Binary compact object populations

The detection of gravitational waves (GWs) from merging binary compact objects (BCOs) by the LIGO–Virgo–KAGRA Collaboration is providing a new window to investigate the lives and deaths of massive stars and compact objects that are members of binary systems, pushing the boundary previously set by electromagnetic observations.. Despite the increasing number of observations, the interpretation of BCOs remains challenging. Many formation channels could be at work, but the uncertainties in stellar and binary processes currently limit our ability to interpret the population properties of GW-merging BCOs. In this thesis, I used the population-synthesis code SEVN to explore the impact of model uncertainties on BCO formation. I devoted particular attention to explore the impact of uncertainties in the physics regulating stellar evolution and mass transfer processes. I compared the results of these studies with the observations of possible progenitors binaries hosting massive stars, focusing on the possible evolution through the pure-helium or Wolf-Rayet (WR) star phase. In the first part of this thesis, I investigated the role of Wolf–Rayet stars with a compact object companion (black hole or neutron star), hereafter WR–CO, as progenitors of BCOs. To constrain model uncertainties, I explored the impact of a wide space of parameters and models, including metallicity, common-envelope efficiency, core-collapse supernova, and natal kicks. I found that WR-COs are a key-progenitor configuration for BCOs, as the majority of BCOs have to experience the WR-CO phase in order to become GW sources, regardless of the combination of models considered. Even if few WR-COs become BCOs, binaries with the orbital properties similar to Cyg X-3 (the only WR-CO observed in the Milky Way) are favoured as BCO progenitors. My results quantify the importance of WR–COs as BCO progenitors, highlighting that the observation and characterization of more of these systems could be crucial to constrain our models on BCO formation. In the second part of this thesis, I explored uncertainties related to orbital angular momentum losses in mass transfer events. I considered four possible models (isotropic winds from the donor, from the accretor, Lagrangian outflows or circumbinary disk formation) and analysed their impact on the formation channels and the demography of GW-merging binary black holes (BBHs). I also considered the impact of tides and stellar physics, for their additional impact on the variation in the orbital separation variations and pair-instability supernovae, respectively. I found that the combination of efficient orbital angular momentum losses (such as Lagrangian outflows or circumbinary disk formation) and non-conservative mass transfer on an Eddington-limited BH accretor allows BBH formation even from binary progenitors that were not completely stripped of their envelope. These systems did not require a prolonged mass transfer to shrink the orbit, thus, completely removing the hydrogen envelope. Instead, their mass transfer was interrupted by the core-collapse supernova event of the donor, that collapsed into a BH while retaining a fraction of its hydrogen envelope. In this scenario, I find that a non-negligible fraction of BBHs (3-11\%) has the most massive BH (primary) that is more massive than 45 solar masses, a threshold generally adopted to indicate BH primaries in BBHs that are not expected to be formed via the isolated binary evolution channel. I find a dearth of BHs above 45 solar masses only for the least massive BH (secondary) in BBHs, in agreement with the GWTC-4.0 data. Eventually, I highlight the need for caution when investigating the effects of mass transfer physics with various computational approaches, because it could be correlated with the impact of stellar evolution uncertainties, an aspect generally poorly explored in population-synthesis studies.