Knowledge about the internal physical structure of stars is crucial to understanding their evolution. The novel binary population synthesis code POSYDON includes a module for interpolating the stellar and binary properties of any system at the end of binary MESA evolution based on a pre-computed set of models. In this work, we present a new emulation method for predicting stellar profiles, i.e., the internal stellar structure along the radial axis, using machine learning techniques. We use principal component analysis for dimensionality reduction and fully-connected feed-forward neural networks for making predictions. We find accuracy to be comparable to that of nearest neighbor approximation, with a strong advantage in terms of memory and storage efficiency. By providing a versatile framework for modeling stellar internal structure, the emulation method presented here will enable faster simulations of higher physical fidelity, offering a foundation for a wide range of large-scale population studies of stellar and binary evolution.

Pair-instability supernovae (PISNe) have long been predicted to be the final fates of near-zero-metallicity very massive stars (Z<Z(circle dot)/3, M-ZAMS greater than or similar to 140M(circle dot)). However, no definite PISN has been observed to date, leaving theoretical modelling validation open. To investigate the observability of these explosive transients, we combine detailed stellar evolution models for PISNe formation, computed from the Binary Population and Spectral Synthesis code suite, BPASS, with the star formation history of all individual computational elements in the Illustris-TNG simulation. This allows us to compute comic PISN rates and predict their host galaxy properties. Of particular importance is that IllustrisTNG galaxies do not have uniform metallicities throughout, with metal-enriched galaxies often harbouring metal-poor pockets of gas where PISN progenitors may form. Accounting for the chemical inhomogeneities within these galaxies, we find that the peak redshift of PISNe formation is z=3.5 instead of the value of z=6 when ignoring chemical inhomogeneities within galaxies. Furthermore, the rate increases by an order of magnitude from 1.9 to 29 PISN Gpc(-3) yr(-1) at z=0, if the chemical inhomogeneities are considered. Using state-of-the-art theoretical PISN light curves, we find an observed rate of 13.8 (1.2) visible PISNe per year for the Euclid-Deep survey, or 83 (7.3) over the six-year lifetime of the mission when considering chemically inhomogeneous (homogenous) systems. Interestingly, only 12 per cent of helium PISN progenitors are sufficiently massive to power a super-luminous supernova event, which can potentially explain why PISN identification in time-domain surveys remains elusive and progress requires dedicated strategies.

Comparing Galactic chemical evolution models to the observed elemental abundances in the Milky Way, we show that neutron star mergers can be a leading r-process site only if at low metallicities such mergers have very short delay times and significant ejecta masses that are facilitated by the masses of the compact objects. Namely, black hole–neutron star mergers, depending on the black hole spins, can play an important role in the early chemical enrichment of the Milky Way. We also show that none of the binary population synthesis models used in this Letter, i.e., COMPAS, StarTrack, Brussels, ComBinE, and BPASS, can currently reproduce the elemental abundance observations. The predictions are problematic not only for neutron star mergers, but also for Type Ia supernovae, which may point to shortcomings in binary evolution models.

ABSTRACT Natal supernova kicks, the linear momentum compact remnants receive during their formation, are an essential part of binary population synthesis (BPS) models. Although these kicks are well supported by evidence, their underlying distributions and incorporation into BPS models are uncertain. In this work, we investigate the nature of natal kicks using a previously proposed analytical prescription where the strength of the kick is given by $v_\text{k}=\alpha \frac{m_\text{ejecta}}{m_\text{remnant}}+\beta \, \mathrm{km\, s}^{-1}$ , for free parameters α and β. We vary the free parameters over large ranges of possible values, comparing these synthetic populations simultaneously against four constraints: the merger rate of compact binary neutron star (BNS) systems, the period–eccentricity distribution of Galactic BNSs, the velocity distribution of single-star pulsars, and the likelihood for low ejecta mass supernovae to produce low-velocity kicks. We find that different samples of the parameter space satisfy each test, and only 1 per cent of the models satisfy all four constraints simultaneously. Although we cannot identify a single best kick model, we report $\alpha =115^{+40}_{-55}\, \mathrm{km\, s}^{-1}, \beta =15^{+10}_{-15}\, \mathrm{km\, s}^{-1}$ as the centre of the region of the parameter space that fulfils all of our constraints, and expect $\beta \ge 0\, \mathrm{km\, s}^{-1}$ as a further constraint. We also suggest further observations that will enable future refinement of the kick model. A sensitive test for the kick model will be the redshift evolution of the BNS merger rate since this is effectively a direct measure of the delay-time distribution for mergers. For our best-fitting values, we find that the peak of the BNS merger rate is the present day.

With the remarkable success of the LVK consortium in detecting binary black hole mergers, it has become possible to use the population properties to constrain our understanding of the progenitor stars' evolution. The most striking features of the observed primary black hole mass distributions are the extended tail up to 100M$_\odot$ and an excess of masses at 35M$_\odot$. Currently, isolated binary population synthesis have difficulty explaining these features. Using the well-tested BPASS detailed stellar binary evolution models to determine mass transfer stability, accretion rates, and remnant masses, we postulate that stable mass transfer with super-Eddington accretion is responsible for the extended tail. Furthermore, that the excess is not due to pulsation-pair instability, as previously thought, but due to stable mass transfer. These systems are able to merge within the Hubble time due to more stable mass transfer with extreme mass ratios that allows the orbits to shrink sufficiently to allow for a merger. These finding are at odds with those from other population synthesis codes but in agreement with other recent studies using detailed binary evolution models.

It was recently suggested that "cosmologically coupled" black holes with masses that increase in proportion to the volume of the Universe might constitute the physical basis of dark energy. We take this claim at face value and discuss its potential astrophysical implications. We show that the gravitational wave emission in binary systems would be significantly enhanced so that the number of black hole mergers would exceed the observed rate by orders of magnitude, with typical masses much larger than those seen by the LIGO-Virgo-KAGRA network. Separately, if the mass growth happens at fixed angular momentum, the supermassive black holes in matter-deficient elliptical galaxies should be slowly rotating. Finally, cosmological coupling would stabilize small black holes against Hawking radiation-induced evaporation.

ABSTRACTThe recent discovery of an unambiguous quiescent black hole (BH) and main-sequence O star companion in VFTS 243 opens the door to new constraints on theoretical stellar evolution and population models looking to reproduce the progenitors of BH–BH binaries. Here, we show that the binary population and spectral synthesis fiducial models natively predict VFTS 243-like systems: We find that VFTS 243 likely originated from a binary system in an ∼15 d orbit with primary mass ranging from 40 to 50 $\mathrm{\, M}_\odot$ and secondary star with initial mass of 24–25 $\mathrm{\, M}_\odot$. Additionally, we find that the death of the primary star must have resulted in a low-energy explosion E < 1050 erg. With a uniform prior, we find that the kick velocity of the newborn BH was ≤10 $\, \text{km s}^{-1}$. The very low eccentricity reported for VFTS 243 and the subsequent conclusion by the authors that the supernova kick must have been very small are in line with the peak in the posterior distribution between 0 and 5 $\, \text{km s}^{-1}$ found from our numerical simulations performed with a uniform prior. Finally, the reduced Hobbs kick distribution commonly used in BH population synthesis is strongly disfavoured.

The detection rate of electromagnetic (EM) and gravitational wave (GW) transients is growing exponentially. As the accuracy of the transient rates will significantly improve over the coming decades, so will our understanding of their evolution through cosmic history. To this end, we present predicted rates for EM and GW transients over the age of the Universe using Binary Population and Spectral Synthesis (BPASS) results combined with four cosmic star formation histories (SFH). These include a widely used empirical SFH of Madau & Dickinson and those from three cosmological simulations: MilliMillennium, EAGLE and IllustrisTNG. We find that the choice of SFH significantly changes our predictions: transients with short delay times are most affected by the star formation rate, while long-delay time events tend to depend on the metallicity evolution of star formation. Importantly we find that the cosmological simulations have very different metallicity evolution that cannot be reproduced by the widely used metallicity model of Langer & Norman, which impacts the binary black hole merger and stripped-envelope supernovae rates in the local Universe most acutely. We recommend against using simple prescriptions for the metallicity evolution of the Universe when predicting the rates of events that can have long delay times and that are sensitive to metallicity evolution.

NOTE 2023-01-23: Not compatible with Python 3.7. A user reported that the dataframes are pickled with protocol 5 only available from python 3.8 (thanks Beth!). EDIT 2022-12-19: Added an extended jupyter notebook with the tutorial presented at the MIAPbP workshop EvE contains a number of tables summarising the BPASSv2.2.1 stellar library to facilitate identifying progenitor systems. If you are a user you only need EvE.hdf5. The rest is the code used to make this version of eve. The code is versioned on github (closed repo as of the publication date) but this acts as a "frozen" version of the code made to create this particular file. This database was created with BPASSv2.2.1 and hoki References BPASSv2.2.1: Eldridge et al. 2017 and Stanway et al. 2018 | hoki: Stevance et al. 2022 Contact: hfstevance@gmail.com

The KM3NeT research infrastructure being built at the bottom of the Mediterranean Sea will host water-Cherenkov telescopes for the detection of cosmic neutrinos. The neutrino telescopes will consist of large volume three-dimensional grids of optical modules to detect the Cherenkov light from charged particles produced by neutrino-induced interactions. Each optical module houses 31 3-inch photomultiplier tubes, instrumentation for calibration of the photomultiplier signal and positioning of the optical module and all associated electronics boards. By design, the total electrical power consumption of an optical module has been capped at seven watts. This paper presents an overview of the front-end and readout electronics system inside the optical module, which has been designed for a 1~ns synchronization between the clocks of all optical modules in the grid during a life time of at least 20 years.