Modelling stellar evolution in mass-transferring binaries and gravitational-wave progenitors with METISSE

Massive binaries are vital sources of various transient processes, including gravitational-wave mergers. However, large uncertainties in the evolution of massive stars, both physical and numerical, present a major challenge to the understanding of their binary evolution. In this paper, we upgrade our interpolation-based stellar evolution code METISSE to include the effects of mass changes, such as binary mass transfer or wind-driven mass loss, not already included within the input stellar tracks. METISSE's implementation of mass loss (applied to tracks without mass loss) shows excellent agreement with the SSE fitting formulae and with detailed MESA tracks, except in cases where the mass transfer is too rapid for the star to maintain equilibrium. We use this updated version of METISSE within the binary population synthesis code BSE to demonstrate the impact of varying stellar evolution parameters, particularly core overshooting, on the evolution of a massive (25M$_\odot$ and 15M$_\odot$) binary system with an orbital period of 1800 days. Depending on the input tracks, we find that the binary system can form a binary black hole or a black hole-neutron star system, with primary(secondary) remnant masses ranging between 4.47(1.36)M$_\odot$ and 12.30(10.89)M$_\odot$, and orbital periods ranging from 6 days to the binary becoming unbound. Extending this analysis to a population of isolated binaries uniformly distributed in mass and orbital period, we show that the input stellar models play an important role in determining which regions of the binary parameter space can produce compact binary mergers, paving the way for predictions for current and future gravitational-wave observatories.Comment: 19 pages, 14 figures, accepted for publication in MNRA

Explaining the differences in massive star models from various simulations

The evolution of massive stars is the basis of several astrophysical investigations, from predicting gravitational-wave event rates to studying star formation and stellar populations in clusters. However, uncertainties in massive star evolution present a significant challenge when accounting for these models' behaviour in stellar population studies. In this work, we present a comparison between five published sets of stellar models from the BPASS (Binary Population and Spectral Synthesis), BoOST (Bonn Optimized Stellar Tracks), Geneva, MIST (MESA Isochrones and Stellar Tracks), and PARSEC (PAdova and TRieste Stellar Evolution Code) simulations at near-solar metallicity. The different sets of stellar models have been computed using slightly different physical inputs in terms of mass-loss rates and internal mixing properties. Moreover, these models also employ various pragmatic methods to overcome the numerical difficulties that arise due to the presence of density inversions in the outer layers of stars more massive than 40 M-circle dot. These density inversions result from the combination of inefficient convection in the low-density envelopes of massive stars and the excess of radiative luminosity to the Eddington luminosity. We find that the ionizing radiation released by the stellar populations can change by up to 18 per cent, the maximum radial expansion of a star can differ between 100 and 1600 R-circle dot, and the mass of the stellar remnant can vary up to 20 M-circle dot between the five sets of simulations. We conclude that any attempts to explain observations that rely on the use of models of stars more massive than 40 M-circle dot should be made with caution

Dust Grain Growth and Dusty Supernovae in Low-metallicity Molecular Clouds

We present 3D hydrodynamical models of the evolution of superbubbles powered by stellar winds and supernovae from young coeval massive star clusters within low-metallicity (Z = 0.02 Z(circle dot)), clumpy molecular clouds. We explore the initial stages of the superbubble evolution, including the occurrence of pair-instability and core-collapse supernovae. Our aim is to study the occurrence of dust grain growth within orbiting dusty clumps, and in the superbubble's swept-up supershell. We also aim to address the survival of dust grains produced by sequential supernovae. The model accounts for the star cluster gravitational potential and self-gravity of the parent cloud. It also considers radiative cooling (including that induced by dust) and a state-of-the-art population synthesis model for the coeval cluster. As shown before, a superbubble embedded into a clumpy medium becomes highly distorted, expanding mostly due to the hot gas streaming through low-density channels Our results indicate that in the case of massive (similar to 10 M-circle dot) molecular clouds, hosting a super star cluster (similar to 5.6 x 10(5) M-circle dot), grain growth increments the dust mass at a rate similar to 4.8 x 10(-5) M-circle dot yr(-1) during the first 2.5 Myr of the superbubble's evolution, while the net contribution of pair-instability and core-collapse supernovae to the superbubble's dust budget is similar to 1200 M circle dot (M-SC/5.6 x 10(5) M-circle dot), where M-SC is the stellar mass of the starburst. Therefore, dust grain growth and dust injection by supernovae lead to the creation of, without invoking a top-heavy initial mass function, massive amounts of dust within low-metallicity star-forming molecular clouds, in accordance with the large dust mass present in galaxies soon after the onset of cosmic reionization

The Impact of Pair-instability Mass Loss on the Binary Black Hole Mass Distribution

A population of binary black hole mergers has now been observed in gravitational waves by Advanced LIGO and Virgo. The masses of these black holes appear to show evidence for a pileup between 30 and 45 M-circle dot and a cutoff above similar to 45 M-circle dot. One possible explanation for such a pileup and subsequent cutoff are pulsational pair-instability supernovae (PPISNe) and pair-instability supernovae (PISNe) in massive stars. We investigate the plausibility of this explanation in the context of isolated massive binaries. We study a population of massive binaries using the rapid population synthesis software COMPAS, incorporating models for PPISNe and PISNe. Our models predict a maximum black hole mass of 40 M-circle dot. We expect similar to 10% of all binary black hole mergers at redshift z = 0 will include at least one component that went through a PPISN (with mass 30-40 M-circle dot), constituting similar to 20%-50% of binary black hole mergers observed during the first two observing runs of Advanced LIGO and Virgo. Empirical models based on fitting the gravitational-wave mass measurements to a combination of a power law and a Gaussian find a fraction too large to be associated with PPISNe in our models. The rates of PPISNe and PISNe track the low metallicity star formation rate, increasing out to redshift z = 2. These predictions may be tested both with future gravitational-wave observations and with observations of superluminous supernovae

X-Ray Emission from Star-cluster Winds in Starburst Galaxies

Inspired by the excess soft X-ray emission recently detected in Green Pea galaxies, we model the soft X-ray emission (0.5-2.0 keV) of hot gas from star-cluster winds. By combining individual star clusters, we estimate the soft X-ray emission expected from the typically unresolved diffuse hot gas in starburst galaxies, devoid of competing emission from, e.g., active galactic nuclei (AGNs) or other unresolved point sources. We use stellar models of subsolar metallicities (0.02 Z (circle dot) and 0.4 Z (circle dot)) and take into account supernova explosions for massive stars. For lower metallicities, we find that stellar winds do not contribute significantly (less than or similar to 3% of the mechanical energy) to the observed soft X-ray emission of normal star-forming galaxies. For higher metallicities and possibly also for larger proportions of massive star clusters in the simulated starburst galaxies, we reproduce well the observed correlation between star formation rate and X-ray luminosity previously reported in the literature. However, we find that no combination of model assumptions is capable of reproducing the substantial soft X-ray emission observed from Green Pea galaxies, indicating that other emission mechanisms (i.e., unusually large quantities of high-/low-mass X-ray binaries, ultraluminous X-ray sources, a modified initial mass function, intermediate-mass black holes, or AGNs) are more likely to be responsible for the X-ray excess

The effect of the metallicity-specific star formation history on double compact object mergers

We investigate the impact of uncertainty in the metallicity-specific star formation rate over cosmic time on predictions of the rates and masses of double compact object mergers observable through gravitational waves. We find that this uncertainty can change the predicted detectable merger rate by more than an order of magnitude, comparable to contributions from uncertain physical assumptions regarding binary evolution, such as mass transfer efficiency or supernova kicks. We statistically compare the results produced by the COMPAS population synthesis suite against a catalogue of gravitational-wave detections from the first two Advanced LIGO and Virgo observing runs. We find that the rate and chirp mass of observed binary black hole mergers can be well matched under our default evolutionary model with a star formation metallicity spread of 0.39 dex around a mean metallicity that scales with redshift z as = 0.035 x 10(-0.23z), assuming a star formation rate of 0.01 x (1 + z)(2.77)/(1 + ((1 + z)/2.9)(4.7)) M-circle dot Mpc(-3) yr(-1). Intriguingly, this default model predicts that 80 per cent of the approximately one binary black hole merger per day that will be detectable at design sensitivity will have formed through isolated binary evolution with only dynamically stable mass transfer, i.e. without experiencing a common-envelope event