The initial mass function (IMF) describes the distribution of stellar masses in a population of newly born stars and is amongst the most fundamental concepts in astrophysics. It is not only the direct result of the star formation process but it also explains the evolution of galaxies' luminosities, metal yields, star-formation efficiencies, and supernova production rates. Because most stars exist in binary systems, however, a full statistical account of stellar mass requires not the IMF but rather the joint distribution of a binary population's primary- and secondary-star masses. This joint distribution must respect the IMF of the stars from which the population has been assembled as well as the distribution of mass ratios that results from the assembly mechanism. Despite its importance, this joint distribution is known only in the case of random pairing. Here we present a method for constructing it in the general case. We also illustrate the use of our method by recovering the known result for random pairing and by finding the previously unknown result for uniform pairing.

Asymptotic giant branch (AGB) stars play a significant role in our understanding of the origin of the elements. They contribute to the abundances of C, N, and approximately 50% of the abundances of the elements heavier than iron. An aspect often neglected in studies of AGB stars is the impact of a stellar companion on AGB stellar evolution and nucleosynthesis. In this study, we update the stellar abundances of AGB stars in the binary population synthesis code binary_c and calibrate our treatment of the third dredge-up using observations of Galactic carbon stars. We model stellar populations of low- to intermediate-mass stars at solar-metallicity and examine the stellar wind contributions to C, N, O, Sr, Ba, and Pb yields at binary fractions between 0 and 1. For a stellar population with a binary fraction of 0.7, we find similar to 20-25% less C and s-process elements ejected than from a population composed of only single stars, and we find little change in the N and O yields. We also compare our models with observed abundances from Ba stars and find our models can reproduce most Ba star abundances, but our population estimates a higher frequency of Ba stars with a surface [Ce/Y] > +0.2dex. Our models also predict the rare existence of Ba stars with masses >10M(circle dot).

Mass changes due to strong stellar winds and binary mass transfer have a dramatic impact on the consequent evolution of stars. This is generally not accounted for in population synthesis codes which are built using single star evolution models from full stellar evolution codes. We produce a new grid of models using the 1D stellar evolution code MESA which includes models with a range of core mass fractions, at each total stellar mass, evolved from core hydrogen exhaustion to the onset of core helium burning. The model grid is used to produce an interpolation table, designed for use in population synthesis codes such as binary_c. We test the interpolation table with a simple integration algorithm to evaluate its capability of reproducing accurate evolutionary tracks. We test our method for stellar masses in the range M=1-16 M_⊙ and show that it successfully reproduces the Hertzpsrung-gap and giant branch lifetime, the core mass at helium ignition and the HR diagram.

Ultrafaint dwarf (UFD) galaxies are dominated by dark matter, the distribution of which may be inferred from the kinematics of that galaxy's stellar population. Star-by-star observations are available for the satellite UFD galaxies of the Milky Way, making them uniquely good laboratories in which to test cosmological predictions at the smallest scales. However, the kinematics of these galaxies are complicated by the presence of binary stars, which alter the stellar velocity distribution. In particular, these binary stars increase the galaxy's stellar velocity dispersion, which is related to the total galactic mass by the virial theorem. Without correctly eliminating or accounting for binary stars, we may therefore overestimate the masses of UFD galaxies or even confuse globular clusters for UFD galaxies. Here, we write down the probability density function for the observed line-of-sight (LOS) velocity of a stellar population containing both visual and spectroscopic binary stars, which we then use to determine the effect of those binary stars on the observed LOS velocity dispersion. For the coldest UFD galaxies, the fractional increase in LOS velocity dispersion is of the order of 1 and for the coldest globular clusters is of the order of 100. However, if the stellar initial mass function is bottom light, as it may be for UFD galaxies and globular clusters, then both of these values increase by half a dex.

ABSTRACT X-ray binaries (XRBs) are thought to regulate cosmic thermal and ionization histories during the Epoch of Reionization and Cosmic Dawn (z ∼ 5–30). Theoretical predictions of the X-ray emission from XRBs are important for modelling such early cosmic evolution. Nevertheless, the contribution from Be-XRBs, powered by accretion of compact objects from decretion discs around rapidly rotating O/B stars, has not been investigated systematically. Be-XRBs are the largest class of high-mass XRBs (HMXBs) identified in local observations and are expected to play even more important roles in metal-poor environments at high redshifts. In light of this, we build a physically motivated model for Be-XRBs based on recent hydrodynamic simulations and observations of decretion discs. Our model is able to reproduce the observed population of Be-XRBs in the Small Magellanic Cloud with appropriate initial conditions and binary stellar evolution parameters. We derive the X-ray output from Be-XRBs as a function of metallicity in the (absolute) metallicity range Z ∈ [10−4, 0.03] with a large suite of binary population synthesis (BPS) simulations. The simulated Be-XRBs can explain a non-negligible fraction ($\gtrsim 30{{\ \rm per\ cent}}$) of the total X-ray output from HMXBs observed in nearby galaxies for Z ∼ 0.0003–0.02. The X-ray luminosity per unit star formation rate from Be-XRBs in our fiducial model increases by a factor of ∼8 from Z = 0.02 to Z = 0.0003, which is similar to the trend seen in observations of all types of HMXBs. We conclude that Be-XRBs are potentially important X-ray sources that deserve greater attention in BPS of XRBs.

ABSTRACT We address the challenge of running thermally pulsing-(super)asymptotic giant branch [TP-(S)AGB] models, with a 1D hydrostatic stellar evolution code, without suffering instabilities that terminate the evolution. We investigate two instabilities that usually occur during the luminosity peak following a thermal pulse: the hydrogen recombination instability and the Fe-peak instability. Both instabilities occur when the stellar mass is significantly reduced ($M \lesssim M_\mathrm{i}/2$) at the end of the TP-(S)AGB in our models with initial mass $M_\mathrm{i}\gtrsim 2~\mathrm{M}_\odot$. The hydrogen recombination instability occurs due to the difficulty of modelling a thermally and dynamically unstable envelope in a 1D hydrostatic code, and is prevented by damping the energy released by hydrogen recombination in the outer envelope. The Fe-peak instability occurs when the radiation pressure drops at the base of the convective envelope and is prevented by boosting the convective energy transport in this region. We provide custom routines to prevent these instabilities in the stellar evolution code mesa. The impact of these routines on the stellar structure is minimized so as to not affect the efficiency of third dredge-up, hot-bottom burning, or the wind mass-loss rate. We find only a modest reduction in third dredge-up efficiency at small envelope masses ($M_\mathrm{env}\lesssim 1.0~\mathrm{M}_\odot$). Consequently, our $M_\mathrm{i}=5~\mathrm{M}_\odot$ star, with hot-bottom burning, becomes a carbon star for the last $\sim 10~{{\ \rm per\ cent}}$ of its thermally pulsing lifetime. The largest stellar radii are reached during the final thermal pulses, which may have important consequences for binary–star interactions.

Precise masses of red giant stars enable a robust inference of their ages, but there are cases where these age estimates are very precise but also very inaccurate. Examples are core-helium-burning (CHeB) stars that have lost more mass than predicted by standard single-star evolutionary models. Members of star clusters in the Kepler database represent a unique opportunity to identify such stars because they combine exquisite asteroseismic constraints with independent age information (members of a star cluster share a similar age and chemical composition). We focus on the single metal-rich (Z approximate to Z(circle dot)) Li-rich low-mass CHeB star KIC4937011, which is a member of the open cluster NGC 6819 (turn-off mass of approximate to 1.6 M-circle dot, i.e. an age of approximate to 2.4 Gyr). This star has a lower mass by approximate to 1 M-circle dot than expected for its age and metallicity, which might be explained by binary interactions or mass loss along the red giant branch (RGB). To infer formation scenarios for this object, we performed a Bayesian analysis by combining the binary stellar evolutionary framework BINARY_C V2.2.3 with the dynamic nested-sampling approach contained in the DYNESTY V2.1.1 package. We find that this star probably is the result of a common-envelope evolution (CEE) phase during the RGB stage of the primary star in which the low-mass (< 0.71 M-circle dot) main-sequence companion does not survive. The mass of the primary star at the zero-age main sequence is in the range [1.46, 1.71] M-circle dot, with a log-orbital period in the range [0.06, 2.4] log(10)(days). During the CEE phase, approximate to 1 M-circle dot of material is ejected from the system, and the final star reaches the CHeB stage after helium flashes as if it were a single star with a mass of approximate to 0.7 M-circle dot, which is what we observe today. Although the proposed scenario is consistent with photometric and spectroscopic observations, a quantitative comparison with detailed stellar evolution calculations is needed to quantify the systematic skewness of the radius, luminosity, and effective temperature distributions towards higher values than observations.

We present detailed implementations of (a) binary stellar evolution (using binary_c) and (b) dust production and destruction into the cosmological semi-analytic galaxy evolution simulation, L-Galaxies. This new version of L-Galaxies is compared to a version assuming only single stars and to global and spatially-resolved observational data across a range of redshifts ($z$). We find that binaries have a negligible impact on the stellar masses, gas masses, and star formation rates of galaxies only if the total mass ejected by massive stars is unchanged. This is because massive stars determine the strength of supernova (SN) feedback, which in turn regulates galaxy growth. Binary effects, such as common envelope ejection and novae, affect carbon and nitrogen enrichment in galaxies, however heavier alpha elements are more affected by the choice of SN and wind yields. Unlike many other simulations, the new L-Galaxies reproduces observed dust-to-metal (DTM) and dust-to-gas (DTG) ratios at $z\sim{}0-4$. This is mainly due to shorter dust accretion timescales in dust-rich environments. However, dust masses are under-predicted at $z>4$, highlighting the need for enhanced dust production at early times in simulations, possibly accompanied by increased star formation. On sub-galactic scales, there is very good agreement between L-Galaxies and observed dust and metal radial profiles at $z=0$. A drop in DTM ratio is also found in diffuse, low-metallicity regions, contradicting the assumption of a universal value. We hope that this work serves as a useful template for binary stellar evolution implementations in other cosmological simulations in future.

Context. The explosive burning that drives nova eruptions results in unique nucleosynthesis that heavily over-produces certain isotopes relative to the solar abundance. However, novae are often ignored when considering the chemical evolution of our Galaxy due to their low ejecta masses. Galactic chemical evolution studies including novae are rare and have previously relied upon simplified treatments for the behaviour of nova populations. Aims. In this work, we use previously computed synthetic nova populations and the galactic chemical evolution code OMEGA+ to assess the impact that novae have on the evolution of stable elemental and isotopic abundances. Methods. We combine populations of novae computed using the binary population synthesis code binary_c with the galactic chemical evolution code OMEGA+ and detailed, white dwarf mass-dependent nova yields to model the nucleosynthetic contributions of novae to the evolution of the Milky Way. We consider three different nova yield profiles, each corresponding to a different set of nova yield calculations. Results. We examine which nova sites contribute most to which isotopes. Despite novae from low-mass white dwarfs (WDs) dominating nova ejecta contributions, we find that novae occurring on massive WDs are still able to contribute significantly to many isotopes, particularly those with high mass numbers. We find that novae can produce up to 35% of the Galactic C-13 and N-15 mass by the time the model Galaxy reaches [Fe/H] = 0, and earlier in the evolution of the Galaxy (between [Fe/H] = -2 and -1) novae may have been the dominant source of N-15. Predictions for [C-13/Fe], [N-15/Fe], C-12/C-13, and N-14/N-15 abundances ratios vary by up to 0.2 dex at [Fe/H] = 0 and by up to 0.7 dex in [N-15/Fe] and N-14/N-15 between [Fe/H] = -2 and -1 (corresponding approximately to Galactic ages of 170 Myr and 1 Gyr in our model). The Galactic evolution of other stable isotopes (excluding Li) is not noticeably affected by including novae. For most isotopes, agreement is generally good between the three different yield profiles we consider. Isotopes where agreement is relatively poor include: He-3 (especially at high M-WD), Li-7, O-18, F-18, and the > 1.3 M-circle dot regime of Si-29, S-33, S-34, Cl-35, and Ar-36.

A common requirement in science is to store and share large sets of simulation data in an efficient, nested, flexible and human-readable way. Such datasets contain number counts and distributions, i.e. histograms and maps, of arbitrary dimension and variable type, e.g. floating-point number, integer or character string. Modern high-level programming languages like Perl and Python have associated arrays, knowns as dictionaries or hashes, respectively, to fulfil this storage need. Low-level languages used more commonly for fast computational simulations, such as C and Fortran, lack this functionality. We present libcdict, a C dictionary library, to solve this problem. Libcdict provides C and Fortran application programming interfaces (APIs) to native dictionaries, called cdicts, and functions for cdicts to load and save these as JSON and hence for easy interpretation in other software and languages like Perl, Python and R.

ABSTRACT The Thermally Pulsing- (Super) Asymptotic Giant Branch is a late stage in the evolution of low- and intermediate-mass stars. These stars undergo strong wind mass-loss and diverse nucleosynthesis. Third dredge-up events, that occur following thermal pulses, are responsible for enriching the surfaces of Asymptotic Giant Branch stars, hence an understanding of this process is crucial for constraining galactic chemical evolution. Using a custom numerical scheme, we investigate the temporal and spatial resolution required to resolve the third dredge-up in the 1D stellar evolution code mesa. With mesa’s default controls, the third dredge-up efficiency is underestimated by as much as $\approx 76~{{\ \rm per\ cent}}$. In stars that undergo hot third dredge-up (M ≳ 6 M⊙), the third dredge-up efficiency is overestimated by $\approx 55~{{\ \rm per\ cent}}$. The Thermally Pulsing- (Super) Asymptotic Giant Branch (TP-(S)AGB) evolution is computed for models with initial masses 1 ≤ Mi/M⊙ ≤ 8 at Solar metallicity (Z = 0.014). The minimum initial mass for carbon stars falls in the range 1.5–1.75 M⊙, compatible with observations. The use of mesa for TP-(S)AGB evolution is validated by comparison to the widely used monash models which show good agreement in the maximum third dredge-up efficiency at initial masses Mi > 2 M⊙. We also compare the third dredge-up efficiency in models produced using two independent stellar evolution codes, fruity and aton, which were computed with various differences in input physics including mass-loss, and which exhibit weaker third dredge-up episodes.

The first population of X-ray binaries (XRBs) is expected to affect the thermal and ionization states of the gas in the early Universe. Although these X-ray sources are predicted to have important implications for high-redshift observable signals, such as the hydrogen 21-cm signal from cosmic dawn and the cosmic X-ray background, their properties are poorly explored, leaving theoretical models largely uninformed. In this paper we model a population of X-ray binaries arising from zero metallicity stars. We explore how their properties depend on the adopted initial mass function (IMF) of primordial stars, finding a strong effect on their number and X-ray production efficiency. We also present scaling relations between XRBs and their X-ray emission with the local star formation rate, which can be used in sub-grid models in numerical simulations to improve the X-ray feedback prescriptions. Specifically, we find that the uniformity and strength of the X-ray feedback in the intergalactic medium is strongly dependant on the IMF. Bottom-heavy IMFs result in a smoother distribution of XRBs, but have a luminosity orders of magnitude lower than more top-heavy IMFs. Top-heavy IMFs lead to more spatially uneven, albeit strong, X-ray emission. An intermediate IMF has a strong X-ray feedback while sustaining an even emission across the intergalactic medium. These differences in X-ray feedback could be probed in the future with measurements of the cosmic dawn 21-cm line of neutral hydrogen, which offers us a new way of constraining population III IMF.

ABSTRACT 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.

Dataset to accompany the publication entitled "The impact of binary stars on the dust and metal evolution of galaxies" by R. M. Yates, D. Hendriks, A. P. Vijayan, R. G. Izzard, P. A. Thomas, and P. Das. The L-Galaxies files included in this dataset were generated from the MM+binaryC and MM+singleStars versions of L-Galaxies, run in snapshot mode on representative sub-volumes of the Millennium-I and Millennium-II simulations, with the LITE_OUTPUT switch turned on. In detail: MM+binaryC version: SA_MM+binaryC_MRI_z0.00_0-5: Zip file containing output files (in binary format) for the MM+binaryC version of L-Galaxies in snapshot mode, run on treefiles 0-5 of Millennium-I at redshift 0.00. SA_MM+binaryC_MRI_z0.00-8.22_0-5: Zip file containing output files (in binary format) for the MM+binaryC version of L-Galaxies in snapshot mode, run on treefiles 0-5 of Millennium-I at nine selected redshifts from 0.00 to 8.22. SA_MM+binaryC_MRII_z#.##_40-79: Zip files containing output files (in binary format) for the MM+binaryC version of L-Galaxies in snapshot mode, run on treefiles 40-79 of Millennium-II at redshift #.## = [0.00,1.04,2.07,3.11,3.95,5.03,5.92,6.97,8.22]. MM+singleStars version: SA_MM+singleStars_MRI_z0.00_0-5: Zip file containing output files (in binary format) for the MM+singleStars version of L-Galaxies in snapshot mode, run on treefiles 0-5 of Millennium-I at redshift 0.00. SA_MM+singleStars_MRI_z0.00-8.22_0-5: Zip file containing output files (in binary format) for the MM+singleStars version of L-Galaxies in snapshot mode, run on treefiles 0-5 of Millennium-I at nine selected redshifts from 0.00 to 8.22. SA_MM+singleStars_MRII_z#.##_40-79: Zip files containing output files (in binary format) for the MM+singleStars version of L-Galaxies in snapshot mode, run on treefiles 40-79 of Millennium-II at redshift #.## = [0.00,1.04,2.07,3.11,3.95,5.03,5.92,6.97,8.22]. Other:L-Galaxies_and_binaryC_processing: a) A set of simple python scripts to read the L-Galaxies outputs, process them into samples (as python "dictionaries"), and make some plots (including those presented in Yates+23). To use, place the desired output files into the /output folder, and execute main_lgals.py. Switches are provided at the top of main_lgals.py to modify functionality. The relevant data structure for python is also provided (LGalaxy_snapshots_liteOutput.py), as are conversions between snapshot number, redshift, and other properties for Millennium-I and Millennium-II snapshots for reference (Millennium_Planck1_Snapshots_Redshifts.txt and MillenniumII_Planck1_Snapshots_Redshifts.txt). For catalogues containing star formation histories, the corresponding times and widths of each bin are read from Database_SFH_table.fits. b) The binaryC ensemble files used in Yates+23 are included, along with a script to read these ensemble files and generate L-Galaxies yield tables from them (process_binaryc_outputs.py). Switches are provided at the top of process_binaryc_outputs.py to modify functionality.

Window to the Stars 2.13 on Raspbian (buster) http://personal.ph.surrey.ac.uk/~ri0005/window.html Username/password: wtts/wtts Please use the latest upload. Others are left here for the interested. Note that the first time you boot, the Pi will do some work to decompress some things and expand the filesystem. Be patient! Just let the Pi do its thing for a few minutes and you'll be fine. Hopefully :)

White dwarfs stars are known to be polluted by their active planetary systems, but little attention has been paid to the accretion of wind from low-mass companions. The capture of stellar or substellar wind by white dwarfs is one of few methods available to astronomers which can assess mass-loss rates from unevolved stars and brown dwarfs, and the only known method to extract their chemical compositions. In this work, four white dwarfs with closely orbiting, L-type brown dwarf companions are studied to place limits on the accretion of a substellar wind, with one case of a detection, and at an extremely non-solar abundance m(Na)/m(Ca) > 900. The mass-loss rates and upper limits are tied to accretion in the white dwarfs, based on limiting cases for how the wind is captured, and compared with known cases of wind pollution from close M dwarf companions, which manifest in solar proportions between all elements detected. For wind captured in a Bondi-Hoyle flow, mass-loss limits (M) overdot less than or similar to 5 x10(-17) M-circle dot yr(-1) are established for three L dwarfs, while for M dwarfs polluting their hosts, winds in the range 10(-13)-10(-16) M-circle dot yr(-1) are found. The latter compares well with the (M) overdot similar to 10(-13)-10(-15) M-circle dot yr(-1) estimates obtained for nearby, isolated M dwarfs using Lyato probe their astropsheres. These results demonstrate that white dwarfs are highly sensitive stellar and substellar wind detectors, where further work on the actual captured wind flow is needed.

Aluminium-26 is a radioactive isotope which can be synthesized within asymptotic giant branch (AGB) stars, primarily through hot bottom burning. Studies exploring 26Al production within AGB stars typically focus on single-stars; however, observations show that low- and intermediate-mass stars commonly exist in binaries. We use the binary population synthesis code binary_c to explore the impact of binary evolution on 26Al yields at solar metallicity both within individual AGB stars and a low/intermediate-mass stellar population. We find the key stellar structural condition achieving most 26Al overproduction is for stars to enter the thermally-pulsing AGB (TP-AGB) phase with small cores relative to their total masses, allowing those stars to spend abnormally long times on the TP-AGB compared to single-stars of identical mass. Our population with a binary fraction of 0.75 has an 26Al weighted population yield increase of 25% compared to our population of only single-stars. Stellar-models calculated from the Mt Stromlo/Monash Stellar Structure Program, which we use to test our results from binary_c and closely examine the interior structure of the overproducing stars, support our binary_c results only when the stellar envelope gains mass after core-He depletion. Stars which gain mass before core-He depletion still overproduce 26Al, but to a lesser extent. This introduces some physical uncertainty into our conclusions as 55% of our 26Al overproducing stars gain envelope mass through stellar wind accretion onto pre-AGB objects. Our work highlights the need to consider binary influence on the production of 26Al.

We develop a rapid algorithm for the evolution of stable, circular, circumbinary discs suitable for parameter estimation and population synthesis modelling. Our model includes disc mass and angular momentum changes, accretion on to the binary stars, and binary orbital eccentricity pumping. We fit our model to the post-asymptotic giant branch (post-AGB) circumbinary disc around IRAS 08544-4431, finding reasonable agreement despite the simplicity of our model. Our best-fitting disc has a mass of about $0.01\, \mathrm{M}_{\odot }$ and angular momentum $2.7\times 10^{52}\, \mathrm{g}\, \mathrm{cm}^{2}\, \mathrm{s}^{-1}\simeq 9 \,\mathrm{M}_{\odot }\, \mathrm{km}\, \mathrm{s}^{-1}\, \mathrm{au}$, corresponding to 0.0079 and 0.16 of the common-envelope mass and angular momentum, respectively. The best-fitting disc viscosity is $\alpha _\mathrm{disc} = 5 \times 10^{-3}$ and our tidal torque algorithm can be constrained such that the inner edge of the disc $R_{\mathrm{in}}\sim 2a$. The inner binary eccentricity reaches about 0.13 in our best-fitting model of IRAS 08544-4431, short of the observed 0.22. The circumbinary disc evaporates quickly when the post-AGB star reaches a temperature of $\sim \! 6\times 10^4\, \mathrm{K}$, suggesting that planetismals must form in the disc in about $10^{4}\, \mathrm{yr}$ if secondary planet formation is to occur, while accretion from the disc on to the stars at about 10 times the inner-edge viscous rate can double the disc lifetime.

ABSTRACT We use the rapid binary stellar evolution code binary_c to estimate the rate of merging neutron stars with numerous combinations of envelope ejection efficiency and natal kick dispersion. We find a peak in the local rate of merging neutron stars around α ≈ 0.3–0.4, depending on the metallicity, where α is the efficiency of utilizing orbital energy to unbind the envelope. The peak height decreases with increasing electron-capture supernova kick dispersion σECSN. We explain the peak as a competition between the total number of systems that survive the common-envelope phase increasing with α and their separation, which increases with α as well. Increasing α reduces the fraction of systems that merge within a time shorter than the age of the Universe and results in different mass distributions for merging and non-merging double neutron stars. This offers a possible explanation for the discrepancy between the Galactic double neutron star mass distribution and the observed massive merging neutron star event GW190425. Within the α–σECSN parameter space that we investigate, the rate of merging neutron stars spans several orders of magnitude up to more than $1\times 10^{3} \, \mathrm{Gpc}^{-3}\, \mathrm{yr}^{-1}$ and can be higher than the observed upper limit or lower than the observed lower limit inferred thus far from merging neutron stars detected by gravitational waves. Our results stress the importance of common-envelope physics for the quantitative prediction and interpretation of merging binary neutron star events in this new age of gravitational wave astronomy.

We present the software package binary_c-python which provides a convenient and easy-to-use interface to the binary_c framework, allowing the user to rapidly evolve individual systems and populations of stars. binary_c-python is available on Pip and on GitLab. binary_c-python contains many useful features to control and process the output of binary_c, like by providing binary_c-python with logging statements that are dynamically compiled and loaded into binary_c. Moreover, we have recently added standardised output of events like Roche-lobe overflow or double compact-object formation to binary_c, and automatic parsing and managing of that output in binary_c-python. binary_c-python uses multiprocessing to utilise all the cores on a particular machine, and can run populations with HPC cluster workload managers like HTCondor and Slurm, allowing the user to run simulations on large computing clusters. We provide documentation that is automatically generated based on docstrings and a suite of Jupyter notebooks. These notebooks consist of technical tutorials on how to use binary_c-python and use-case scenarios aimed at doing science. Much of binary_c-python is covered by unit tests to ensure reliability and correctness, and the test coverage is continually increased as the package is improved.