arXiv subjects

F. R. N. Schneider

Publications and source records attributed to F. R. N. Schneider.

At least 19 recordsLinked to original sources

arXiv2026

Multiplicity of Massive Stars at Low Metallicity: Early Results from the BLOeM Campaign

Massive stars at low metallicity (Z) play a central role in shaping the high-redshift Universe, yet their multiplicity remains poorly constrained. The Binarity at Low Metallicity (BLOeM) campaign is a two-year survey of 929 stars in the Small Magellanic Cloud with the Fibre Large Array Multi Element Spectrograph (FLAMES) instrument at ESO's Very Large Telescope, providing the first large-scale spectroscopic monitoring of massive stars at low Z (1/5 solar). Analysis of the initial nine epochs reveals high intrinsic binary fractions (>70%) on the main sequence and a steep decline in evolved objects. Analysis of the full dataset will yield orbital solutions, identify black-hole companions, and allow a derivation of the initial mass function for single and binary stars at low Z.

physics.gen-ph↗

arXiv2025

NLTE spectral modelling for a carbon-oxygen and helium white-dwarf merger as a Ca-rich transient candidate

We carry out NLTE (non local thermodynamic equilibrium) radiative transfer simulations to determine whether explosion during the merger of a carbon-oxygen (CO) white dwarf (WD) with a helium (He) WD can reproduce the characteristic Ca II/[Ca II] and He I lines observed in Ca-rich transients. Our study is based on a 1D representation of a hydrodynamic simulation of a 0.6 $M_{\odot}$ CO + 0.4 $M_{\odot}$ He WD merger. We calculate both photospheric and nebular-phase spectra including treatment for non-thermal electrons, as is required for accurate modelling of He I and [Ca II]. Consistent with Ca-rich transients, our simulation predicts a nebular spectrum dominated by emission from [Ca II] 7291, 7324 angstrom and the Ca II near-infrared (NIR) triplet. The photospheric-phase synthetic spectrum also exhibits a strong Ca II NIR triplet, prominent optical absorption due to He I 5876 angstrom and He I 10830 angstrom in the NIR, as is commonly observed for Ca-rich transients. Overall, our results therefore suggest that CO+He WD mergers are a promising channel for Ca-rich transients. However, the current simulation overpredicts some He I features, in particular both He I 6678 and 7065 angstrom and shows a significant contribution from Ti II, which results in a spectral energy distribution that is substantially redder than most Ca-rich transients at peak. Additionally the Ca II nebular emission features are too broad. Future work should investigate if these discrepancies can be resolved by considering full 3D models and exploring a range of CO+He WD binary configurations.

astro-ph.HE↗

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.

astro-ph.SR↗

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.

astro-ph.SR↗

arXiv2025

New gravitational-wave data support a bimodal black-hole mass distribution

Detailed stellar evolution and supernova models yield a bimodal black-hole mass distribution with a narrow peak around 10 solar masses from stars within a narrow range of progenitor properties and a second broader peak starting around 20 solar masses from very massive progenitors. This bimodal black-hole mass distribution leads to a characteristic distribution of chirp masses of merging binary black holes, with two main peaks arising from the merger of two black holes where both come either from the low- or the high-mass peak and a smaller peak in between from the mixed merger of a low-mass and a high-mass black hole. We carry out a population synthesis study of binary black hole formation and compare the results to the observed chirp masses of gravitational-wave events. We find that only the bimodal black-hole mass prescription is able to reproduce the structure of peaks and gaps in the observed chirp-mass distribution, which is not matched by predictions from other remnant mass prescriptions in the literature.

astro-ph.HE↗

arXiv2025

Explosions of pulsating red supergiants: a natural pathway for the diversity of Type II-P/L supernovae

Red supergiants (RSGs), which are progenitors of hydrogen-rich Type II supernovae (SNe), have been known to pulsate from both observations and theory. The pulsations can be present at core collapse and affect the resulting SN. However, SN light curve models of such RSGs commonly use hydrostatic progenitor models and ignore pulsations. Here, we model the final stages of a 15 solar-mass RSG and self-consistently follow the hydrodynamical evolution. We find the growth of large amplitude radial pulsations in the envelope. After a transient phase where the envelope restructures, the pulsations settle to a steady and periodic oscillation with a period of 817 days. We show that they are driven by the $κγ$-mechanism, which is an interplay between changing opacities and the release of recombination energy of hydrogen and helium. This leads to complex and non-coherent expansion and contraction in different parts of the envelope, which greatly affect the SN progenitor properties, including its location in the Hertzsprung-Russell diagram. We simulate SN explosions of this model at different pulsations phases. Explosions in the compressed state result in a flat light curve (Type II-P). In contrast, the SN light curve in the expanded state declines rapidly, reminiscent of a Type II-L SN. For cases in between, we find light curves with various decline rates. Features in the SN light curves are directly connected to features in the density profiles. These are in turn linked to the envelope ionization structure, which is the driving mechanism of the pulsations. We predict that some of the observed diversity in Type II SN light curves can be explained by RSG pulsations. For more massive RSGs, we expect stronger pulsations that might even lead to dynamical mass ejections of the envelope and to an increased diversity in SN light curves.

astro-ph.SR↗

arXiv2025

Explodability criteria for the neutrino-driven supernova mechanism

Massive stars undergoing iron core-collapse at the end of their evolution terminate their lives either in successful or failed supernovae (SNe). The physics of core-collapse supernovae (CCSNe) is complex, and their understanding requires computationally expensive simulations. Using these to predict CCSN outcomes over large, densely sampled parameter spaces of SN progenitors, as is needed e.g. for population synthesis studies, is thus not feasible. To remedy this situation, we present explodability criteria that allow us to predict the final fates of stars by evaluating stellar structure variables at the onset of core-collapse. The criteria are calibrated to predictions of a semi-analytical SN model, evaluated over a set of $\sim$~3,900 heterogeneous stellar progenitors (single, binary-stripped and accretor stars). Over these, the criteria achieve an accuracy of >99\% agreement with the semi-analytical model. The criteria are tested on 29 state-of-the-art 3D CCSN simulation outcomes from two different groups. Furthermore, we find that all explodability proxies needed for our pre-SN structure-based criteria have two distinct peaks and intervening valleys as a function of the carbon-oxygen (CO) core mass $M_\mathrm{CO}$, which coincide with failed and successful SNe, respectively. The CO core masses of explodability peaks shift systematically with metallicity, $Z$, and with timing of hydrogen-rich envelope removal by binary mass transfer. With these, we identify critical values in $M_\mathrm{CO}$ that define windows over which black holes form by direct collapse and formulate a CCSN recipe based on $M_\mathrm{CO}$ and $Z$, applicable for rapid binary population synthesis and other studies. Our explodability formalism is consistent with observations of Type~IIP, IIb/Ib and Ic supernova progenitors and partially addresses the missing Red Supergiant Problem by direct black hole formation.

astro-ph.SR↗

arXiv2025

Supernovae from stellar mergers and accretors of binary mass transfer: Implications for Type IIP, 1987A-like and interacting supernovae

As most massive stars are born in binary and other multiple-star systems, many are expected to exchange mass with a companion star or merge with it during their lives. This means that most supernovae (SNe) are from such binary products. Here, we focus on hydrogen-rich Type II SNe from accretors of binary mass transfer and stellar mergers. We compute various SN properties such as the explosion energies, nickel yields, and neutron star (NS) kick velocities, but also consider NS masses. We find tight correlations between these parameters and, e.g., the central specific entropy and core compactness. However, there is no obvious relation between these explosion properties and the evolutionary history of the pre-SN stars. We find linear relations between the nickel mass and the SN explosion energy and the NS remnant mass. We further group our models into progenitors of SNe IIP, SN 1987A-like and interacting SNe, predict their SN and SN-progenitor properties and compare to observations. Accretors of binary mass transfer and stellar mergers naturally produce SNe IIP with long plateau durations from progenitors with relatively small CO-cores but large envelope masses (c.f. SN 2015ba). Our models give rise to tight relations between the plateau luminosity and the nickel mass as well as the SN ejecta velocity as inferred observationally for SNe IIP. We speculate that cool/red supergiants at $\log\,L/L_\odot\,{\geq}\,5.5$ encounter enhanced mass loss due to envelope instabilities and could then explode in interacting SNe IIn. The rate of such SNe from our models seems compatible with observations. Some of our binary models explode as $10^6\,L_\odot$ blue supergiants that may have encountered enhanced and/or eruptive mass loss shortly before their SNe and could thus help understand interacting SNe such as SN 1961V and SN 2005gl but also superluminous Type II SNe such as SN 2010jl. [abridged]

astro-ph.HE↗

arXiv2025

Binarity at LOw Metallicity (BLOeM): Pipeline-Determined Physical Properties of OB Stars

We aim to determine the physical properties of OB stars from the multi-epoch VLT/FLAMES BLOeM spectroscopic survey of the Small Magellanic Cloud. We apply a pipeline designed to analyse large spectroscopic samples of OB stars to the co-added, initial 9 epochs of the BLOeM survey, utilising grids of synthetic model spectra computed with the stellar atmosphere code FASTWIND. 69 OB stars are excluded from the analysis owing to disk emission or significant contamination by secondaries in SB2 binaries. We determine physical properties of 778 OB stars, including Teff, log g, log L/Lsun and v_e sin i. There appears to be a bimodality in v_e sin i of single O stars, while v_e sin i distributions of OB stars are strikingly different for single (median 78 km/s) and binary (median 200 km/s) systems. Inferred temperatures are broadly in agreement with literature results for stars in common, plus results from a grid-based automization tool for a subset of O and early B stars, although uncertainties are larger for surface gravities. Rotational velocities are broadly in line with an independent tool applied to the same subset. We recover the anticipated lower mass cutoff at 8 Msun from the survey design using a Bayesian inference method coupled with SMC metallicity evolutionary models, with median masses of 12.6 Msun (19.8 Msun) for B-type (O-type) stars. Spectroscopic masses exceed evolutionary masses, albeit with large uncertainties in surface gravities. We also provide an updated catalogue of O stars in the SMC since half of the 159 BLOeM O stars are newly classified as O-type stars.

astro-ph.SR↗

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.

astro-ph.SR↗

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.

astro-ph.SR↗

arXiv2025

Asteroseismic predictions for a massive main-sequence merger product

The products of stellar mergers between two massive main-sequence (MS) stars appear as seemingly normal MS stars after a phase of thermal relaxation, if not for certain peculiarities. Since these peculiarities are not limited to the merger product's surface, we use asteroseismology to predict how the differences in the internal structure of a merger product and a genuine single star manifest via properties of non-radial stellar pulsations. We mapped the result of a 3D MHD stellar merger simulation between a 9 and an 8 solar-mass MS star to 1D and evolved it through the MS. We compare the predicted pressure (p) and gravity (g) modes for the merger product model with those predicted for a corresponding genuine single-star model. The p-mode frequencies are consistently lower for the merger product than for the genuine single star, and the differences between them are more than a thousand times larger than the current best observational uncertainties for measured mode frequencies of this kind. Even though g-mode period spacing differences vary in value and sign throughout the MS, they, too, are larger than the current best observational uncertainties for such long-period modes. This, combined with additional variability in the merger product's period spacing patterns, shows the potential of identifying merger products in future-forward modelling. We also attempt to replicate the merger product's structure using three widely applied 1D merger prescriptions and repeat the asteroseismic analysis. Although none of the 1D prescriptions reproduces the entire merger product's structure, we conclude that the prescription with shock heating shows the highest potential, provided that it can be calibrated on binary-evolution-driven 3D merger simulations. Our work should be expanded to encompass the various possible merger product structures predicted to exist in the Universe. (abridged)

astro-ph.SR↗

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.

astro-ph.SR↗

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.

astro-ph.SR↗

arXiv2024

Going from 3D common-envelope simulations to fast 1D simulations

One-dimensional (1D) methods for simulating the common-envelope (CE) phase offer advantages over three-dimensional (3D) simulations regarding their computational speed and feasibility. We present the 1D CE method from Bronner et al. (2024), including the results of the CE simulations of an asymptotic giant branch star donor. We further test this method in the massive star regime by computing the CE event of a red supergiant with a neutron-star mass and a black-hole mass companion. The 1D model can reproduce the orbital evolution and the envelope ejection from 3D simulations when choosing suitable values for the free parameters in the model. The best-fitting values differ from the expectations based on the low mass simulations, indicating that the free parameters depend on the structure of the giant star. The released recombination energy from hydrogen and helium helps to expand the envelope, similar to the low-mass CE simulations.

astro-ph.SR↗

arXiv2024

From spherical stars to disk-like structures: 3D common-envelope evolution of massive binaries beyond inspiral

Three-dimensional simulations usually fail to cover the entire dynamical common-envelope phase of gravitational wave progenitor systems due to the vast range of spatial and temporal scales involved. We investigated the common-envelope interactions of a $10\,M_\odot$ red supergiant primary star with a black hole and a neutron star companion, respectively, until full envelope ejection (${\gtrsim}\,97 \,\mathrm{\%}$ of the envelope mass). We find that the dynamical plunge-in of the systems determines largely the orbital separations of the core binary system, while the envelope ejection by recombination acts only at later stages of the evolution and fails to harden the core binaries down to orbital frequencies where they qualify as progenitors of gravitational-wave-emitting double-compact object mergers. As opposed to the conventional picture of a spherically symmetric envelope ejection, our simulations show a new mechanism: The rapid plunge-in of the companion transforms the spherical morphology of the giant primary star into a disk-like structure. During this process, magnetic fields are amplified, and the subsequent transport of material through the disk around the core binary system drives a fast jet-like outflow in the polar directions. While most of the envelope material is lost through a recombination-driven wind from the outer edge of the disk, about $7\,\mathrm{\%}$ of the envelope leaves the system via the magnetically driven outflows. We further explored the potential evolutionary pathways of the post-common-envelope systems given the expected remaining lifetime of the primary core ($2.97\,M_\odot$) until core collapse ($6{\times}10^{4}\,\mathrm{yr}$), most likely forming a neutron star. We find that the interaction of the core binary system with the circumbinary disk increases the likelihood of giving rise to a double-neutron star merger. (abridged)

astro-ph.SR↗

arXiv2024

It's written in the massive stars: The role of stellar physics in the formation of black holes

In the age of gravitational-wave (GW) sources and newly discovered local black holes (BH) and neutron stars (NS), understanding the fate of stars is a key question. Not every massive star is expected to successfully explode as a supernova and leave behind a NS; some stars form BHs. The remnant depends on explosion physics but also on the final core structure, often summarized by the compactness parameter or iron core mass, where high values have been linked to BH formation. Several groups have reported similar patterns in these parameters as a function of mass, characterized by a prominent compactness peak followed by another peak at higher masses, pointing to a common underlying physical mechanism. Here, we investigate its origin by computing single-star models from 17 to 50 solar masses with MESA. The first and second compactness increases originate from core carbon and neon burning, respectively, becoming neutrino dominated, which enhances the core contraction and ultimately increases the iron-core mass and compactness. An early core neon ignition during carbon burning, and an early silicon ignition during oxygen burning help counter the core contraction and decrease the final iron core mass and compactness. Shell mergers between C/Ne and O-burning shells further decrease the compactness and we show that they are due to an enhanced entropy production in these layers. We find that the final structure of massive stars is not random but already written in their cores at core helium exhaustion. The same mechanisms determine the final structure of any star in this core mass range, including binary products, though binary interactions systematical shift the range of expected BH formation. Finally, we discuss the role of stellar physics uncertainties and how to apply these findings to studies of GW sources. [Abridged]

astro-ph.SR↗

arXiv2024

Towards a self-consistent model of the convective core boundary in upper main sequence stars. Part I: 2.5D and 3D simulations

There is strong observational evidence that the convective cores of intermediate-mass and massive main sequence stars are substantially larger than those predicted by standard stellar-evolution models. However, it is unclear what physical processes cause this phenomenon or how to predict the extent and stratification of stellar convective boundary layers. Convective penetration is a thermal-timescale process that is likely to be particularly relevant during the slow evolution on the main sequence. We use our low-Mach-number Seven-League Hydro code to study this process in 2.5D and 3D geometries. Starting with a chemically homogeneous model of a $15\,\mathrm{M}_\odot$ zero-age main sequence star, we construct a series of simulations with the luminosity increased and opacity decreased by the same factor, ranging from $10^3$ to $10^6$. After reaching thermal equilibrium, all of our models show a clear penetration layer; its thickness becomes statistically constant in time and it is shown to converge upon grid refinement. The penetration layer becomes nearly adiabatic with a steep transition to a radiative stratification in simulations at the lower end of our luminosity range. This structure corresponds to the adiabatic `step overshoot' model often employed in stellar-evolution calculations. The simulations with the highest and lowest luminosity differ by less than a factor of two in the penetration distance. The high computational cost of 3D simulations makes our current 3D data set rather sparse. Depending on how we extrapolate the 3D data to the actual luminosity of the initial stellar model, we obtain penetration distances ranging from $0.09$ to $0.44$ pressure scale heights, which is broadly compatible with observations.

astro-ph.SR↗