms for common envelope evolution. None of these models succeed in reproducing the observed component mass distributions or the absolute number of WD+MS binaries. The data are best reproduced by a model in which post-AGB binaries remain wide only if the accretor-to-donor mass ratio exceeds $\sim 0.4$. Our model allows us to constrain the intrinsic population demographics of intermediate-separation WD+MS binaries. The inferred period distribution is close to flat, with ${\rm d}N/{\rm d}P_{{\rm orb}}\propto P_{{\rm orb}}^{0.12}$, while the WD mass distribution is sharply peaked at $0.6\,M_{\odot}$. The model implies that $\sim 0.4\%$ of solar-type stars have WD companions with periods of $100 - 1000\,$d.