Widening of Binaries via Non-conservative Mass Transfer as a Formation Channel for Gaia Black Hole System

Abstract: The detected Gaia systems hosting compact objects challenge standard models of binary star evolution. In particular, if the observed black hole (BH) systems evolved in isolation, it is expected that they underwent a mass transfer phase. Given their highly unequal mass ratios, such mass transfer is dynamically unstable within standard models, leading to either a stellar merger or a final binary with a very short orbital period. In contrast, the observed systems have much wider orbits than predicted, making their formation within conventional evolutionary frameworks difficult to reconcile. With the aid of detailed binary evolution calculations, we test whether fully non-conservative mass transfer, where mass is lost from the system carrying the specific angular momentum of the donor's center of mass, can explain the properties of two of the Gaia BH systems. This mass-loss geometry differs from the standard isotropic re-emission model, which assumes mass loss from the accretor's vicinity. We find that our mass-loss model, without the need for fine-tuning, reproduces the observed orbital periods of the two Gaia BH systems remarkably well across a wide range of initial conditions. This scenario, therefore, offers a plausible formation pathway for these systems. We speculate that orbital widening during mass loss could result from the unequal Roche-lobe sizes of the components and eruptive mass loss driven by the donor's high-opacity subsurface layers. Similar mass loss may also be relevant for all other classes of post-mass-transfer binaries that face analogous evolutionary challenges, including Gaia neutron star and white dwarf systems, binaries hosting stripped-envelope Wolf-Rayet stars, and low-mass X-ray binaries.