Forming merging double compact objects with stable mass transfer

Context. Merging double compact objects (COs) are one of the possible endpoints of the evolution of stellar binary systems. As they represent the inferred sources of every detected gravitational wave (GW) signal, modeling their progenitors is of paramount importance both to gain a better understanding of gravitational physics and to constrain stellar evolution theory.
Aims: Stable mass transfer (MT) between a donor star and a black hole (BH) is one of the proposed tightening mechanisms to form binary BHs that merge within the lifetime of the universe. We aim to assess the potential of stable non-conservative MT to produce different pairings of compact objects including BHs, neutron stars (NSs) and white dwarfs (WDs).
Methods: We investigated the conditions (orbital periods and mass ratios) required for MT between a star and a CO to be stable and to lead to binary COs that merge within a Hubble time. We use published results on the response of the stellar radii to rapid mass loss, covering different evolutionary stages and masses. Coupled with analytical models of orbital evolution, we determined the boundary for unstable MT as well as the post-interaction properties of binaries undergoing stable MT. In addition, we investigated the impact of the angular momentum loss prescription in the resulting hardening by accounting for both the isotropic re-emission from the accretor's vicinity and mass outflow from the second Lagrangian point.
Results: Stable MT in systems with a CO + Roche lobe-filling star, in the completely non-conservative limit of isotropic re-emission from the vicinity of the accretor, is shown to be able to form any pair of merging double COs, with the exception of WD + BH and with a limited parameter space for NS + NS. Considering the possibility of mass outflow from the Lagrangian point L₂, the resulting parameter space for GW progenitors is shifted toward smaller initial mass ratios (defined as the ratio of the donor mass over the CO mass), consequently ruling out the formation of NS + NS pairs while allowing the production of merging WD + BH binaries. We compare our results with observations of single-degenerate binaries and find that the conditions for the stable MT channel to operate are present in nature. We then show that stable MT in the isotropic re-emission channel can produce merging binary BHs with mass ratios > 0.1, consistent with the majority of inferred sources of the third gravitational wave transient catalogue. Enhanced angular momentum loss from L₂ increases the minimum final mass ratio achievable by stable MT.