arXiv subjects

Georges Meynet

Publications and source records attributed to Georges Meynet.

At least 19 recordsLinked to original sources

arXiv2026

Evolution of low-mass He stars and implications for electron-capture supernova formation in close binaries

The evolution of low-mass helium (He) stars ($\sim2.5$--$5\,M_\odot$) with neutron-star (NS) companions in close binaries has been extensively studied, but the combined effects of rotation and tidal interaction remain poorly understood. We investigate how rotation, mass transfer, and tidal interactions affect the evolution of low-mass He stars, the formation of electron-capture supernovae (ECSNe), and the properties of the resulting NSs. Using detailed stellar and binary evolution calculations that include mass loss, differential rotation, and tidal interactions, we systematically explore the initial binary parameter space leading to ECSNe. We find that rotation has only a modest effect on the evolution of low-mass He stars. ECSNe occur within a narrow initial He-star mass range of $2.42$--$2.67\,M_\odot$ at solar metallicity ($Z_\odot$) and $2.37$--$2.62\,M_\odot$ at $0.01\,Z_\odot$. The resulting NSs have spin periods of $7.7$--$83.8\,\mathrm{ms}$, magnetic fields of order $10^{12}\,\mathrm{G}$, and rotational energies of $2.6\times10^{48}$--$2.5\times10^{50}\,\mathrm{erg}$, although these values would be substantially reduced if efficient angular-momentum transport mechanisms, such as the Spruit--Tayler dynamo, were included. We further show that the evolutionary outcome is highly sensitive to the initial orbital period, with shorter-period systems undergoing Roche-lobe overflow at earlier evolutionary stages and experiencing stronger binary interactions. Finally, comparison with Galactic double NS systems indicates that most observed binaries can be reproduced in the eccentricity--orbital-period plane by adopting relatively large natal kick velocities.

astro-ph.HE↗

arXiv2026

On the origin of the rotation of massive stars

We explore the origin of the rotation rates of massive stars. Contrary to their low-mass siblings, most massive stars do not have detectable magnetic fields, so that star-disk interaction models used for the formation of rotating low-mass stars do not apply. We investigate whether the magnetic fields of protostellar jets present in the parent molecular cloud prevent the protostar from reaching the critical angular velocity. Starting from the gravitational collapse of a molecular cloud, we run two two-dimensional radiation-gravito-magnetohydroynamical simulations to study the formation of an accretion disk and the launching of magnetically-driven protostellar outflows (of particular interest is the formation of a magnetocentrifugal jet originating from the protostar and inner disk). We then study the angular momentum transfer from the disk and jet onto the protostar. Finally, we compute one-dimensional stellar evolution models of the pre-main sequence including our results from the disk-jet simulations and follow the angular momentum redistribution within the structure of the protostar. We find that the angular momentum transported outwards by the magnetically-driven protostellar outflows is sufficient for keeping the protostar below the critical speed at all times. Moreover, we are able to link the strength of the jet, and thus the rotation rate at the end of the accretion epoch, to the initial conditions for star formation. Our results show that the jet strength produces a variety of stellar rotation rates, suggesting that protostellar jets fix the rotation rate of massive stars.

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arXiv2026

Probing the first generations of massive stars through fluorine in CEMP-no stars

We investigate whether the first discovered fluorine-rich CEMP-no star, CS 29498$-$043, can be explained by a very metal-poor rotating massive star. We consider single rotating stellar models of 20 $M_{\odot}$ at a metallicity of $Z = 10^{-5}$, exploring initial rotation rates from $\upsilon_{\rm ini}/\upsilon_{\rm crit} = 0$ to $0.7$ in increments of $0.1$ ($0<\upsilon_{\rm ini}<644$ km s$^{-1}$). Rotational mixing enhances the production of light elements in the H--He layers, including fluorine. The ejected material can be nitrogen-rich without being fluorine-rich, whereas fluorine-rich ejecta are always predicted to be nitrogen-rich. The model providing the best fit to the abundances of CS 29498$-$043 is the $\upsilon_{\rm ini}/\upsilon_{\rm crit} = 0.6$ model ($\upsilon_{\rm ini} = 547$ km s$^{-1}$), which reproduces C, N, O, Na, Mg, and Al within the observational uncertainties. However, the predicted [F/Fe] $=2.8$ exceeds the observed value of [F/Fe] $=2.0 \pm 0.4$. By simultaneously varying the $^{15}$N($\alpha,\gamma$)$^{19}$F and $^{19}$F($\alpha,p$)$^{22}$Ne reaction rates within their acceptable ranges, the [F/Fe] ratio in the $\upsilon_{\rm ini}/\upsilon_{\rm crit} = 0.6$ model can be reduced to 2.2, providing a plausible solution to the abundance pattern of CS 29498$-$043. Our results support the hypothesis that fluorine-rich CEMP-no stars may originate from material enriched by a single, metal-poor, rotating massive star. A potential observational test of this scenario may be to check whether the nitrogen and fluorine abundances observed at the surface of CEMP-no stars are correlated.

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arXiv2026

Chemical evolution of close massive binaries -- tidally-enhanced or tidally-suppressed mixing?

One of the largest source of uncertainties in the predictions of stellar models comes from the internal transport mechanisms. In close massive binaries, previous theoretical studies suggest that tides systematically boost chemical mixing. However, observations do not reveal any clear period-nitrogen enrichment trend, challenging these predictions. In addition, comprehensive examinations of the interplay between tidal interactions, angular momentum and chemicals transport have so far been scarce. We investigate the interplay between tidal interactions and rotational mixing, and the impact of the angular moment transport (AMT) assumptions. We examine whether tidal interactions enhance or suppress chemical mixing by computing grids of genec binary models with various AMT treatments. In order to independently assess the role of tidal interactions, we systematically compute model variations of single stars with identical initial conditions. Our investigations reveal that tides can either enhance or suppress mixing relative to single-star models, and that the outcome is highly sensitive to the AMT assumptions. We identify a key contrast between the two types of computed models: in close systems subject to tides, magnetic models predict that the mixing efficiency is mostly determined by the orbital configuration, whereas in hydrodynamic models it also depends on the assumed initial velocity. As a result, hydro models may display non-monotonic period-enrichment trends, or even period-enrichment correlations. These results highlight the importance of the AMT assumptions in modeling binaries with tidal interactions. The sensitivity of the predictions of hydro models to initial conditions extends the size of the period-enrichment parameter space they cover, allowing them to accommodate for peculiar observed systems, i.e., with mild enrichment at short periods, or high enrichment at longer periods.

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arXiv2025

Diagnosing the Properties and Evolutionary Fates of Black Hole and Wolf-Rayet X-ray Binaries as Potential Gravitational Wave Sources for the LIGO-Virgo-KAGRA Network

IC 10 X-1, NGC 300 X-1, and Cyg X-3 constitute a unique class of X-ray binaries in which a stellar-mass black hole (BH) accretes material from a Wolf-Rayet (WR). These systems are particularly intriguing because of their short orbital periods, which make them promising progenitors of gravitational-wave (GW) sources detectable by the LIGO-Virgo-KAGRA (LVK) network. Adopting a revised accretion efficiency within the standard Bondi-Hoyle-Lyttleton framework, we perform detailed binary evolution calculations using \texttt{MESA} to characterize their properties at different evolutionary stages and to assess their ultimate fates as potential LVK-detectable GW sources. By applying additional constraints from the observed properties of IC 10 X-1 and NGC 300 X-1, we find that the upper limits on the BH masses in these systems ($M_{\rm BH} \lesssim 25\, M_\odot$ for IC 10 X-1 and $M_{\rm BH} \lesssim 15\, M_\odot$ for NGC 300 X-1) are significantly lower than previous estimates. Both systems are expected to form binary black holes (BBHs) that will merge within a Hubble time, except in the case where the BH in NGC 300 X-1 has a mass of $9\,M_\odot$, corresponding to the lower limit inferred in a previous study using the continuum-fitting method with a relativistic slim-disc model. For Cyg X-3, we find that the BH spin magnitude is constrained to be $\lesssim$ 0.6. Moreover, the WR star in Cyg X-3 is likely to form a lower-mass-gap BH, and the resulting BBH system is also expected to merge within a Hubble time.

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arXiv2025

Reassessing the Spin of Second-born Black Holes in Coalescing Binary Black Holes and Its Connection to the chi_eff-q Correlation

The mass ratio q and effective inspiral spin chi_eff of binary black hole (BBH) mergers in GWTC-4.0 show a weaker anti-correlation than in GWTC-3.0, motivating investigation of its physical origin. Within the isolated binary evolution framework, we adopt a recently proposed He-star wind prescription to study the spin of the second-born BH and its impact on the q-chi_eff relation. Using \texttt{MESA}, including the updated He-star wind, internal differential rotation, and tidal interactions, we examine how initial conditions and key processes determine the BH spin. We also perform rapid population synthesis with \texttt{COMPAS} to predict the population-level q-chi_eff correlation. The updated wind prescription is significantly weaker than the standard Dutch scheme, particularly at subsolar metallicity. Detailed binary models of He stars with BH companions show that the resulting BH spin is largely insensitive to the He star's evolutionary stage at the onset of tidal interaction and to the companion mass. Instead, wind mass loss dominates: more massive He-star progenitors produce lower-spinning BHs. Initial stellar rotation has only a minor effect, especially under strong tidal coupling. We provide a fitting formula for the spin of the second-born BH. Combining this formula with rapid population synthesis under default assumptions, we find that 85.8% of BBHs formed via stable mass transfer undergo mass-ratio reversal, compared to only 2.8% in the common-envelope channel. Notably, no correlation between q and chi_eff is found in either channel. Future work will explore alternative physical prescriptions and compare our predictions with BBH mergers reported by the LIGO-Virgo-KAGRA Collaboration.

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arXiv2025

pySTARBURST99: The Next Generation of STARBURST99

STARBURST99 is a population synthesis code tailored to predict the integrated properties or observational characteristics of star-forming galaxies. Here we present an update to STARBURST99 where we port the code to python, include new evolutionary tracks both rotating and non-rotating at a range of low metallicity environments. We complement these tracks with a corresponding grid of new synthetic SEDs. Additionally we include both evolutionary and spectral models of stars up to 300-500Msol. Synthesis models made with the python version of the code and new input stellar models are labelled pySTARBURST99. We make new predictions for many properties, such as ionising flux, SED, bolometric luminosity, wind power, hydrogen line equivalent widths and the UV beta-slope. These properties are all assessed over wider coverage in metallicity, mass and resolution than in previous versions of STARBURST99. A notable finding from these updates is an increase in H I ionising flux of 0.3 dex in the first 2Myr when increasing the upper mass limit from 120 to 300Msol. Changing metallicity has little impact on H I in the first 2Myr (range of 0.015 dex from Z = 0.02 to 0.0) but lower metallicities have higher H I by 1 dex (comparing Z = 0.02 to 0.0004) at later times, with Z = 0.0 having even higher H I at later times. Rotating models have significantly higher H I than their equivalent non-rotating models at any time after 2Myr. Similar trends are found for He I and He II, bolometric luminosity and wind momentum, with more complex relations found for hydrogen line equivalent widths and UV beta-slopes.

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arXiv2025

Fluorine production in He-burning regions of massive stars during cosmic history

The origin of fluorine is still a debated question. AGB stars synthesise this element and likely contribute significantly to its synthesis in the present-day Universe. However, it is not clear whether other sources contribute, especially in the early Universe. We discuss variations of the surface abundances of fluorine coming from our massive star models and compare them with available present-day observations. We compute the contribution of massive stars in producing 19F over metallicities covering the whole cosmic history. We used models in the mass range of 9Msol < Mini < 300Msol at metallicities from Pop III up to super-solar while accounting for the required nuclear network to follow the evolution of 19F during the core H- and He-burning phases. Results from models with and without rotational mixing are presented. We find that rotating models predict a slight depletion of fluorine at their surface at the end of the MS phase. In more advanced evolutionary phases, only models with an initial mass larger than 25Msol at metallicities Z > 0.014 show phases where the abundance of fluorine is enhanced. This occurs when the star is a WR star of the WC type. WC stars can show surface abundances of fluorine ten times larger than their initial abundance. However, we obtained that the winds of massive stars at metallicities larger than Z=0.006 do not significantly contribute to fluorine production, confirming previous findings. In contrast, very metal-poor rapidly rotating massive star models may be important sources of fluorine through the mass expelled at the time of their SN explosion. Observations of WC stars at solar or super-solar metallicities may provide very interesting indications on the nuclear pathways that lead to fluorine production in massive stars. The possibility of observing fluorine-rich CEMPs is also a way to put constrains in present models at very low metallicities.

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arXiv2025

Effects of chemically homogeneous evolution of the first stars on the 21-cm signal and reionization

The first generation of stars, known as Population III (Pop III), played a crucial role in the early Universe through their unique formation environment and metal-free composition. These stars can undergo chemically homogeneous evolution (CHE) due to fast rotation, becoming more compact and hotter/bluer than their (commonly assumed) non-rotating counterparts. In this study, we investigate the impact of Pop III CHE on the 21-cm signal and cosmic reionization under various assumptions on Pop III star formation, such as their formation efficiency, initial mass function, and transition to metal-enriched star formation. We combine stellar spectra computed by detailed atmosphere models with semi-numerical simulations of Cosmic Dawn and the Epoch of Reionization ($z\sim 6-30$). The key effect of CHE arises from the boosted ionizing power of Pop III stars, which reduces the Pop III stellar mass density required to reproduce the observed Thomson scattering optical depth by a factor of $\sim 2$. Meanwhile, the maximum 21-cm global absorption signal is shallower by up to $\sim 15$ mK (11%), partly due to the reduced Lyman-band emission from CHE, and the large-scale ($k\sim 0.2\ \rm cMpc^{-1}$) power drops by a factor of a few at $z\gtrsim 25$. In general, the effects of CHE can be comparable to those of Pop III star formation parameters, showing an interesting interplay with distinct features in different epochs. These results highlight the importance of metal-free/poor stellar evolution in understanding the early Universe and suggest that future studies should consider joint constraints on the physics of star/galaxy formation and stellar evolution.

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arXiv2025

Strong nebular HeII emission induced by He$^+$ ionizing photons escaping through the clumpy winds of massive stars

The origin of nebular HeII-emission in both local and high-redshift galaxies remains an unsolved problem. Various theories have been proposed to explain it, including HeII-ionization by high mass X-ray binaries, ultra-luminous X-ray sources, or "stripped" He stars, shock ionization, and hidden AGNs. All these theories have shortcomings, however, leaving the cause of nebular HeII emission unclear. We investigate the hypothesis that the photons responsible for driving nebular HeII emissions are produced by the evolution of single massive stars and/or WR stars. We combine models of stellar evolution with population synthesis and nebular models to identify the most favorable scenarios for producing nebular HeII via this channel. We find that, if WR winds are clumpy enough to become close to optically thin, stellar populations with a wide range of metallicities and rotation rates can produce HeII ionizing photons at rates sufficient to explain the observed nebular $I(HeII)/I(\mathrm{H}\beta)$ ratio $\sim 0.004-0.07$ found in HeII-emitting galaxies. Metal-poor, rapidly rotating stellar populations ($[\mathrm{Fe}/\mathrm{H}]=-2.0$, $v/v_\mathrm{crit}=0.4$) also reach these levels of HeII production even for partially clumpy winds. These scenarios also yield HeII, H$\beta$, and "Blue-Bump" line equivalent widths comparable to those observed in HeII emitters. Only for laminar, non-clumpy winds, do we fail to find combinations of metallicity and stellar rotation rate that yield $I(HeII)/I(\mathrm{H}\beta)$ values as high as those observed in HeII-emitters. Contrary to previous findings, we conclude that single WR stars can be a strong source for nebular HeII emission if their winds are sufficiently clumpy allowing significant escape of hard ionizing photons.

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arXiv2024

Signatures of Rapidly Rotating Stars with Chemically Homogeneous Evolution in the First Galaxies

The James Webb Space Telescope (JWST) has revealed an unexpectedly high abundance of UV luminous galaxies at redshifts $z\gtrsim 10$, challenging `standard' galaxy formation models. This study investigates the role of rapidly rotating (massive) stars undergoing chemically homogeneous evolution (CHE) in reconciling this potential tension. These stars are more compact, hotter, and exhibit enhanced UV emission. We find that the rest-frame UV luminosity of star-forming galaxies can be significantly enhanced by a factor of $\sim 3-6$ when CHE stars above a minimum initial mass of $m_{\star,\min}^{\rm CHE}\sim 2-10\ \rm M_\odot$ account for more than half of the total stellar mass following a Salpeter initial mass function. As a result, the UV luminosity functions observed at $z\sim 12-16$ can be reproduced with less extreme values of star formation efficiency and UV luminosity stochastic variability. Our results highlight the potential of CHE in explaining the UV-bright galaxy populations detected by JWST and call for future work to explore the broader astrophysical implications of CHE and its associated phenomena in the early universe, such as gamma-ray bursts, compact object binaries, and metal enrichment.

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arXiv2024

Chemical Evolution of R-process Elements in Stars (CERES) II. The impact of stellar evolution and rotation on light and heavy elements

Context. Carbon, nitrogen, and oxygen are the most abundant elements throughout the universe, after hydrogen and helium. Studying these elements in low-metallicity stars can provide crucial information on the chemical composition in the early Galaxy and possible internal mixing processes that can alter the surface composition of the stars. Aims. This work aims to investigate the chemical abundance patterns for CNO elements and Li in a homogeneously analyzed sample of 52 metal-poor halo giant stars. Methods. We used high-resolution spectra with a high signal-to-noise ratio (S/N) to carry out a spectral synthesis to derive detailed C, N, O, and Li abundances for a sample of stars with metallicities in the range of -3.58 <= [Fe/H] <= -1.79 dex. Our study was based on the assumption of one-dimensional (1D) local thermodynamic equilibrium (LTE) atmospheres. Results. Based on carbon and nitrogen abundances, we investigated the deep mixing taking place within stars along the red giant branch (RGB). The individual abundances of carbon decrease towards the upper RGB while nitrogen shows an increasing trend, indicating that carbon has been converted into nitrogen. No signatures of ON-cycle processed material were found for the stars in our sample. We computed a set of galactic chemical evolution (GCE) models, implementing different sets of massive star yields, both with and without including the effects of stellar rotation on nucleosynthesis. We confirm that stellar rotation is necessary to explain the highest [N/Fe] and [N/O] ratios observed in unmixed halo stars. The predicted level of N enhancement varies sensibly in dependence of the specific set of yields that are adopted. For stars with stellar parameters similar to those of our sample, heavy elements such as Sr, Y, and Zr appear to have unchanged abundances despite the stellar evolution mixing processes.

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arXiv2024

Origin of Black Hole Spin in Lower-Mass-Gap Black Hole-Neutron Star Binaries

During the fourth observing run, the LIGO-Virgo-KAGRA Collaboration reported the detection of a coalescing compact binary (GW230529$_{-}$181500) with component masses estimated at $2.5-4.5\, M_\odot$ and $1.2-2.0\, M_\odot$ with 90\% credibility. Given the current constraints on the maximum neutron star (NS) mass, this event is most likely a lower-mass-gap (LMG) black hole-neutron star (BHNS) binary. The spin magnitude of the BH, especially when aligned with the orbital angular momentum, is critical in determining whether the NS is tidally disrupted. An LMG BHNS merger with a rapidly spinning BH is an ideal candidate for producing electromagnetic counterparts. However, no such signals have been detected. In this study, we employ a detailed binary evolution model, incorporating new dynamical tide implementations, to explore the origin of BH spin in an LMG BHNS binary. If the NS forms first, the BH progenitor (He-rich star) must begin in orbit shorter than 0.35 days to spin up efficiently, potentially achieving a spin magnitude of $\chi_{\rm BH} > 0.3$. Alternatively, if a non-spinning BH (e.g., $M_{\rm BH} = 3.6\, M_\odot$) forms first, it can accrete up to $\sim 0.2\, M_\odot$ via Case BA mass transfer (MT), reaching a spin magnitude of $\chi_{\rm BH} \sim 0.18$ under Eddington-limited accretion. With a higher Eddington accretion limit (i.e., 10.0 $\Dot{M}_{\rm Edd}$), the BH can attain a significantly higher spin magnitude of $\chi_{\rm BH} \sim\,0.65$ by accreting approximately $1.0\, M_\odot$ during Case BA MT phase.

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arXiv2024

Stable Case BB/BC Mass Transfer to Form GW190425-like Massive Binary Neutron Star Mergers

On April 25th, 2019, the LIGO-Virgo Collaboration discovered a Gravitational-wave (GW) signal from a binary neutron star (BNS) merger, i.e., GW190425. Due to the inferred large total mass, the origin of GW190425 remains unclear. We perform detailed stellar structure and binary evolution calculations that take into account mass-loss, internal differential rotation, and tidal interactions between a He-rich star and a NS companion. We explore the parameter space of the initial binary properties, including initial NS and He-rich masses and initial orbital period. We find that the immediate post-common-envelope progenitor system, consisting of a primary $\sim2.0\,M_\odot$ ($\sim1.7\,M_\odot$) NS and a secondary He-rich star with an initial mass of $\sim3.0-5.5\,M_\odot$ ($\sim5.5-6.0\,M_\odot$) in a close binary with an initial period of $\sim0.08-0.5\,{\rm{days}}$ ($\sim 0.08-0.4\,{\rm{days}}$), that experiences stable Case BB/BC mass transfer (MT) during binary evolution, can reproduce the formation of GW190425-like BNS events. Our studies reveal that the secondary He-rich star of the GW190425's progenitor before its core collapse can be efficiently spun up through tidal interaction, finally remaining as a NS with rotational energy even reaching $\sim10^{52}\,{\rm{erg}}$, which is always much higher than the neutrino-driven energy of the supernova (SN) explosion. If the newborn secondary NS is a magnetar, we expect that GW190425 can be the remnant of a magnetar-driven SN, e.g., a magnetar-driven ultra-stripped SN, a superluminous SN, or a broad-line Type Ic SN. Our results show that GW190425 could be formed through the isolated binary evolution, which involves a stable Case BB/BC MT just after the common envelope phase. On top of that, we show the He-rich star can be tidally spun up, potentially forming a spinning magnetized NS (magnetar) during the second SN explosion.

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arXiv2024

Evolving massive stars to core collapse with GENEC: Extension of equation of state, opacities and effective nuclear network

Stars with initial mass above roughly 8 solar masses will evolve to form a core made of iron group elements at which point no further exothermic nuclear reactions between charged nuclei may prevent the core collapse. Electron captures, neutrino losses, and the photo-disintegration of heavy nuclei trigger the collapse of these stars. Models at the brink of core collapse are produced using stellar evolution codes and these pre-collapse models may be used in the study of the subsequent dynamical evolution (including their explosion as supernovae and the formation of compact remnants such as neutron stars or black holes). We upgrade the physical ingredients employed by the GENeva stellar Evolution Code, GENEC, so that it may cover the regime of high temperatures and high densities required to produce progenitors of core-collapse. We have improved GENEC in three directions, equation of state, the nuclear reaction network and the radiative and conductive opacities adapted for the computation of the advanced phases of evolution. We produce a small grid of pre-supernova models of stars with zero-age main sequence masses of 15, 20 and 25 solar masses at solar and less than half solar metallicities. The results are compared with analogous models produced with the MESA code. The global properties of our new models, particularly of their inner cores, are comparable to models computed with MESA and pre-existing progenitors in the literature. Between codes the exact shell structure varies impacting explosion predictions. Using GENEC with state-of-the-art physics, we have produced massive stellar progenitors prior to collapse. These progenitors are suitable for follow-up studies, including the dynamical collapse and supernova phases. Larger grids of supernova progenitors are now feasible, with potential for further dynamical evolution.

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arXiv2024

Grids of stellar models with rotation VIII: Models from 1.7 to 500 $M_\odot$ at metallicity $Z = 10^{-5}$

Grids of stellar evolution models with rotation using the Geneva stellar evolution code (Genec) have been published for a wide range of metallicities. We introduce the last remaining grid of Genec models, with a metallicity of $Z=10^{-5}$. We study the impact of this extremely metal-poor initial composition on various aspects of stellar evolution, and compare it to the results from previous grids at other metallicities. We provide electronic tables that can be used to interpolate between stellar evolution tracks and for population synthesis. Using the same physics as in the previous papers of this series, we computed a grid of stellar evolution models with Genec spanning masses between 1.7 and 500 $M_\odot$, with and without rotation, at a metallicity of $Z=10^{-5}$. Due to the extremely low metallicity of the models, mass-loss processes are negligible for all except the most massive stars. For most properties (such as evolutionary tracks in the Hertzsprung-Russell diagram, lifetimes, and final fates), the present models fit neatly between those previously computed at surrounding metallicities. However, specific to this metallicity is the very large production of primary nitrogen in moderately rotating stars, which is linked to the interplay between the hydrogen- and helium-burning regions. The stars in the present grid are interesting candidates as sources of nitrogen-enrichment in the early Universe. Indeed, they may have formed very early on from material previously enriched by the massive short-lived Population III stars, and as such constitute a very important piece in the puzzle that is the history of the Universe.

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arXiv2024

The Evolution of Accreting Population III Stars at 10$^{-6}$-10$^3$ M$_\odot$/yr

The first stars formed over five orders of magnitude in mass by accretion in primordial dark matter halos. We study the evolution of massive, very massive and supermassive primordial (Pop III) stars over nine orders of magnitude in accretion rate. We use the stellar evolution code GENEC to evolve accreting Pop III stars from 10$^{-6}$ - 10$^3$ M$_\odot$/yr and study how these rates determine final masses. The stars are evolved until either the end of central Si burning or until they encounter the general relativistic instability (GRI). We also examine how metallicity affects the evolution of the stars. At rates below $2.5 x 10^{-5}$ M$_\odot$/yr the final mass of the star falls below that required for pair-instability supernovae. The minimum rate required to produce black holes with masses above 250 M$_\odot$ is $5 x 10^{-5}$ M$_\odot$/yr, well within the range of infall rates found in numerical simulations of halos that cool via H$_2$, $10^{-3}$ M$_\odot$/yr. At rates of $5 x 10^{-5}$ M$_\odot$/yr to $4 x 10^{-2}$ \Ms\ yr$^{-1}$, like those expected for halos cooling by both H$_2$ and Ly-alpha, the star collapses after Si burning. At higher accretion rates the GRI triggers the collapse of the star during central H burning. Stars that grow at above these rates are cool red hypergiants with effective temperatures $log(T_{\text{eff}}) = 3.8$ and luminosities that can reach 10$^{10.5}$ L$_\odot$. At accretion rates of 100 - 1000 M$_\odot$/yr the gas encounters the general relativistic instability prior to the onset of central hydrogen burning and collapses to a black hole with a mass of 10$^6$ M$_\odot$ without ever having become a star. We reveal for the first time the critical transition rate in accretion above which catastrophic baryon collapse, like that which can occur during galaxy collisions in the high-redshift Universe, produces supermassive black holes via dark collapse.

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arXiv2024

The First Billion Years, According to JWST

With stunning clarity, JWST has revealed the Universe's first billion years. The scientific community is analyzing a wealth of JWST imaging and spectroscopic data from that era, and is in the process of rewriting the astronomy textbooks. Here, 1.5 years into the JWST science mission, we provide a snapshot of the great progress made towards understanding the initial chapters of our cosmic history. We highlight discoveries and breakthroughs, topics and issues that are not yet understood, and questions that will be addressed in the coming years, as JWST continues its revolutionary observations of the Early Universe. While this compendium is written by a small number of authors, invited to ISSI Bern in March 2024 as part of the 2024 ISSI Breakthrough Workshop, we acknowledge the work of a large community that is advancing our collective understanding of the evolution of the Early Universe.

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