A hidden population of white dwarfs in Gaia astrometric binaries: Insights and the mystery of missing massive white dwarfs

The third data release of Gaia was the first to include orbital solutions assuming non-single stars. By applying the astrometric triage technique of Shahaf et al. combined with Gaia's synthetic photometry we uncovered a population of nearly 3200 binaries with main-sequence primaries, characterized by orbital separations on the order of an astronomical unit, in which the faint astrometric companion is probably a white dwarf (WD). This sample increases the number of orbitally solved binary systems of this type by about two orders of magnitude. It is not currently represented in synthetic binary populations, and is not easily reproduced by available binary population synthesis codes. Such systems are likely to have undergone a phase of stable mass transfer while the WD progenitor was on the asymptotic giant branch.
We find that the number of massive WDs relative to the total number of WDs in a volume-complete subsample of these systems with K/M-dwarf primaries is smaller by an order of magnitude compared to their occurrence among single WDs in the field. One possible reason can be an implicit selection of the WD mass range if these are indeed post-stable-mass-transfer systems. Another reason can be the lack of merger products in our sample compared to the field, due to the relatively tight orbital separations of these systems.
This sample offers a unique opportunity to gain insights into the processes governing WD formation, binary evolution, and mass transfer.