Ebraheem Farag

The understanding of the mechanism(s) by which massive stars collapse and possibly explode is rapidly maturing. Uncertainties in the structure of the stellar core at the onset of collapse are often dominant in numerical simulations, and a limited number of progenitor grids are available. This is especially true for explosions where rotation and magnetic fields play a significant or primary role. We present a grid of 113 single-star models with initial masses M _ZAMS = 30–90 M _⊙ and initial rigid rotation ω _ZAMS = 0.5–0.99 ω _crit computed at Z = 0.001 with the open-source stellar evolution code MESA . We adopt a 128-isotope nuclear reaction network capable of following the weak reactions deleptonizing the core during and after silicon core burning. By construction, these models experience rotationally induced chemically homogeneous evolution, and reach the onset of collapse ( v _infall ≲ −300 km s ⁻¹ ) with large and structured amounts of angular momentum, possibly sufficient to form accretion disks on a protocompact object. Therefore, these progenitor structures provide a homogeneous set of models with updated input physics and improved algorithmic accuracy to understand stellar explosions of (some types of) stripped-envelope supernovae, possibly jetted and/or broad-lined, collapsars or magnetar-powered, and/or long gamma-ray bursts.

Fast and accurate stellar evolution emulators—surrogate models that approximate expensive simulation outputs with machine learning (ML)—are powerful tools for modern stellar characterization, hierarchical inference, and population synthesis. We analyze the grid density required for reliable emulation by training ML algorithms on main-sequence models with masses M ∈ [0.7, 1.2] M _⊙ . This range is challenging to emulate due to rapidly varying evolutionary behavior caused by the radiative-to-convective core transition, as well as the requirement to match the part-per-thousand seismic precision that has been delivered for such stars from the NASA Kepler mission. Generating grids from analytical models, as well as MESA , YREC , MIST , and ASTEC , we compare linear interpolation, k-nearest neighbors, random forests, and neural networks (NNs) in interpolating the stellar observables T _eff , L , Δ ν , and ν max . While NNs outperform other methods, sparse grids induce localized failures in the core-transition region, resulting in unstable derivatives, ensemble disagreement, and fragmented posterior distributions during inference. Performance gains from denser grids are nonuniform, suggesting that adaptive grid generation should be favored over uniform refinement. Finally, we show that NN ensembles allow for localized uncertainty propagation, more accurately reflecting emulator reliability across parameter space than global uncertainty estimates. As we consider only the 2D case of varying only stellar mass and age along the main sequence, these results represent a lower bound on the challenge in emulating stellar evolution simulations for precision asteroseismology.

We extend the time-dependent convection (TDC) treatment in MESA by introducing eddy-viscous dissipation. This software change brings MESA-TDC into closer alignment with the radial stellar pulsation (RSP) framework of the MESA-RSP module. We demonstrate that the inclusion of the eddy viscosity in hydrodynamic stellar models remains stable on evolutionary timescales. We then present the first self-consistent integration of large-amplitude, nonlinear classical Cepheid pulsations directly within a MESA-star evolutionary run, demonstrating that the TDC formalism implemented in MESA-star and the MESA-RSP module are physically identical. Starting from a 6 M _⊙ blue-loop stellar evolution model, we demonstrate evolving the entire stellar model through pulsations as well as pausing the evolution, excising the core, and remeshing the envelope to match the grid used by MESA-RSP . We compare the pulsation properties (e.g., period, light and radius curves, and growth rate) with a matched MESA-RSP run, and find reasonable agreement between the two modules. This unified approach eliminates the reliance on separate postprocessing workflows and enables fully coupled evolution–pulsation simulations. This approach enables future studies of stellar pulsations with the inclusion of composition gradients, mass loss, or rotation. It also enables future studies of the ϵ mechanism as well as providing a physical source of viscosity for other science cases explored using MESA ’s hydrodynamics solver. We have integrated these modifications into the MESA-star module, enabling open-source use by the community.

Low-mass stars from the first epoch of star formation may still persist in the Milky Way and its satellite dwarf galaxies today; however, their detection is confounded by surface pollution from interstellar accretion and internal mixing, which obscure their primordial composition and blur their distinction from second-generation stars. Asteroseismology offers a probe of the internal structure and evolutionary state of stars, and hence may aid in the search for primordial stars. In this second paper of the series, we present the first nonradial adiabatic pulsation analysis of low-mass, metal-free stellar models. We use a 0.85 M _⊙ red giant as a case study and compare its seismic signatures with those of higher-metallicity models. At the same central hydrogen fractions, Population III (Pop III) main-sequence models display systematically higher r ₀₂ ≡ δν ₀₂ /Δ ν ratio and lower Δ ν than metal-enriched analogues, a direct consequence of their larger internal sound speeds and steeper core-envelope stratification. To interpret the structural dependence during giant evolution, we introduce a composite asteroseismic diagnostic, ψ ≡ Δ ν /ΔΠ ₁ , which traces how metallicity influences the balance between acoustic and buoyancy cavities through its imprint on opacity, core contraction, and mean molecular-weight gradients. Pop III models occupy a distinct locus in the ψ − ΔΠ ₁ plane due to their radiative interiors with lower mean densities and delayed development of core mean molecular weight gradients. We find that asteroseismology is a powerful diagnostic for identifying relic Pop III stars despite potentially polluted surfaces, providing a clear pathway for future searches of the Galaxy’s oldest surviving stars with upcoming surveys.

The first stars likely formed from pristine clouds, marking a transformative epoch after the dark ages by initiating reionization and synthesising the first heavy elements. Among these, low-mass Population III (Pop III) stars are of particular interest, as their long lifespans raise the possibility that some may survive to the present day in the Milky Way’s stellar halo or satellite dwarfs. As the first paper in a series, we present hydrodynamic evolutionary models for 0.7–1 M _⊙ stars evolved up to the white dwarf phase, utilising the MESA software instrument. We systematically vary mass-loss efficiencies, convective transport, and overshooting prescriptions, thereby mapping (i) how uncertain physics influences nucleosynthetic yields; (ii) surface enrichment, including nitrogen-rich post-main-sequence stars arising from convective shell mergers; (iii) remnant properties, such as low-mass helium or carbon-oxygen white dwarfs ( M _WD ∼ 0.45−0.55 M _⊙ ) and transient UV-bright phases; and (iv) potential observational signatures, including neutrino emission during shell mergers and helium flashes. These models establish a predictive framework for identifying surviving Pop III stars and their descendants, providing both evolutionary and observational constraints that were previously unexplored.

Context. It has been theorized that the formation of extremely massive and supermassive stars (> 10 ³ M _⊙ ) could plausibly be the outcome of stellar mergers in low metallicity ( Z < 10 ⁻¹ Z _⊙ ) and dense (≳10 ³ M _⊙ pc ⁻³ ) stellar environments. These objects remain relevant as they can serve as the progenitors of intermediate-mass black holes and they are also formidable chemical polluter candidates, as evidenced by the peculiar abundances seen across cosmic history. Our understanding of the formation of these objects depends on the physical processes involved in their mass accretion and loss, via gas inflows and stellar winds, as well as the stellar mergers themselves. Aims. This work investigates merger-induced mass loss in extremely massive stars within a hydrodynamic framework and provides a prescription derived from the simulations to estimate both the mass loss and the outcome of the interaction. Methods. We adapted the 1D hydrodynamic, stellar structure, and evolution code MESA to simulate stellar inspirals. In our simulations, we considered stars of > 1000 M _⊙ with inspiraling companions of < 100 M _⊙ ; hence, with mass ratios of < 0.1. As the inspiral progresses, the orbital energy of the system is lost through the hydrodynamic and gravitational drag forces. This energy gets deposited as thermal energy in the extremely massive star’s envelope. The reaction of the star can be followed by solving Euler’s hydrodynamical equations. Results. The extremely massive star experiences mass loss from pulsations produced by the inspiral. By scaling the mass lost through such pulsations based on the time it takes for the extremely massive star to radiate away the injected thermal energy, we find that the total ejected mass is ∼10–30% of the system’s mass. Our results point out that most of the energy deposited by the inspiral is used to eject mass. This is consistent with the fact that the extremely massive star models that we study are barely bound, as their mean adiabatic exponent is ∼4/3. Conclusions. These findings demonstrate that merger-induced mass loss is non-negligible for the considered configurations. Thus, it is an important process to account for when investigating the formation of extremely massive stars and predicting their possible role throughout cosmic history.

The first stars likely formed from pristine clouds, marking a transformative epoch after the dark ages by initiating reionisation and synthesising the first heavy elements. Among these, low-mass Population III stars are of particular interest, as their long lifespans raise the possibility that some may survive to the present day in the Milky Way's stellar halo or satellite dwarfs. As the first paper in a series, we present hydrodynamic evolutionary models for 0.7 - 1 MSun stars evolved up to the white dwarf phase, utilising the MESA software instrument. We systematically vary mass-loss efficiencies, convective transport, and overshooting prescriptions, thereby mapping how uncertain physics influences nucleosynthetic yields; surface enrichment, including nitrogen-rich post-main sequence stars arising from convective shell mergers; remnant properties, such as low-mass helium or carbon-oxygen white dwarfs (M_WD ~ 0.45-0.55 MSun) and transient UV-bright phases; and potential observational signatures, including neutrino emission during shell mergers and helium flashes. These models establish a predictive framework for identifying surviving Pop III stars and their descendants, providing both evolutionary and observational constraints that were previously unexplored.

The origin of the first supermassive black holes (SMBHs) observed at redshifts z ≥ 9 remains one of the most challenging open questions in astrophysics. Their rapid emergence suggests that massive “heavy seeds” must have formed early, possibly through the direct collapse of pristine gas clouds in the first galaxies. We present Modules for Experiments in Stellar Astrophysics (MESA)-QUEST, a new framework built on the MESA code, designed to model the structure and evolution of quasi-stars—massive, radiation-supported envelopes hosting accreting black holes (BHs) at their cores—believed to be the progenitors of direct collapse BH seeds. Our implementation introduces flexible boundary conditions representing both Bondi accretion and saturated convection regimes and explores the impact of several stellar wind and mass-loss prescriptions, including Reimers, Dutch, and super-Eddington radiation-driven winds. We find that quasi-stars can grow central BHs to ≥10 ³ M _⊙ under favorable conditions, with saturated convection models yielding black-hole-to-total mass ratios up to 0.55 M _* —five times higher than Bondi-limited cases. However, strong radiation-driven winds can dramatically curtail growth, potentially quenching heavy-seed formation unless balanced by sustained envelope accretion. Our results delineate the physical limits under which quasi-stars can remain stable and produce heavy seeds capable of evolving into the earliest SMBHs detected by JWST and Chandra. Future extensions will incorporate rotation, magnetic fields, and general relativistic radiation hydrodynamics to refine accretion physics and constrain the viability of the quasi-star pathway for early SMBH formation.

JWST has revealed a population of red, compact, high-redshift ( z ∼ 3–10) objects referred to as “Little Red Dots” (LRDs). These objects exhibit unusual spectral features reminiscent of stellar spectra with blackbody-like spectral energy distributions, large hydrogen Balmer breaks, Balmer line absorption, and classical stellar absorption features such as calcium H&K and the calcium triplet. Following the recent suggestion that these may be actively accreting direct-collapse black holes in the process of assembly, i.e., quasi-stars, we present evolutionary models of quasi-stars using our recently released, publicly available MESA-QUEST modeling framework. We compute a grid of models spanning a range of black hole masses and predict the luminosities, temperatures, surface gravities, and lifetimes of these objects. We find that these models lie along a Hayashi track once they hit their “late stage,” which constitutes the majority of their lives (∼20 Myr). We present scaling relations for estimating the mass of a quasi-star as a function of the bolometric luminosity, as well as the bolometric luminosity as a function of the effective temperature for the Hayashi track. The short lifetimes in tandem with the observed number density of LRDs imply the possibility that every supermassive black hole detected at these epochs was once a quasi-star. We compare synthetic spectra of our quasi-star models to observations of LRDs, and show that these models are broadly capable of reproducing the continuum spectra of observed LRDs. These results indicate that quasi-stars are promising candidates for the origin of supermassive black holes via direct collapse in the early Universe.

We present theoretical predictions for solar-like oscillators in the pre-main-sequence (pre-MS) phase of stellar evolution. Our pre-MS models start from a stellar seed of 0.01 solar masses that gains mass through accretion, offering an alternative description to the classical approach segmented into the Hayashi and Henyey tracks. Evolutionary models are calculated using the MESA stellar evolution code with a custom accretion routine, and pulsation properties are investigated using the GYRE oscillation code. We present evolutionary tracks and internal structures for accreting pre-MS solar-like stars in a mass range from 0.7 to 1.6 solar masses, adopting 35 mass accretion histories previously extracted from two-dimensional magneto-hydrodynamical simulations. Atmospheric parameters of our models constrain characteristic frequencies of pre-MS solar-like oscillators to be generally greater than 500 μ Hz. We highlight the imprint of accretion on the buoyancy and Lamb profiles, and illustrate the effects on the small- and large-frequency separations. We additionally quantify individual frequency differences across the 35 accretion histories at the zero-age MS, showcasing differences no larger than 20 μ Hz for an exemplary model. Finally, we discuss the potential of detecting solar-like oscillations in pre-MS stars with the upcoming ESA PLATO mission.