As the number of merging binary black holes observed with ground-based gravitational-wave detectors grows, increasingly accurate theoretical models are required to compare them to the observed sample and disentangle contributions from multiple channels. In formation models involving isolated binary stars, important uncertainties remain regarding the stability of mass transfer and common-envelope evolution. To study some of these uncertainties, we have computed binary simulations using the MESA code consisting of a 30 M⊙ star in a low metallicity (Z⊙/10) environment with a black-hole companion. We have developed an updated prescription to compute mass transfer rates including the possibility of outflows from outer Lagrangian points, as well as a method to self-consistently determine the core-envelope boundary in cases where there is common-envelope evolution. We find that binaries survive common-envelope evolution only if unstable mass transfer happens after the formation of a deep convective envelope, resulting in a narrow range (0.2 dex) in period for successful envelope ejection. All cases where binary interaction is initiated with a radiative envelope have large binding energies (∼1050 erg), and they result in mergers during the common-envelope phase even under the assumption that all the internal and recombination energy of the envelope, as well as the energy from an inspiral, is used to eject the envelope. This is independent of whether or not helium is ignited in the core of the donor, conditions under which various rapid-population synthesis calculations assume a successful envelope ejection is possible. Moreover, we find that the critical mass ratio for instability is such that across a large range in initial orbital periods (∼1−1000 days), merging binary black holes can be formed via stable mass transfer. A large fraction of these systems undergo overflow of their L2 equipotential, in which case we find that stable mass transfer produces merging binary black holes even under extreme assumptions of mass and angular momentum outflows. Our conclusions are limited to the study of one donor mass at a single metallicity, but they suggest that population synthesis calculations overestimate the formation rate of merging binary black holes produced by common-envelope evolution and that stable mass transfer could dominate the formation rate from isolated binaries. This is in agreement with a few other recent studies. Further work is required to extend these results to different masses and metallicities as well as to understand how they can be incorporated into rapid population synthesis calculations.

Rapid binary population synthesis codes are often used to investigate the evolution of compact-object binaries. They typically rely on analytical fits of single-star evolutionary tracks and parameterized models for interactive phases of evolution (e.g., mass transfer on a thermal timescale, determination of dynamical instability, and common envelope) that are crucial to predict the fate of binaries. These processes can be more carefully implemented in stellar structure and evolution codes such as MESA. To assess the impact of such improvements, we compare binary black hole mergers as predicted in models with the rapid binary population synthesis code COSMIC to models ran with MESA simulations through mass transfer and common-envelope treatment. We find that results significantly differ in terms of formation paths, the orbital periods and mass ratios of merging binary black holes, and consequently merger rates. While common-envelope evolution is the dominant formation channel in COSMIC, stable mass transfer dominates in our MESA models. Depending upon the black hole donor mass, and mass-transfer and common-envelope physics, at subsolar metallicity, COSMIC overproduces the number of binary black hole mergers by factors of 2–35 with a significant fraction of them having merger times orders of magnitude shorter than the binary black holes formed when using detailed MESA models. Therefore we find that some binary black hole merger rate predictions from rapid population syntheses of isolated binaries may be overestimated by factors of ∼ 5–500. We conclude that the interpretation of gravitational-wave observations requires the use of detailed treatment of these interactive binary phases.

We use a simple framework to calculate the time evolution of the composition of the fallback material onto a supermassive black hole arising from the tidal disruption of main sequence stars. We study stars with masses between 0.8 and 3.0 M , at evolutionary stages from zero-age main sequence to terminal-age main sequence, built using the Modules for Experiments in Stellar Astrophysics code. We show that most stars develop enhancements in nitrogen ( 14 N) and depletions in carbon ( 12 C) and oxygen ( 16 O) over their lifetimes, and that these features are more pronounced for higher mass stars. We find that, in an accretion-powered tidal disruption flare, these features become prominent only after the time of peak of the fallback rate and appear at earlier times for stars of increasing mass. We postulate that no severe compositional changes resulting from the fallback material should be expected near peak for a wide range of stellar masses and, as such, are unable to explain the extreme helium-to-hydrogen line ratios observed in some TDEs. On the other hand, the resulting compositional changes could help explain the presence of nitrogen-rich features, which are currently only detected after peak. When combined with the shape of the light curve, the time evolution of the composition of the fallback material provides a clear method to help constrain the nature of the disrupted star. This will enable a better characterization of the event by helping break the degeneracy between the mass of the star and the mass of the black hole when fitting tidal disruption light curves.

Intermediate and massive stars drive fast and powerful isotropic winds that interact with the winds of nearby stars in star clusters and the surrounding interstellar medium (ISM). Wind–ISM collisions generate astrospheres around these stars that contain hot T ∼ 107 K gas that adiabatically expands. As individual bubbles expand and collide they become unstable, potentially driving turbulence in star clusters. In this Letter we use hydrodynamic simulations to model a densely populated young star cluster within a homogeneous cloud to study stellar wind collisions with the surrounding ISM. We model a mass-segregated cluster of 20 B-type young main-sequence stars with masses ranging from 3 to 17 M ⊙. We evolve the winds for ∼11 kyr and show that wind–ISM collisions and overlapping wind-blown bubbles around B-stars mix the hot gas and ISM material, generating Kolmogorov-like turbulence on small scales early in its evolution. We discuss how turbulence driven by stellar winds may impact the subsequent generation of star formation in the cluster.

Gravitational-wave observations of binary black hole (BBH) systems point to black hole spin magnitudes being relatively low. These measurements appear in tension with high spin measurements for high-mass X-ray binaries (HMXBs). We use grids of MESA simulations combined with the rapid population-synthesis code COSMIC to examine the origin of these two binary populations. It has been suggested that Case-A mass transfer while both stars are on the main sequence can form high-spin BHs in HMXBs. Assuming this formation channel, we show that depending on the critical mass ratios for the stability of mass transfer, 48%–100% of these Case-A HMXBs merge during the common-envelope phase and up to 42% result in binaries too wide to merge within a Hubble time. Both MESA and COSMIC show that high-spin HMXBs formed through Case-A mass transfer can only form merging BBHs within a small parameter space where mass transfer can lead to enough orbital shrinkage to merge within a Hubble time. We find that only up to 11% of these Case-A HMXBs result in BBH mergers, and at most 20% of BBH mergers came from Case-A HMXBs. Therefore, it is not surprising that these two spin distributions are observed to be different.