Detections of gravitational waves are now starting to probe the mass distribution of stellar mass black holes (BHs). Robust predictions from stellar models are needed to interpret these. Theory predicts the existence of a gap in the BH mass distribution because of pair-instability supernovae. The maximum BH mass below the gap is the result of pulsational mass loss. We evolve massive helium stars through their late hydrodynamical phases of evolution using the open-source MESA stellar evolution code. We find that the location of the lower edge of the mass gap at 45 M  is remarkably robust against variations in the metallicity (≈3 M ), the treatment of internal mixing (≈1 M ), and stellar wind mass loss (≈4 M ), making it the most robust predictor for the final stages of the evolution of massive stars. The reason is that the onset of the instability is dictated by the near-final core mass, which in turn sets the resulting BH mass. However, varying the a g C , O 12 16 () reaction rate within its 1σ uncertainties shifts the location of the gap between 40 M  and 56 M . We provide updated analytic fits for population synthesis simulations. Our results imply that the detection of merging BHs can provide constraints on nuclear astrophysics. Furthermore, the robustness against metallicity suggests that there is a universal maximum for the location of the lower edge of the gap, which is insensitive to the formation environment and redshift for first-generation BHs. This is promising for the possibility to use the location of the gap as a "standard siren" across the universe.

Double neutron stars (DNSs) have been observed as Galactic radio pulsars, and the recent discovery of gravitational waves from the DNS merger GW170817 adds to the known DNS population. We perform rapid population synthesis of massive binary stars and discuss model predictions, including DNS formation rates, mass distributions, and delay time distributions. We vary assumptions and parameters of physical processes such as mass transfer stability criteria, supernova natal kick distributions, remnant mass prescriptions and common-envelope energetics. We compute the likelihood of observing the orbital period-eccentricity distribution of the Galactic DNS population under each of our population synthesis models, allowing us to quantitatively compare the models. We find that mass transfer from a stripped post-helium-burning secondary (case BB) onto a neutron star is most likely dynamically stable. We also find that a natal kick distribution composed of both low (Maxwellian σ = 30 km s −1 ) and high (σ = 265 km s −1 ) components is preferred over a single high-kick component. We conclude that the observed DNS mass distribution can place strong constraints on model assumptions.

Pair-instability and pulsational pair-instability supernovae (PPISN) have not been unambiguously observed so far. They are, however, promising candidates for the progenitors of the heaviest binary black hole (BBH) mergers detected. If these BBHs are the product of binary evolution, then PPISNe could occur in very close binaries. Motivated by this, we discuss the implications of a PPISN happening with a close binary companion, and what impact these explosions have on the formation of merging BBHs through binary evolution. For this, we have computed a set of models of metal-poor (Z /10) helium stars using the MESA software instrument. For PPISN progenitors with pre-explosion masses > 50M we find that, after a pulse, heat deposited throughout the layers of the star that remain bound cause it to expand to more than 100R for periods of 10 2 − 10 4 yrs depending on the mass of the progenitor. This results in long-lived phases of Roche-lobe overflow or even common-envelope events if there is a close binary companion, leading to additional electromagnetic transients associated to PPISN eruptions. If we ignore the effect of these interactions, we find that mass loss from PPISNe reduces the final black hole spin by ∼ 30%, induces eccentricities that can be detected by the LISA observatory, and can produce a double-peaked distribution of measured chirp masses in BBH mergers observed by ground-based detectors.