Soon after the recent first ever detection of gravitational waves from merging black holes it has been suggested that their origin is primordial. Appealingly, a sufficient number of primordial black holes (PBHs) could also partially or entirely constitute the dark matter (DM) in the Universe. However, recent studies on PBHs in ultrafaint dwarf galaxies (UFDGs) suggest that they would dynamically heat up the stellar component due to two-body relaxation processes. From the comparison with the observed stellar velocity dispersions and the stellar half-light radii it was claimed that only PBHs with masses 10 M can significantly contribute to the DM. In this work, we improve the latter constraints by considering the largest observational sample of UFDGs and by allowing the PBH masses to follow an extended (log-normal) distribution. By means of collisional Fokker-Planck simulations, we explore a wide parameter space of UFDGs containing PBHs. The analysis of the half-light radii and velocity dispersions resulting from the simulations leads to three general findings that exclude PBHs with masses ∼ O(1-100) M from constituting all of the DM: (i) We identify a critical sub-sample of UFDGs that only allows for ∼ O(1) M PBH masses; (ii) for any PBH mass, there is an UFDG in our sample that disfavours it; (iii) for a majority of UFDGs, dynamical heating by PBHs would be too efficient to match the observed stellar half-light radii.

Close stellar binaries are prone to undergo a phase of stable mass transfer in which a star loses mass to its companion. Assuming that the donor star loses mass along the instantaneous interstellar axis, we derive the orbit-averaged equations of motion describing the evolution of the donor rotational angular momentum vector (spin) that accompanies the transfer of mass. We consider: (i) a model in which the mass transfer rate is constant within each orbit and (ii) a phase-dependent rate in which all mass per orbit is lost at periapsis. In both cases, we find that the ejection of ≳30 percent of the donor's initial mass causes its spin to nearly flip onto the orbital plane of the binary, independently of the initial spin-orbit alignment. Moreover, we show that the spin flip due to mass transfer can easily dominate over tidal synchronization in any giant stars and main-sequence stars with masses ∼1.5 to 5 M ⊙ . Finally, the general equations of motion, including tides, are used to evolve a realistic population of massive binary stars, leading to the formation of binary black holes. Assuming that the stellar core and envelope are fully coupled, the resulting tilt of the first-born black hole reduces its spin projection onto the orbit normal by a factor ∼Oð0.1Þ. This result supports previous studies in favor of an insignificant contribution to the effective spin projection, χ eff , in binary black holes formed from the evolution of field binaries.

Most back hole and neutron star progenitors are found in triples or higher multiplicity systems. Here, we present a new triple stellar evolution code, ${\tt TSE}$, which simultaneously takes into account the physics of the stars and their gravitational interaction. ${\tt TSE}$ is used to simulate the evolution of massive stellar triples in the galactic field from the zero-age-main-sequence until they form compact objects. To this end, we implement initial conditions that incorporate the observed high correlation between the orbital parameters of early-type stars. We show that the interaction with a tertiary companion can significantly impact the evolution of the inner binary. High eccentricities can be induced by the third-body dynamical effects, leading to a Roche lobe overflow or even to a stellar merger from initial binary separations  103 – $10^5\, \rm R_\odot$. In $\sim 5\, {{\ \rm per\ cent}}$ of the systems the tertiary companion itself fills its Roche lobe, while $\sim 10\, {{\ \rm per\ cent}}$ of all systems become dynamically unstable. We find that between $0.3{{\ \rm per\ cent}}$ and $5{{\ \rm per\ cent}}$ of systems form a stable triple with an inner compact object binary, where the exact fraction depends on metallicity and the natal kick prescription. Most of these triples are binary black holes with black hole companions. We find no binary neutron star in any surviving triple, unless zero natal kicks are assumed. About half of all black hole binaries formed in our models are in triples, where in the majority the tertiary black hole can perturb their long-term evolution. Our results show that triple interactions are key to a full understanding of massive star evolution and compact object binary formation.

The majority of massive stars are found in close binaries which: (i) are prone to merge and (ii) are accompanied by another distant tertiary star (triples). Here, we study the evolution of the stellar postmerger binaries composed of the merger product and the tertiary companion. We find that postmerger binaries originating from compact stellar triples with outer semimajor axes aout,init 10 1 -10 2 AU provide a new way to form binary black hole mergers in the galactic field. By means of a population synthesis, we estimate their contribution to the total black hole merger rate to be R(z = 0) = 0.3 -25.2 Gpc −3 yr −1 . Merging binary black holes that form from stellar postmerger binaries have exceptionally low mass ratios. We identify a critical mass ratio q 0.5 below which they dominate the total black hole merger rate in the field. We show that after including their additional contribution, the mass ratio distribution of binary black hole mergers in the galactic field scenario is in better agreement with that inferred from gravitational wave detections.-Outer binary channel: The two stars in the inner binary merge and the postmerger star and tertiary companion subsequently form a stable BBH.• Isolated binary population: Starting from an isolated binary population the binary stars form stable BBHs which subsequently merge within a Hubble time due to the emission of gravitational waves. This is a standard population model used in the literature for which the effect of a tertiary companion is not considered [20][21][22][23][27][28][29][30][31][32].