arXiv subjects

S. E. de Mink

Publications and source records attributed to S. E. de Mink.

At least 19 recordsLinked to original sources

arXiv2025

Binarity at LOw Metallicity (BLOeM): a spectroscopic VLT monitoring survey of massive stars in the SMC

Surveys in the Milky Way and Large Magellanic Cloud revealed that the majority of massive stars will interact with companions during their lives. However, knowledge of the binary properties of massive stars at low metallicity, which approaches the conditions of the Early Universe, remains sparse. We present the Binarity at LOw Metallicity (BLOeM) campaign - an ESO large programme designed to obtain 25 epochs of spectroscopy for 929 massive stars in the SMC - the lowest metallicity conditions in which multiplicity is probed to date (Z = 0.2 Zsun). BLOeM will provide (i) the binary fraction, (ii) the orbital configurations of systems with periods P < 3 yr, (iii) dormant OB+BH binaries, and (iv) a legacy database of physical parameters of massive stars at low metallicity. The stars are observed with the LR02 setup of the giraffe instrument of the Very Large Telescope (3960-4570A, resolving power R=6200; typical signal-to-noise ratio S/N=70-100). This paper utilises the first 9 epochs obtained over a three-month time. We describe the survey and data reduction, perform a spectral classification of the stacked spectra, and construct a Hertzsprung-Russell diagram of the sample via spectral-type and photometric calibrations. The sample covers spectral types from O4 to F5, spanning the effective temperature and luminosity ranges 6.5<Teff/kK<45 and 3.7<log L/Lsun<6.1 and initial masses 8<Mini/Msun<80. It comprises 159 O-type stars, 331 early B-type (B0-3) dwarfs and giants (luminosity classes V-III), 303 early B-type supergiants (II-I), and 136 late-type supergiants. At least 82 stars are Oe/Be stars: 20 O-type and 62 B-type (13% and 11% of the respective samples). In addition, it includes 4 high-mass X-ray binaries, 3 stars resembling luminous blue variables, 2 bloated stripped-star candidates, 2 candidate magnetic stars, and 74 eclipsing binaries.

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arXiv2025

Binary mass transfer in 3D: Mass Transfer Rate and Morphology

Mass transfer is crucial in binary evolution, yet its theoretical treatment has long relied on analytic models whose key assumptions remain debated. We present a direct and systematic evaluation of these assumptions using high-resolution 3D hydrodynamical simulations including the Coriolis force. We simulate streams overflowing from both the inner and outer Lagrangian points, quantify mass transfer rates, and compare them with analytic solutions. We introduce scaling factors, including the overfilling factor, to render the problem dimensionless. The donor-star models are simplified, with either an isentropic initial stratification and adiabatic evolution or an isothermal structure and evolution, but the scalability of this formulation allows us to extend the results for a mass-transferring system to arbitrarily small overfilling factors for the adiabatic case. We find that the Coriolis force -- often neglected in analytic models -- strongly impacts the stream morphology: breaking axial symmetry, reducing the stream cross section, and shifting its origin toward the donor's trailing side. Contrary to common assumptions, the sonic surface is not flat and does not always intersect the Lagrangian point: instead, it is concave and shifted, particularly toward the accretor's trailing side. Despite these structural asymmetries, mass transfer rates are only mildly suppressed relative to analytic predictions and the deviation is remarkably small -- within a factor of two (ten) for the inner (outer) Lagrangian point over seven orders of magnitude in mass ratio. We use our results to extend the widely-used mass-transfer rate prescriptions by Ritter(1988) and Kolb&Ritter(1990), for both the inner and outer Lagrangian points. These extensions can be readily adopted in stellar evolution codes like MESA, with minimal changes where the original models are already in use.

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arXiv2025

A high fraction of close massive binary stars at low metallicity

At high metallicity, a majority of massive stars have at least one close stellar companion. The evolution of such binaries is subject to strong interaction processes, heavily impacting the characteristics of their life-ending supernova and compact remnants. For the low-metallicity environments of high-redshift galaxies constraints on the multiplicity properties of massive stars over the separation range leading to binary interaction are crucially missing. Here we show that the presence of massive stars in close binaries is ubiquitous, even at low metallicity. Using the Very Large Telescope, we obtained multi-epoch radial velocity measurements of a representative sample of 139 massive O-type stars across the Small Magellanic Cloud, which has a metal content of about one fifth of the solar value. We find that 45% of them show radial velocity variations which demonstrate that they are members of close binary systems, and predominantly have orbital periods shorter than one year. Correcting for observational biases indicates that at least 70[+11:-6]% of the O stars in our sample are in close binaries, and that at least 68[+7:-8]% of all O stars interact with a companion star during their lifetime. We found no evidence supporting a statistically significant trend of the multiplicity properties with metallicity. Our results indicate that multiplicity and binary interactions govern the evolution of massive stars and determine their cosmic feedback and explosive fates.

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arXiv2025

Populations of evolved massive binary stars in the Small Magellanic Cloud I: Predictions from detailed evolution models

Context. The majority of massive stars are born with a close binary companion. How this affects their evolution and fate is still largely uncertain, especially at low metallicity. Aims. We derive synthetic populations of massive post-interaction binary products and compare them with corresponding observed populations in the Small Magellanic Cloud (SMC). Methods. We analyse 53298 detailed binary evolutionary models computed with MESA. Our models include the physics of rotation, mass and angular momentum transfer, magnetic internal angular momentum transport, and tidal spin-orbit coupling. They cover initial primary masses of 5-100Msun, initial mass ratios of 0.3-0.95, and all initial periods for which interaction is expected. They are evolved through the first mass transfer and the donor star death, a possible ensuing Be/X-ray binary phase, and they end when the mass gainer leaves the main sequence. Results.In our fiducial synthetic population, 8% of the OB stars in the SMC are post-mass transfer systems, and 7% are merger products. In many of our models, the mass gainers are spun up and form Oe/Be stars. While our model underpredicts the number of Be/X-ray binaries in the SMC, it reproduces the main features of their orbital period distribution and the observed number of SMC binary WR stars. We expect $\sim$50 OB+BH binaries below and $\sim$170 above 20d orbital period. The latter might produce merging double BHs. However, their progenitors, the predicted long-period WR+OB binaries, are not observed. Conclusions. While the comparison with the observed SMC stars supports many physics assumptions in our high-mass binary models, a better match of the large number of observed OBe stars and Be/X-ray binaries likely requires a lower merger rate and/or a higher mass transfer efficiency during the first mass transfer. The fate of the initially wide O star binaries remains uncertain.

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arXiv2025

Binarity at LOw Metallicity (BLOeM): Enhanced multiplicity of early B-type dwarfs and giants at $Z=0.2\,{\rm Z}_\odot$

Early B-type stars ($M_i=8-15$ M$_\odot$) are frequently in multiple systems, as evidenced by spectroscopic campaigns in the Milky Way (MW) and the Large Magellanic Cloud (LMC). Previous studies have shown no strong metallicity dependence in the close-binary (a>10 au) fraction or orbital-period distributions between the MW's solar metallicity (Z$_\odot$) and that of the LMC (Z=0.5 Z$_\odot$). However, similar analyses in more metal-poor environments are still scarce. We focus on 309 early B-type stars (luminosity classes III-V) from the Binarity at LOw Metallicity campaign in the Small Magellanic Cloud (SMC, Z=0.2 Z$_\odot$) using VLT/FLAMES multi-epoch spectroscopy. By applying binary detection criteria consistent with previous works, we identify 153 stars (91 SB1, 59 SB2, 3 SB3) exhibiting significant radial-velocity (RV) variations, resulting in an observed multiplicity fraction of $f^{obs}_{mult}=50\pm3\%$. Using Monte Carlo simulations to account for observational biases, we infer an intrinsic close-binary fraction of $f_{mult}=80\pm8\%$. A Markov chain Monte Carlo analysis of the peak-to-peak RV distribution ($Δ{\rm RV}_{\rm max}$) confirms a high multiplicity fraction of $f_{mult}=78\pm5\%$. These findings suggest a possible anti-correlation between metallicity and the fraction of close B-type binaries, with the SMC multiplicity fraction significantly exceeding previous measurements in the LMC and MW. The enhanced fraction of close binaries at SMC's low metallicity may have broad implications for massive-star evolution in the early Universe. More frequent mass transfer and envelope stripping could boost the production of exotic transients, stripped supernovae, gravitational-wave progenitors, and sustained UV ionising flux, potentially affecting cosmic reionisation. Theoretical predictions of binary evolution under metal-poor conditions will provide a key test of our results.

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arXiv2025

Binarity at LOw Metallicity (BLOeM): Multiplicity of early B-type supergiants in the Small Magellanic Cloud

The blue supergiant (BSG) domain contains a large variety of stars whose past and future evolutionary paths are still highly uncertain. Since binary interaction plays a crucial role in the fate of massive stars, investigating the multiplicity among BSGs helps shed light on the fate of such objects. We aim to estimate the binary fraction of a large sample of BSGs in the Small Magellanic Cloud within the Binarity at LOw Metallicity (BLOeM) survey. In total, we selected 262 targets with spectral types B0-B3 and luminosity classes I-II. This work is based on spectroscopic data collected by the GIRAFFE instrument, mounted on the Very Large Telescope, which gathered nine epochs over three months. Our spectroscopic analysis for each target includes the individual and peak-to-peak radial velocity measurements, an investigation of the line profile variability, and a periodogram analysis to search for possible short- and long-period binaries. By applying a 20 km s$^{-1}$ threshold on the peak-to-peak radial velocities above which we would consider the star to be binary, the resulting observed spectroscopic binary fraction for our BSG sample is 23 $\pm$ 3$\%$. In addition, we derived reliable orbital periods for 41 spectroscopic binaries and potential binary candidates, among which there are 17 eclipsing binaries, including 20 SB1 and SB2 systems with periods of less than 10 days. We reported a significant drop in the binary fraction of BSGs with spectral types later than B2 and effective temperatures less than 18 kK, which could indicate the end of the main sequence phase in this temperature regime. We found no metallicity dependence in the binary fraction of BSGs, compared to existing spectroscopic surveys of the Galaxy and Large Magellanic Cloud.

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arXiv2025

Binarity at Low Metallicity (BLOeM) -- Multiplicity properties of Oe and Be stars

Rapidly rotating classical OBe stars have been proposed as the products of binary interactions, and the fraction of Be stars with compact companions implies that at least some are. However, to constrain the interaction physics spinning up the OBe stars, a large sample of homogeneously analysed OBe stars with well-determined binary characteristics and orbital parameters are required. We investigate the multiplicity properties of a sample of 18 Oe, 62 Be, and two Of?p stars observed within the BLOeM survey in the Small Magellanic Cloud. We analyse the first nine epochs of spectroscopic observations obtained over approximately three months in 2023. Radial velocities (RVs) of all stars are measured. Applying commonly-used binarity criteria we classify objects as binaries, binary candidates, and apparently single (RV stable) objects. We further inspect the spectra for double-lined spectroscopic binaries and cross-match with catalogues of X-ray sources and photometric binaries. We classify 14 OBe stars as binaries, and an additional 11 as binary candidates. The two Of?p stars are apparently single. Two more objects are most likely currently interacting binaries. Without those, the observed binary fraction for the OBe sample (78 stars) is f_OBe_obs=0.18+/-0.04 (f_obs_cand=0.32+/-0.05 including candidates). This fraction is less than half of that measured for OB stars in BLOeM. Combined with the lower fraction of SB2s, this suggests that OBe stars have indeed fundamentally different binary properties than OB stars. We find no evidence for OBe binaries with massive compact companions, in contrast to expectations from binary population synthesis. Our results support the binary scenario as an important formation channel for OBe stars, as post-interaction binaries may have been disrupted or the stripped companions of OBe stars are harder to detect.

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arXiv2025

Binarity at LOw Metallicity (BLOeM): The multiplicity properties and evolution of BAF-type supergiants

Given the uncertain evolutionary status of blue supergiant stars, their multiplicity properties hold vital clues to better understand their origin and evolution. As part of The Binarity at LOw Metallicity (BLOeM) campaign in the Small Magellanic Cloud we present a multi-epoch spectroscopic survey of 128 supergiant stars of spectral type B5--F5, which roughly correspond to initial masses in the range 6 to 30 solar masses. The observed binary fraction for the B5-9 supergiants is 25+/-6 % (10+/-4 %) and 5+/-2 % (0 %) for the A-F stars, using a radial velocity (RV) variability threshold of 5 kms (10 kms) as a criterion for binarity. Accounting for observational biases we find an intrinsic multiplicity fraction of less than 18% for the B5-9 stars and 8$^{+9}_{-7}$% for the AF stars, for the orbital periods up to 10$^{3.5}$day and mass-ratios (q) in the range 0.1 < q < 1. The large stellar radii of these supergiant stars prevent short orbital periods but we demonstrate that this effect alone cannot explain our results. We assess the spectra and RV time series of the detected binary systems and find that only a small fraction display convincing solutions. We conclude that the multiplicity fractions are compromised by intrinsic stellar variability such that the true multiplicity fraction may be significantly smaller. Our main conclusions from comparing the multiplicity properties of the B5-9 and AF supergiants to that of their less evolved counterparts is that such stars cannot be explained by a direct evolution from the main sequence. Furthermore, by comparing their multiplicity properties to red supergiant stars we conclude that the AF supergiant stars are neither progenitors nor descendants of red supergiants.

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arXiv2024

Investigating the Chemically Homogeneous Evolution Channel and its Role in the Formation of the Enigmatic Binary Black Hole Progenitor Candidate HD 5980

Chemically homogeneous evolution (CHE) is a promising channel for forming massive binary black holes. The enigmatic, massive Wolf-Rayet (WR) 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. Using MESA, 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 day orbit as a result of mass loss -- correspondingly, they have very high initial stellar masses ($\gtrsim$150 M$_\odot$). We explore variations in stellar wind-mass loss and internal mixing efficiency, and find that models assuming enhanced mass-loss are greatly favored to explain HD 5980, while enhanced mixing is only slightly favoured over our fiducial assumptions. Our most probable models slightly underpredict the hydrogen surface abundances. Regardless of its prior history, this system is a likely binary black hole progenitor. We model its further evolution under our fiducial and enhanced wind assumptions, finding that both stars produce black holes with masses ~19-37 M$_\odot$. 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.

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arXiv2023

Solar evolution models with a central black hole

Hawking (1971) proposed that the Sun may harbor a primordial black hole whose accretion supplies some of the solar luminosity. Such an object would have formed within the first 1 s after the Big Bang with the mass of a moon or an asteroid. These light black holes are a candidate solution to the dark matter problem, and could grow to become stellar-mass black holes (BHs) if captured by stars. Here we compute the evolution of stars having such a BH at their center. We find that such objects can be surprisingly long-lived, with the lightest black holes having no influence over stellar evolution, while more massive ones consume the star over time to produce a range of observable consequences. Models of the Sun born about a BH whose mass has since grown to approximately $10^{-6}~\rm{M}_\odot$ are compatible with current observations. In this scenario, the Sun would first dim to half its current luminosity over a span of 100 Myr as the accretion starts to generate enough energy to quench nuclear reactions. The Sun would then expand into a fully-convective star, where it would shine luminously for potentially several Gyr with an enriched surface helium abundance, first as a sub-subgiant star, and later as a red straggler, before becoming a sub-solar-mass BH. We also present results for a range of stellar masses and metallicities. The unique internal structures of stars harboring BHs may make it possible for asteroseismology to discover them, should they exist. We conclude with a list of open problems and predictions.

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arXiv2023

The B-type Binaries Characterisation Programme II. VFTS 291: A stripped star from a recent mass transfer phase

Recent studies of massive binaries with putative black hole companions have uncovered a phase of binary evolution that has not been observed before, featuring a bloated stripped star that very recently ceased transferring mass to a main-sequence companion. In this study, we focus on the candidate system VFTS 291, a binary with an orbital period of 108 d and a high semi-amplitude velocity ($K_1=93.7\pm0.2$ km s$^{-1}$). Through our analysis of the disentangled spectra of the two components, together with dynamical and evolutionary arguments, we identify a narrow-lined star of ~1.5-2.5 $M_\odot$ dominating the spectrum, and an early B-type main-sequence companion of $13.2\pm1.5$ $M_\odot$. The low mass of the narrow-lined star, and the high mass ratio, suggest that VFTS 291 is a post-mass-transfer system, with the narrow-lined star being bloated and stripped of its hydrogen-rich envelope, sharing many similarities with other recently discovered stripped stars. Our finding is supported by our detailed binary evolution models, which indicate that the system can be well explained by an initial configuration consisting of an 8.1 $M_\odot$ primary with an 8 $M_\odot$ companion in a 7 d orbital period. While some open questions remain, particularly concerning the surface helium enrichment of the stripped star and the rotational velocity of the companion, we expect that high-resolution spectroscopy may help reconcile our estimates with theory. Our study highlights the importance of multi-epoch spectroscopic surveys to identify and characterize binary interaction products, and provides important insights into the evolution of massive binary stars.

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arXiv2023

The young massive SMC cluster NGC 330 seen by MUSE III. Stellar parameters and rotational velocities

The origin of initial rotation rates of stars, and how a star's surface rotational velocity changes during the evolution, either by internal angular momentum transport or due to interactions with a binary companion, remain open questions in stellar astrophysics. Here, we aim to derive the physical parameters and study the distribution of (projected) rotational velocities of B-type stars in the 35 Myr-old, massive cluster NGC 330 in the Small Magellanic Cloud. NGC 330 is in an age range where the number of post-interaction binaries is predicted to be high near the cluster turnoff (TO). We develop a simultaneous photometric and spectroscopic grid-fitting method adjusting atmosphere models on multi-band Hubble Space Telescope photometry and Multi Unit Spectroscopic Explorer spectroscopy. This allows us to homogeneously constrain the physical parameters of over 250 B and Be stars, brighter than mF814W = 18.8 mag. The rotational velocities of Be stars in NGC 330 are significantly higher than the ones of B stars. The rotational velocities vary as a function of the star's position in the color-magnitude diagram, qualitatively following predictions of binary population synthesis. A comparison to younger clusters shows that stars in NGC 330 rotate more rapidly on average. The rotational velocities of the 35 Myr old population in NGC 330 quantitatively agree with predictions for a stellar population that underwent significant binary interactions: the bulk of the B stars could be single stars or primaries in pre-interaction binaries. The rapidly spinning Be stars could be mass and angular momentum gainers in previous interactions, while those Be stars close to the TO may be spun-up single stars. The slowly rotating, apparently single stars above the TO could be merger products. The different vsini-characteristics of NGC 330 compared to younger populations can be understood in this framework.

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arXiv2023

Discovery of the missing intermediate-mass helium stars stripped in binaries

The theory of binary evolution predicts that many massive stars should lose their hydrogen-rich envelopes via interaction with a companion -- revealing hot helium stars with masses of $\sim$2--8M$_{\odot}$. However, only one candidate system had been identified, leaving a large discrepancy between theory and observation. Here, we present a new sample of stars -- identified via excess ultraviolet emission -- whose luminosities, colors, and spectral morphologies are consistent with predictions for the missing population. We detect radial velocity variations indicative of binary motion and measure high temperatures ($T_{\rm eff}\sim60-100$kK), high surface gravities ($\log(g)\sim5$) and depleted surface hydrogen mass fractions ($X_{\rm{H,surf}}\lesssim0.3$), which match expectations for stars with initial masses between 8--25 M$_{\odot}$ that have been stripped via binary interaction. These systems fill the helium star mass gap between subdwarfs and Wolf-Rayet stars, and are thought to be of large astrophysical significance as ionizing sources, progenitors of stripped-envelope supernovae and merging double neutron stars.

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arXiv2023

Stellar properties of observed stars stripped in binaries in the Magellanic Clouds

Massive stars (~8-25Msun) stripped of their hydrogen-rich envelopes via binary interaction are thought to be the main progenitors for merging neutron stars and stripped-envelope supernovae. We recently presented the discovery of the first set of such stripped stars in a companion paper. Here, we fit the spectra of ten stars with new atmosphere models in order to constrain their stellar properties precisely. We find that the stellar properties align well with the theoretical expectations from binary evolution models for helium-core burning envelope-stripped stars. The fits confirm that the stars have high effective temperatures (Teff~50-100kK), high surface gravities (log g ~5), and hydrogen-poor/helium-rich surfaces (X(H, surf)~0-0.4) while showing for the first time a range of bolometric luminosities (10^3-10^5 Lsun), small radii (~0.5-1Rsun), and low Eddington factors (Gamma_e~0.006-0.4). Using these properties, we derive intermediate current masses (~1-8Msun), which suggest that their progenitors were massive stars (~5-25Msun) and that a subset will reach core-collapse, leaving behind neutron stars or black holes. Using the model fits, we also estimate the emission rates of ionizing photons for these stars, which agree well with previous model expectations. Further, by computing models for a range of mass-loss rates, we find that the stellar winds are weaker than predicted by any existing scheme (Mdot(wind)<~ 1e-9 Msun/yr). The properties of this first sample of intermediate mass helium stars suggest they both contain progenitors of type Ib and IIb supernovae, and provide important benchmarks for binary evolution and population synthesis models.

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arXiv2023

X-Shooting ULLYSES: massive stars at low metallicity. I. Project Description

Observations of individual massive stars, super-luminous supernovae, gamma-ray bursts, and gravitational-wave events involving spectacular black-hole mergers, indicate that the low-metallicity Universe is fundamentally different from our own Galaxy. Many transient phenomena will remain enigmatic until we achieve a firm understanding of the physics and evolution of massive stars at low metallicity (Z). The Hubble Space Telescope has devoted 500 orbits to observe 250 massive stars at low Z in the ultraviolet (UV) with the COS and STIS spectrographs under the ULLYSES program. The complementary ``X-Shooting ULLYSES'' (XShootU) project provides enhanced legacy value with high-quality optical and near-infrared spectra obtained with the wide-wavelength coverage X-shooter spectrograph at ESO's Very Large Telescope. We present an overview of the XShootU project, showing that combining ULLYSES UV and XShootU optical spectra is critical for the uniform determination of stellar parameters such as effective temperature, surface gravity, luminosity, and abundances, as well as wind properties such as mass-loss rates in function of Z. As uncertainties in stellar and wind parameters percolate into many adjacent areas of Astrophysics, the data and modelling of the XShootU project is expected to be a game-changer for our physical understanding of massive stars at low Z. To be able to confidently interpret James Webb Space Telescope (JWST) spectra of the first stellar generations, the individual spectra of low Z stars need to be understood, which is exactly where XShootU can deliver.

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arXiv2023

Nucleosynthesis of binary-stripped stars

The cosmic origin of the elements, the fundamental chemical building blocks of the Universe, is still uncertain. Binary interactions play a key role in the evolution of many massive stars, yet their impact on chemical yields is poorly understood. Using the MESA stellar evolution code we predict the chemical yields ejected in wind mass loss and the supernovae of single and binary-stripped stars. We do this with a large 162 isotope nuclear network at solar-metallicity. We find that binary-stripped stars are more effective producers of the elements than single stars, due to their increased mass loss and an increased chance to eject their envelopes during a supernova. This increased production by binaries varies across the periodic table, with Fluorine and Potassium being more significantly produced by binary-stripped stars than single stars. We find that the C12/C13 could be used as an indicator of the conservativeness of mass transfer, as C13 is preferentially ejected during mass transfer while C12 is preferentially ejected during wind mass loss. We identify a number of gamma-ray emitting radioactive isotopes that may be used to help constrain progenitor and explosion models of core-collapse supernovae with next-generation gamma-ray detectors. For single stars we find V44 and Mn52 are strong probes of the explosion model, while for binary-stripped stars it is Cr48. Our findings highlight that binary-stripped stars are not equivalent to two single stars and that detailed stellar modelling is needed to predict their final nucleosynthetic yields.

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arXiv2023

Spectroscopic and evolutionary analyses of the binary system AzV 14 outline paths toward the WR stage at low metallicity

The origin of the observed population of Wolf-Rayet (WR) stars in low-metallicity (low-Z) galaxies, such as the Small Magellanic Cloud (SMC), is not yet understood. Standard, single-star evolutionary models predict that WR stars should stem from very massive O-type star progenitors, but these are very rare. On the other hand, binary evolutionary models predict that WR stars could originate from primary stars in close binaries. We conduct an analysis of the massive O star, AzV 14, to spectroscopically determine its fundamental and stellar wind parameters, which are then used to investigate evolutionary paths from the O-type to the WR stage with stellar evolutionary models. Multi-epoch UV and optical spectra of AzV 14 are analyzed using the non-LTE stellar atmosphere code PoWR. An optical TESS light curve was extracted and analyzed using the PHOEBE code. The obtained parameters are put into an evolutionary context, using the MESA code. AzV 14 is a close binary system consisting of two similar main sequence stars with masses of 32 Msol. Both stars have weak stellar winds with mass-loss rates of log $\dot{M}$ = -7.7. Binary evolutionary models can explain the empirically derived stellar and orbital parameters. The model predicts that the primary will evolve into a WR star with T = 100 kK, while the secondary, which will accrete significant amounts of mass during the first mass transfer phase, will become a cooler WR star with T = 50 kK and are predicted to have compared to other WR stars increased oxygen abundances. This model prediction is supported by a spectroscopic analysis of a WR star in the SMC. We hypothesize that the populations of WR stars in low-Z galaxies may have bimodal temperature distributions. Hotter WR stars might originate from primary stars, while cooler WR stars are the evolutionary descendants of the secondary stars if they accreted a significant amount of mass.

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arXiv2023

The locations of features in the mass distribution of merging binary black holes are robust against uncertainties in the metallicity-dependent cosmic star formation history

New observational facilities are probing astrophysical transients such as stellar explosions and gravitational wave (GW) sources at ever increasing redshifts, while also revealing new features in source property distributions. To interpret these observations, we need to compare them to predictions from stellar population models. Such models require the metallicity-dependent cosmic star formation history ($\mathcal{S}(Z,z)$) as an input. Large uncertainties remain in the shape and evolution of this function. In this work, we propose a simple analytical function for $\mathcal{S}(Z,z)$. Variations of this function can be easily interpreted, because the parameters link to its shape in an intuitive way. We fit our analytical function to the star-forming gas of the cosmological TNG100 simulation and find that it is able to capture the main behaviour well. As an example application, we investigate the effect of systematic variations in the $\mathcal{S}(Z,z)$ parameters on the predicted mass distribution of locally merging binary black holes (BBH). Our main findings are: I) the locations of features are remarkably robust against variations in the metallicity-dependent cosmic star formation history, and II) the low mass end is least affected by these variations. This is promising as it increases our chances to constrain the physics that governs the formation of these objects.

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