Binary neutron stars and binary black holes: from the Milky Way to the high-redshift Universe

The majority (≳ 70 %) of massive stars lie in a binary. The role played by binary interactions is thus critical towards the understanding of current observations. In this thesis, I use the SEVN population synthesis code to explore several major challenges in the physics of binary evolution. In my first work I incorporate pulsars’ spins and magnetic fields evolution in SEVN. I evolve binary neutron star systems in a Milky-Way model to reproduce the merger rates, orbital properties and pulsars’ properties of the Galactic binary neutron star population. The analysis reveals a strong dependence of the final observed population on the birth distributions of spin period and magnetic fields. In my second work, I address the impact of the metal-dependent star formation rate (SFR) on the binary black hole (BBH) merger rate density. To this end, I adopt the most up-to-date observational scaling relations and I study how the BBH merger rate density varies over a wide grid of galaxy and binary evolution parameters. I show that assuming a realistic metal-dependent SFR evolution yields a BBH merger rate density higher than that inferred from gravitational wave data, even when accounting for uncertainties from low-mass galaxies. The analysis suggests that stellar evolution uncertainties are the main responsible for the discrepancy between the BBH merger rate density inferred from data and theoretical models. Lastly, I evaluate envelope binding energies directly from the PARSEC stellar tracks for a vast range of stellar masses and metallicities, investigating the impact of different core-envelope boundary criteria and energy sources. I then implement the new binding energies in SEVN. The results show that the merger rate densities of binary compact objects, when using these new consistent binding energies, can differ by more than an order of magnitude compared to models employing precomputed fits.