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Chemically homogeneous evolution (CHE) is a promising channel for forming massive binary black holes. The enigmatic, massive Wolf–Rayet binary HD 5980 A&B has been proposed to have formed through this channel. We investigate this claim by comparing its observed parameters with CHE models. UsingMESA, we simulate grids of close massive binaries, then use a Bayesian approach to compare them with the stars’ observed orbital period, masses, luminosities, and hydrogen surface abundances. The most probable models, given the observational data, have initial periods ∼3 days, widening to the present-day ∼20 days orbit as a result of mass loss—correspondingly, they have very high initial stellar masses (≳150M_⊙). We explore variations in stellar-wind mass loss and internal mixing efficiency, and find that models assuming enhanced mass loss are greatly favored to explain HD 5980, while enhanced mixing is only slightly favored over our fiducial assumptions. Our most probable models slightly underpredict the hydrogen surface abundances. Regardless of its prior history, this system is a likely binary black hole progenitor. We model its further evolution under our fiducial and enhanced wind assumptions, finding that both stars produce black holes with masses ∼19–37M_⊙. The projected final orbit is too wide to merge within a Hubble time through gravitational waves alone. However, the system is thought to be part of a 2+2 hierarchical multiple. We speculate that secular effects with the (possible) third and fourth companions may drive the system to promptly become a gravitational-wave source.

 

Regular, automated testing is a foundational principle of modern software development. Numerous widely used continuous integration systems exist, but they are often not suitable for the unique needs of scientific simulation software. Here we describe the testing infrastructure developed for and used by the Modules for Experiments in Stellar Astrophysics (MESA) project. This system allows the computationally demanding MESA test suite to be regularly run on a heterogeneous set of computers and aggregates and displays the testing results in a form that allows for the rapid identification and diagnosis of regressions. Regularly collecting comprehensive testing data also enables longitudinal studies of the performance of the software and the properties of the models it generates.

 

Current observations of binary black hole (BBH) merger events show support for a feature in the primary BH-mass distribution at $\sim \, 35 \ \mathrm{M}_{\odot }$, previously interpreted as a signature of pulsational pair-instability supernovae (PPISNe). Such supernovae are expected to map a wide range of pre-supernova carbon–oxygen (CO) core masses to a narrow range of BH masses, producing a peak in the BH mass distribution. However, recent numerical simulations place the mass location of this peak above $50 \ \mathrm{M}_{\odot }$. Motivated by uncertainties in the progenitor’s evolution and explosion mechanism, we explore how modifying the distribution of BH masses resulting from PPISN affects the populations of gravitational-wave (GW) and electromagnetic (EM) transients. To this end, we simulate populations of isolated BBH systems and combine them with cosmic star formation rates. Our results are the first cosmological BBH-merger predictions made using the binary_c rapid population synthesis framework. We find that our fiducial model does not match the observed GW peak. We can only explain the $35 \ \mathrm{M}_{\odot }$ peak with PPISNe by shifting the expected CO core-mass range for PPISN downwards by $\sim {}15 \ \mathrm{M}_{\odot }$. Apart from being in tension with state-of-the art stellar models, we also find that this is likely in tension with the observed rate of hydrogen-less super-luminous supernovae. Conversely, shifting the mass range upward, based on recent stellar models, leads to a predicted third peak in the BH mass function at $\sim {}64 \ \mathrm{M}_{\odot }$. Thus we conclude that the $\sim {}35 \ \mathrm{M}_{\odot }$ feature is unlikely to be related to PPISN.

 

Thorne–Żytkow objects (TŻO) are potential end products of the merger of a neutron star with a non-degenerate star. In this work, we have computed the first grid of evolutionary models of TŻOs with the MESA stellar evolution code. With these models, we predict several observational properties of TŻOs, including their surface temperatures and luminosities, pulsation periods, and nucleosynthetic products. We expand the range of possible TŻO solutions to cover $3.45 \lesssim \rm {\log \left(T_{eff}/K\right)}\lesssim 3.65$ and $4.85 \lesssim \rm {\log \left(L/L_{\odot }\right)}\lesssim 5.5$. Due to the much higher densities our TŻOs reach compared to previous models, if TŻOs form we expect them to be stable over a larger mass range than previously predicted, without exhibiting a gap in their mass distribution. Using the GYRE stellar pulsation code we show that TŻOs should have fundamental pulsation periods of 1000–2000 d, and period ratios of ≈0.2–0.3. Models computed with a large 399 isotope fully coupled nuclear network show a nucleosynthetic signal that is different to previously predicted. We propose a new nucleosynthetic signal to determine a star’s status as a TŻO: the isotopologues $\mathrm{^{44}Ti} \rm {O}_2$ and $\mathrm{^{44}Ti} \rm {O}$, which will have a shift in their spectral features as compared to stable titanium-containing molecules. We find that in the local Universe (∼SMC metallicities and above) TŻOs show little heavy metal enrichment, potentially explaining the difficulty in finding TŻOs to-date.

 

Abstract Common envelope (CE) evolution is an outstanding open problem in stellar evolution, critical to the formation of compact binaries including gravitational-wave sources. In the “classical” isolated binary evolution scenario for double compact objects, the CE is usually the second mass transfer phase. Thus, the donor star of the CE is the product of a previous binary interaction, often stable Roche lobe overflow (RLOF). Because of the accretion of mass during the first RLOF, the main-sequence core of the accretor star grows and is “rejuvenated.” This modifies the core-envelope boundary region and decreases significantly the envelope binding energy for the remaining evolution. Comparing accretor stars from self-consistent binary models to stars evolved as single, we demonstrate that the rejuvenation can lower the energy required to eject a CE by ∼42%–96% for both black hole and neutron star progenitors, depending on the evolutionary stage and final orbital separation. Therefore, binaries experiencing first stable mass transfer may more easily survive subsequent CE events and result in possibly wider final separations compared to current predictions. Despite their high mass, our accretors also experience extended “blue loops,” which may have observational consequences for low-metallicity stellar populations and asteroseismology. 

We update the capabilities of the open-knowledge software instrument Modules for Experiments in Stellar Astrophysics (MESA). The newauto_diffmodule implements automatic differentiation inMESA, an enabling capability that alleviates the need for hard-coded analytic expressions or finite-difference approximations. We significantly enhance the treatment of the growth and decay of convection inMESAwith a new model for time-dependent convection, which is particularly important during late-stage nuclear burning in massive stars and electron-degenerate ignition events. We strengthenMESA’s implementation of the equation of state, and we quantify continued improvements to energy accounting and solver accuracy through a discussion of different energy equation features and enhancements. To improve the modeling of stars inMESA, we describe key updates to the treatment of stellar atmospheres, molecular opacities, Compton opacities, conductive opacities, element diffusion coefficients, and nuclear reaction rates. We introduce treatments of starspots, an important consideration for low-mass stars, and modifications for superadiabatic convection in radiation-dominated regions. We describe new approaches for increasing the efficiency of calculating monochromatic opacities and radiative levitation, and for increasing the efficiency of evolving the late stages of massive stars with a new operator-split nuclear burning mode. We close by discussing major updates toMESA’s software infrastructure that enhance source code development and community engagement.

 

Abstract The collapse of degenerate oxygen–neon cores (i.e., electron-capture supernovae or accretion-induced collapse) proceeds through a phase in which a deflagration wave (“flame”) forms at or near the center and propagates through the star. In models, the assumed speed of this flame influences whether this process leads to an explosion or to the formation of a neutron star. We calculate the laminar flame speeds in degenerate oxygen–neon mixtures with compositions motivated by detailed stellar evolution models. These mixtures include trace amounts of carbon and have a lower electron fraction than those considered in previous work. We find that trace carbon has little effect on the flame speeds, but that material with electron fraction has laminar flame speeds that are times faster than those at . We provide tabulated flame speeds and a corresponding fitting function so that the impact of this difference can be assessed via full star hydrodynamical simulations of the collapse process.