Triage of the Gaia DR3 astrometric orbits. II. A census of white dwarfs

Abstract

The third data release of Gaia was the first to include orbital solutions assuming non-single stars. Here, we apply the astrometric triage technique of Shahaf et al. to identify binary star systems with companions that are not single main-sequence stars. Gaia’s synthetic photometry of these binaries is used to distinguish between systems likely to have white-dwarf companions and those that may be hierarchical triples. The study uncovered a population of nearly 3 200 binaries, characterised by orbital separations on the order of an astronomical unit, in which the faint astrometric companion is probably a white dwarf. This sample increases the number of orbitally solved binary systems of this type by about two orders of magnitude. Remarkably, over 110 of these systems exhibit significant ultraviolet excess flux, confirming this classification and, in some cases, indicating their relatively young cooling ages. We show that the sample is not currently represented in synthetic binary populations, and is not easily reproduced by available binary population synthesis codes. Therefore, it challenges current binary evolution models, offering a unique opportunity to gain insights into the processes governing white-dwarf formation, binary evolution, and mass transfer.

A Deficit of Massive White Dwarfs in Gaia Astrometric Binaries

“…Based on Gaia DR3 data, Shahaf et al (2023) complied a sample of high-probability class III binary systems consisting of an MS and a compact object. Using Gaia synthetic photometry, calculated based on Gaia's low-resolution BP/ RP spectra, Shahaf et al (2024) identified systems with a significant excess red color to further differentiate between class II hierarchical MS triple systems and binaries with a compact-object companion. While an MS companion(s) is expected to contribute flux in the redder wavelengths, the light contribution of a WD companion to the Gaia band is expected to be negligible (due to the extreme size ratio), unless it is very hot and young; then it contributes to the bluer end of the spectrum (see Shahaf et al 2024 for details).…”

“…The orbital separations of these systems, on the order of 1 au, place them in a largely unexplored region of parameter space, with only a few such systems known previously (Anguiano et al 2022; Parsons et al 2023). In addition, these systems are not easily reproduced by standard binary population synthesis codes (see discussion in Shahaf et al 2024).…”

“…Here we focus on a volume-complete sample of likely MS +WD binaries, following the method of Shahaf et al (2024; see Section 2.2 below). We should note that this triage method relies on Gaia's astrometric solutions (Halbwachs et al 2023) along with the primary mass estimates reported by Gaia (Gaia Collaboration et al 2023a).…”

“…We should note that this triage method relies on Gaia's astrometric solutions (Halbwachs et al 2023) along with the primary mass estimates reported by Gaia (Gaia Collaboration et al 2023a). It is possible that despite the precautions taken by Shahaf et al (2023Shahaf et al ( , 2024, some spurious results still got through. However, for the mass ratios discussed in this paper and for MS primaries, the contamination by spurious detections is expected to be small.…”

“…The third data release (DR3) of Gaia (Gaia Collaboration et al 2023b) was the first to include a catalog of unresolved sources with non-single-star solutions (Gaia Collaboration et al 2023a). Recently, Shahaf et al (2024) have identified nearly 3200 systems that are likely MS+WD binaries. This unprecedentedly large sample of WDs in solved astrometric binaries offers a unique opportunity to derive a model-independent mass distribution of a volume-complete WD sample.…”