arXiv subjects

Devesh Nandal

Publications and source records attributed to Devesh Nandal.

At least 19 recordsLinked to original sources

arXiv2026

Little Red Dots are Direct-Collapse Black Hole-Forming Galaxies

"Little Red Dots" (LRDs) at $4 < z < 8$ are one of the most challenging discoveries by JWST to date because their distinctive V-shaped spectra and compact morphologies (100 - 200 pc) defy conventional astrophysical interpretation. Previous attempts to explain LRDs as compact stellar systems, heavily-cocooned black holes with differential flows, supermassive stars, or more exotic objects like 'black-hole stars' either cannot show how they formed, explain the origin of the dense shells needed for the absorption features in their spectra, or account for their observed abundances or inferred lifetimes. Here we show that LRDs are simply direct-collapse black hole galaxies in which the BH is still shrouded by the massive disk that created it. Our cosmological simulations yield spectra that are good matches to those of LRDs because high densities at the center of the disk trap X-rays from the BH and produce the observed Balmer absorption features while allowing UV, optical and reprocessed IR flux to partly escape. The host galaxy forms a dense 10$^8$ M$_{\odot}$ cluster of stars with a radius of 150 pc next to the BH, consistent with observations of LRDs. Our models reproduce a wide variety of LRD spectra from typical objects like RUBIES-EGS-42046 at $z = 5.28$ to those with the strongest Balmer breaks such as MoM-BH$^*$-1 at $z = 7.76$ and those at the highest redshifts like CAPERS-LRD-z9 at $z = 9.29$.

astro-ph.GA↗

arXiv2026

Can Little Red Dots Contribute To The Early Universe Cosmic Dust Budget?

The origin of dust in the early Universe remains uncertain. We explore whether Little Red Dots (LRDs) could provide a high-redshift dust-production channel. Motivated by an analogy with Type IIn supernovae, we investigate whether LRDs may share the efficient dust-forming conditions. We consider dust formation in outer winds and later in a shielded cold dense shell after the central source fades. Such dust may avoid a global remnant-phase reverse shock, while longer LRD lifetimes may promote grain growth. We model the SEDs of two low-redshift LRD analogs as a thermal pseudo-photosphere plus optically thin dust components, obtaining dust reservoirs of order $10^{2}$-$10^{3}\,M_{\odot}$. Although photometry cannot exclude pre-existing dust, the narrow Balmer components of both analogs are close to the Case B ratio and their narrow-line environments are metal-poor, disfavoring a dominant diffuse host-ISM origin for the inferred reservoir. Formation within the LRD outflow is consistent with our optical-depth, sublimation, and energy-balance checks. In this scenario, the large dust masses do not produce a strong optical attenuation because a clumpy or asymmetric distribution can leave the dominant optical sightlines relatively unobscured. Our population calculation shows that the LRD contribution to early-Universe dust production can range from negligible to dominant. At higher efficiencies, LRDs can approach or exceed the lower CCSN contribution above $z\approx5$, whereas less favorable assumptions yield a minor contribution. We therefore propose that LRDs can act as early-Universe dust factories and, under some conditions, may dominate over CCSNe or provide seed grains for subsequent growth in the early ISM.

astro-ph.GA↗

arXiv2026

Planet Formation at Cosmic Dawn: Planetesimals in H$_2$O-Rich Disks Around Low-Mass Stars

Primordial, or Pop III, supernovae (SNe) were the first, great nucleosynthetic engines in the Universe, forging the heavy elements required for the later formation of planets, and life. Past studies suggest that the rise of planet formation was gradual, and did not peak until about half of the present age of the Universe after cosmic mean metallicities exceeded a critical value. However, Pop III pair-instability (PI) SNe, which can eject over 100 M$_{\odot}$ of metals, locally enriched gas to metallicities of up to 1 Z$_{\odot}$ at Cosmic Dawn, just 100 Myr after the Big Bang. Here we show that planetesimals, the precursors of terrestrial planets, can form around low-mass, long-lived stars in the debris of such explosions, before the first galaxies and far earlier than previously thought. We modeled the collapse of a dense core with a Jeans mass of just 1 - 2 M$_{\odot}$ from a PI SN remnant and found that a protoplanetary disk formed with several Earth masses of planetesimals 0.5 - 1.0 AU from their parent star, within its water snow line. The disk has H$_2$O mass fractions that are only a factor of a few less than in the Solar System today, raising the possibility of enrichment of the first planets in the Universe with water in direct analogy to Earth in the Solar system.

astro-ph.GA↗

arXiv2026

Little Red Dots as Intermediate Mass, Super-Eddington Engines: Insights from Type IIn Supernovae and The 1837-1856 Great Eruption of $η$ Carinae

JWST's Little Red Dots (LRDs) display a unique constellation of features that do not occur simultaneously in any other class of galaxies or AGN. Here we observe that many of these features find parallels in the 19th century Great Eruption (GE) of $η$ Carinae and a sub-class of supernovae (Type IIn). Drawing on these stellar phenomena -- outflows trapped by dense circumstellar gas envelopes -- we sketch a possible scenario for LRDs. Outflows from the central engine produce an enshrouding envelope of gas that may be thought of as a slow wind. This dense wind and its enormous extent produce an opacity so high that a pseudo-photosphere forms within the wind, obscuring the central engine and manifesting as a blackbody-like continuum. Radiation from the buried engine powers the system. The engine may also launch fast winds that crash into the existing envelope to generate shocks. Lines form within the wind above the photosphere -- electron scattering and absorption in the clumpy (ionized + neutral) medium account for broad wings and P-Cygni cores. A key implication is that inferences of ``overmassive black holes" may be interpreting this wind-like physics as a virial broad-line region. We propose an escape velocity argument to constrain the mass of the engine, which yields $M<10^{5} M_\odot$ for the typical LRD. The lack of variability and low surface gravity of the photosphere provide further support for intermediate mass ($M\approx10^{3-6} M_\odot$), but very luminous super-Eddington ($L_{\rm{bol}}/L_{\rm{edd}}\gtrsim5$) systems harboring a supermassive star or intermediate mass black hole. Paralleling the evolution of IIn SNe, dust production in the envelope may mark the beginnings of classical AGN. This paper explores a possible self-consistent explanation for the entire life-cycle of LRDs, from their enshrouding in dense gas to their fates as seeds of massive black holes.

astro-ph.GA↗

arXiv2026

Pulsational mass loss from supermassive stars creates the compact shells of Little Red Dots

Little Red Dots (LRDs) have emerged as one of the central puzzles of the JWST era. Their spectra increasingly require dense gas close to the source, yet the physical origin of that cocoon-like structure remains unclear. We examine whether late pulsational mass loss from supermassive stars (SMS)leads to dense gas cocoons. We analyze five accreting GENEC models at different metallicities with characteristic masses of order $10^5\,M_\odot$, following them through post-accretion evolution with radial pulsation calculations and general relativistic (GR) stability diagnostics. Mass loss during the final stages of evolution occurs not as a steady wind, but through discrete strange-mode ejection episodes. In the $Z=10^{-2}\,Z_\odot$ model, which provides the clearest LRD analogue, four late episodes last $41$--$282$ yr and eject $10$--$348\,M_\odot$ each, for a total loss of $(4.8-10)\times10^2\,M_\odot$; the final episode alone contributes $\simeq 73\%$ of that budget. Since the last episode dominates the mass-loss, it is the only event sufficiently massive enough to leave behind a compact, optically thick shell extending out to 0.4 pc that reproduces the LRD dense gas cocoon. The final ejecta are H/He dominated but chemically distinctive, with a robust nitrogen-rich composition, $\log(\mathrm{N/O})\simeq0.13$ and $\log(\mathrm{C/O})\simeq-0.23$. The SMS reaches GR instability at an age of $\sim 1$ Myr and collapses in $\sim10^4$ s, retaining $\sim 99\%$ all of its mass. Across the full metallicity range from Pop III to $10^{-2}\,Z_\odot$, this shell-ejection channel persists. Pulsational mass-loss from SMSs therefore provides a physically motivated origin for the compact cocoon-like structure implied by LRDs, while remaining the natural progenitors of the massive black hole seeds invoked in direct collapse scenario.

astro-ph.HE↗

arXiv2026

Habitable Zones Around Massive Stars: From the Main Sequence to Supergiants

Massive stars dominate the feedback of young stellar populations, yet their ultraviolet fields and winds are often presumed to preclude Earth like habitability. We test this by mapping time dependent habitable zones (HZs) for solar metallicity stars of $0.8$--$120\,M_\odot$. Using rotating and non rotating \textsc{GENEC} tracks, we compute bolometric climate HZ boundaries and impose XUV energy limited escape and wind ram pressure constraints for a dipole-magnetized Earth analogue. The retention limited inner edge is the most restrictive limit. We measure annulus lifetime, longest fixed orbit residence, and maximum dynamically packed terrestrial multiplicity, finding a sharp main-sequence ceiling. A rotating $9\,M_\odot$ star sustains a retention limited HZ for $\sim 30$ Myr at $\sim 70$--$130$ AU, but becomes brief and narrow by $12\,M_\odot$ and disappears by $15\,M_\odot$. Post main-sequence evolution can reopen HZs up to $\sim 25$--$30\,M_\odot$, but only for $\sim 0.03$--$1.5$ Myr at hundreds to $\sim 10^3$ AU, disappearing by $\sim 40\,M_\odot$. Stellar rotation modestly increases habitable lifetimes near the upper main sequence without altering the high mass ceiling. IMF weighting shows that massive stars contribute only $\sim 10^{-4}$ of the habitable planet time budget. Even so, for the fiducial occurrence normalization and if rocky planets form or survive at the required wide separations, they add a few $10^5$ Earth analogues satisfying the adopted criteria to the Milky Way at any instant. Massive star systems do not dominate the Galaxy-wide habitability budget, but may provide short-lived, distinct targets for biosignature searches.

astro-ph.SR↗

arXiv2026

SEEDZ: Rapid Galaxy Assembly as a Pathway to Supermassive Stars, Dense Stellar Environments and Massive Black Hole Seeds

We investigate the assembly history of early galaxies in the SEEDZ hydrodynamic simulations, to investigate the high inflow rates believed to be required for the formation of supermassive stars (SMSs), dense stellar clusters and subsequently heavy seed black holes. Using a heavy seed formation criteria of $>$1 M$_\odot$ yr$^{-1}$ flowing into 10 pc regions, we find that heavy seeds form in halos that grow rapidly compared to those halos that never meet the criteria. Halos with growth rates of $\gtrsim$1 M$_\odot$ yr$^{-1}$ at their virial radius (scales of a few hundred pc) are able to sustain a flow rate of 0.1 M$_\odot$ yr$^{-1}$ into the inner 1 pc of the halo, maintaining higher density environments within the central 10 - 100~pc. These halos continue to grow rapidly after their initial collapse, typically forming heavy seeds $\sim$100 Myr after forming their first stars and stellar mass black holes. By $z=10$, most heavy seeds form in regions of near-solar metallicity, although a minority of heavy seeds do continue to form in low metallicity (10$^{-2}$ Z$_\odot$) regions. Under the assumption that a SMS forms as the progenitor to a heavy seed if it forms in a region of low (10$^{-2}$ Z$_\odot$) metallicity, and can sustain high accretion rates above 0.02 M$_\odot$ yr$^{-1}$ throughout the SMS lifetime of 2 Myr, we find a number density of SMSs of 0.1 cMpc$^{-3}$, meaning that only a fraction of 10$^{-4}$ of these SMSs would need to be visible to JWST to account for the observed population of Little Red Dot galaxies.

astro-ph.GA↗

arXiv2026

The formation of supermassive black holes from Population III.1 seeds. IV. Self-regulated seeding from supermassive star ionizing feedback

Supermassive Population III.1 stars, i.e., formed from pristine, metal-free gas leading to conditions where dark matter annihilation heating is significant, have been proposed as the progenitors of supermassive black holes (SMBHs) in the early universe ($z \sim 20-40$). Since such Pop III.1 stars only form from non-irradiated gas in dark matter minihalos, they are predicted to appear isolated from each other and other sources of feedback. The previous papers in this series used the isolation distance of Pop III.1 stars as a free parameter to seed SMBH in cosmological simulations of dark matter halos. Here we develop a feedback-regulated model of Pop III.1 isolation, based on the growth of HII regions around each Pop III.1 star and lower-mass, irradiated Pop III.2 stars. Our model considers the time delay between the formation of a minihalo and its Pop III.1 star, R-type expansion of HII regions that expand into the intergalactic medium (IGM), and the redshift dependence of Strömgren spheres for longer-lived ionizing sources. For a fiducial Pop III.1 star H-ionizing photon luminosity of $10^{53}\:{\rm s}^{-1}$ and lifetime of $10\:$Myr we find an R-type HII region radius of $R_{\rm R}\simeq1.3\:$cMpc, approximately independent of redshift. The median formation redshift is $\sim20$, with the process essentially complete by $z\sim16$. The overall number density of SMBHs produced in this model is then $n_{\rm SMBH}\simeq 3 ϕ_V/(4πR_{\rm R}^3)\simeq 0.2\:{\rm cMpc}^{-3}$. We also discuss predictions for the abundance of binary SMBHs, which may appear as dual active galactic nuclei (AGN; $\lesssim 0.3\%$ for $z>6$), and SMBH binary merger rates, measurable by the forthcoming LISA mission.

astro-ph.GA↗

arXiv2026

Growth of Metal-Enriched Supermassive Stars by Accretion and Collisions

Supermassive stars (SMSs) are candidate progenitors of massive black hole seeds and may contribute to anomalous abundance patterns in high-redshift galaxies and globular clusters. Recent radiation-hydrodynamic simulations indicate that SMSs can form at finite metallicity, not only in metal-free direct-collapse conditions. We model SMS growth with \textsc{GENEC} over $Z/Z_\odot=10^{-5}$-$10^{-2}$ using simulation-motivated accretion histories. The final masses reach $\sim7.2\times10^{4}\,M_\odot$ at $10^{-5}\,Z_\odot$ and $\sim2.3\times10^{3}\,M_\odot$ at $10^{-2}\,Z_\odot$. Models are evolved through the pre-main sequence and core H-burning phases, terminating at the onset of general-relativistic instability for $Z\lesssim10^{-4}\,Z_\odot$ or at core He exhaustion for $Z\gtrsim10^{-3}\,Z_\odot$. The dominant mass growth channel transitions from collision-driven to accretion-driven between $Z=10^{-4}$ and $10^{-3}$. With stellar lifetimes remaining nearly constant at $1.8$-$2.0$ Myr, collisions do not significantly rejuvenate the star, implying that collision driven runaway collapse cannot proceed in isolation and must be supplemented, and likely dominated by gas accretion. We further compute the critical inflow rate required to keep the stellar envelope inflated, $\dot{M}_{\rm crit}$, which decreases with increasing $Z$ and decreasing central mass fraction of hydrogen ($X_{\rm c}$). The critical rate falls below $10^{-5}\,M_\odot\,{\rm yr^{-1}}$ at $X_{\rm c}=0.60$ for $10^{-2}Z_\odot$. This indicates that SMSs with $0.01~Z_\odot$ are cool supergiants during most of their lifetimes, where UV photon emissivity and radiative feedback is strongly suppressed. Overall, SMS evolution remains viable up to $Z\simeq0.01\,Z_\odot$, supporting SMS formation in proto-globular clusters and other metal-enriched dense environments.

astro-ph.SR↗

arXiv2025

Supermassive Stars Match the Spectral Signatures of JWST's Little Red Dots

The James Webb Space Telescope (JWST) has unveiled a population of enigmatic, compact sources at high redshift known as ``Little Red Dots'' (LRDs), whose physical nature remains a subject of intense debate. Concurrently, the rapid assembly of the first supermassive black holes (SMBHs) requires the formation of heavy seeds, for which supermassive stars (SMSs) are leading theoretical progenitors. In this work, we perform the first quantitative test of the hypothesis that LRDs are the direct observational manifestation of these primordial SMSs. We present a novel, first-principles pipeline generating synthetic spectra for a non-rotating, metal-free SMS up to $10^6 \, M_\odot$. We establish that its luminosity ($L_λ\approx 1.7 \times 10^{44} \, \text{erg} \, \text{s}^{-1} \, μ\text{m}^{-1}$ at 4050\,Å) provides a decisive constraint, matching prominent LRDs. Our model self-consistently reproduces their defining spectral features: the V-shaped Balmer break morphology is shown to be an intrinsic photospheric effect, while the complex line phenomenology, strong H$β$ in emission with other Balmer lines in absorption arises from non-LTE effects in a single stellar atmosphere. With wind and macroturbulent broadening, we match LRD spectra at $z=7.76$ and $z=3.55$, including the H$β$ width of MoM-BH*-1 to within 4\%. We predict a luminosity-dependent observability window, $\sim10^{4}$ yr for the most luminous systems and $10^{5}$--$10^{6}$ yr if $L_λ(4050\,\textÅ)$ is lower by 1--2 dex. These results provide a self-consistent alternative to multi-component obscured AGN scenarios and suggest JWST may be witnessing luminous stages of SMBH progenitors before collapse.

astro-ph.GA↗

arXiv2025

Dark-Matter-Powered Population III Evolution: Lifetimes, Rotation, and Quasi-Homogeneity in massive Stars

Population III stars supplied the first light and metals in the Universe, setting the pace of re-ionisation and early chemical enrichment. In dense haloes their evolution can be strongly influenced by the energy released when WIMPs annihilate inside the stellar core. We follow the evolution of a $20\,M_\odot$ Population III model with the \textsc{genec} code, adding a full treatment of spin dependent WIMP capture and annihilation. Tracks are calculated for six halo densities from $10^{8}$ to $3\times10^{10}\,\mathrm{GeV\,cm^{-3}}$ and three initial rotation rates between zero and $0.4\,v/v_{\mathrm{crit}}$. As soon as the capture product reaches $ρ_χσ_{\mathrm{SD}}\simeq2\times10^{-28}\,\mathrm{GeV\,cm^{-1}}$, the dark-matter luminosity rivals hydrogen fusion, stretching the main-sequence lifetime from about ten million years to more than a gigayear. The extra time allows meridional circulation to smooth out differential rotation; a star that begins at $0.4\,v/v_{\mathrm{crit}}$ finishes core hydrogen burning with near solid-body rotation and a helium core almost twice as massive as in the dark-matter-free case. Because the nuclear timescale is longer, chemically homogeneous evolution now sets in at only $0.2\,v/v_{\mathrm{crit}}$, rather than the $\gtrsim0.5\,v/v_{\mathrm{crit}}$ required without WIMPs. For a star with $0.4\,v/v_{\mathrm{crit}}$, the surface hydrogen fraction drops to $X\!\sim\!0.27$, helium rises to $Y\!\sim\!0.73$, and primary $^{14}\mathrm N$ increases by four orders of magnitude at He exhaustion. Moderate rotation combined with plausible dark-matter densities can therefore drive primordial massive stars towards long-lived, quasi-homogeneous evolution with distinctive chemical and spectral signatures.

astro-ph.SR↗

arXiv2025

1000-10,000 M$_\odot$ Primordial Stars Created the Nitrogen Excess in GS 3073 at $z = 5.55$

The advent of the James Webb Space Telescope has revealed a wealth of new galaxies just a few hundred Myr after the Big Bang. Some of these galaxies exhibit unusual N/O ratios that are difficult to explain with stellar populations today. While Wolf-Rayet stars in multiple-burst populations, very massive or rapidly-rotating primordial stars, general relativistic explosions of metal-enriched supermassive stars, or the precursors of globular clusters can in principle account for the nitrogen excess in the galaxies GN-z11 and CEERS 1019, no known stars or supernovae can explain the far higher N/O ratio of 0.46 in GS 3073 at redshift $z =$ 5.55. Here we show that the extreme nitrogen abundances in GS 3073 can be produced by 1000 - 10,000 M$_{\odot}$ primordial (Pop III) stars. We find that these are the only candidates that can account for its large N/O ratios and its C/O and Ne/O ratios. GS 3073 is thus the first conclusive evidence in the fossil abundance record of the existence of supermassive Pop III stars at cosmic Dawn.

astro-ph.GA↗

arXiv2025

The Evolution of Pop III.1 Protostars Powered by Dark Matter Annihilation. II. Dependence on WIMP Properties

The rapid appearance of supermassive black holes (SMBHs) at $z\gtrsim7$ requires efficient pathways to form massive black hole seeds. We investigate whether annihilation of weakly interacting massive particles (WIMPs) can alter primordial (Pop III.1) protostellar evolution sufficiently to enable formation of such `heavy'' seeds. Using the one-dimensional Geneva stellar-evolution code (GENEC) with an implemented Gould single-scatter capture module, we compute a grid of protostellar evolution models covering ambient WIMP mass densities $ρ_χ=10^{12}$-$10^{16}\ \mathrm{GeV\,cm^{-3}}$, WIMP masses $m_χ=30$-$3000\ \mathrm{GeV}$, spin-dependent cross sections $σ_{\rm SD}=10^{-42}$-$10^{-40}\ \mathrm{cm^2}$, and baryonic accretion rates $\dot{M_*}=(1-3)\times10^{-3}\, M_\odot \,{\rm yr}^{-1}$. We find a robust bifurcation of outcomes. For sufficiently high ambient dark matter density ($ρ_χ\gtrsim5\times10^{14}\ \mathrm{GeV\,cm^{-3}}$) and capture efficiency ($σ_{\rm SD}\gtrsim10^{-41}\ \mathrm{cm^2}$) WIMP annihilation supplies enough energy to inflate protostars onto extended, cool (Hayashi-track) configurations that dramatically suppress ionizing feedback and permit uninterrupted growth to $\sim10^{5}\,M_\odot$. Lighter WIMPs and larger $σ_{\rm SD}$ favour earlier and stronger annihilation support; heavier WIMPs delay the effect. For our fiducial case, WIMP masses $<$3 TeV are essential for allowing growth to the supermassive regime, otherwise the protostar evolves to the compact, feedback-limited regime that results in `light'' seeds. These results indicate that, under plausible halo conditions, DM annihilation provides a viable channel for forming heavy black hole seeds.

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arXiv2025

Rotating Supermassive Pop III Stars On The Main Sequence

The detection of billion-solar-mass supermassive black holes (SMBHs) within the first billion years of cosmic history challenges conventional theories of black hole formation and growth. Simultaneously, recent JWST observations revealing exceptionally high nitrogen-to-oxygen abundance ratios in galaxies at high redshifts raise critical questions about rapid chemical enrichment mechanisms operating in the early universe. Supermassive stars (SMSs) with masses of 1000 to 10000 M$_{\odot}$ are promising candidates to explain these phenomena, but existing models have so far neglected the pivotal role of stellar rotation. Here, we present the first comprehensive evolutionary models of rotating Pop III SMSs computed using the GENEC stellar evolution code, including detailed treatments of rotation-induced chemical mixing, angular momentum transport, and mass loss driven by the $ΩΓ$ limit. We demonstrate that rotation significantly enlarges the convective core and extends stellar lifetimes by up to 20%, with moderate enhancement of mass-loss rates as stars approach critical rotation thresholds. Our results further indicate that the cores of SMSs rotate relatively slowly (below $\sim 200$ km s$^{-1}$), resulting in dimensionless spin parameters $a* < 0.1$ for intermediate-mass black hole (IMBH) remnants that are notably lower than theoretical maximum spins. These findings highlight rotation as a key factor in determining the structural evolution, chemical yields, and black hole spin properties of SMSs, providing critical insights to interpret observational signatures from the high-redshift universe.

astro-ph.SR↗

arXiv2025

The Evolution of Pop III.1 Protostars Powered by Dark Matter Annihilation. I. Fiducial model and first results

The existence of billion-solar-mass quasars at redshifts $z \gtrsim 7$ poses a formidable challenge to theories of black hole formation, requiring pathways for the rapid growth of massive seeds. Population III.1 stars, forming in pristine, dense dark matter (DM) minihalos, are compelling progenitors. This study presents a suite of stellar evolution models for accreting Pop III.1 protostars, calculated with the \textsc{GENEC} code. We systematically explore a wide parameter space, spanning ambient WIMP densities of $ρ_χ\sim 10^{12}\mbox{-}10^{16}\,\mathrm{GeV\,cm^{-3}}$ and gas accretion rates of $10^{-3}\mbox{-}10^{-1}\,M_\odot\,\mathrm{yr^{-1}}$, to quantify the effects of DM annihilation. A central finding is that for a protostar to grow to supermassive scales ($\gtrsim 10^5 \, M_{\odot}$), the ambient DM density in the immediate vicinity of the star must exceed a critical threshold of $ρ_χ \gtrsim 5 \times 10^{14} \, \text{GeV cm}^{-3}$. The energy injected by WIMP annihilation inflates the protostar, lowering its surface temperature, which suppresses the ionizing feedback that would otherwise halt accretion and significantly delays the onset of hydrogen fusion. This heating also governs the star's final fate: in dense halos ($ρ_χ\gtrsim 10^{15}\,\mathrm{GeV\,cm^{-3}}$), stars remain stable against general relativistic instability beyond $10^6 \, M_{\odot}$, whereas at lower densities ($ρ_χ\lesssim 10^{13}\,\mathrm{GeV\,cm^{-3}}$), they collapse at masses of $\sim 5 \times 10^5 \, M_{\odot}$. Once the DM fuel is exhausted and core burning commences, the protostar contracts and its ionising photon output can reach very high levels $\sim 10^{53} s^{-1}$. These distinct evolutionary phases offer clear observational signatures for the JWST, providing a robust, physically-grounded pathway for forming heavy black hole seeds in the early universe.

astro-ph.SR↗

arXiv2025

The Future of Solar modelling: requirements for a new generation of solar models

Helioseismology and solar modelling have enjoyed a golden era thanks to decades-long surveys from ground-based networks such as for example GONG, BiSON, IRIS and the SOHO and SDO space missions which have provided high-quality helioseismic observations that supplemented photometric, gravitational, size and shape, limb-darkening and spectroscopic constraints as well as measurements of neutrino fluxes. However, the success of solar models is also deeply rooted in progress in fundamental physics (equation of state of the solar plasma, high-quality atomic physics computations and opacities, description of convection and the role of macroscopic transport processes of angular momentum and chemicals, such as for example meridional circulation, internal gravity waves, shear-induced turbulence or even convection. In this paper, we briefly outline some key areas of research that deserve particular attention in solar modelling. We discuss the current uncertainties that need to be addressed, how these limit our predictions from solar models and their impact on stellar evolution in general. We outline potential strategies to mitigate them and how multidisciplinary approaches will be needed in the future to tackle them.

astro-ph.SR↗

arXiv2024

The Origin of Supermassive Black Holes from Pop III.1 Seeds

The origin of supermassive black holes (SMBHs) is a key open question for contemporary astrophysics and cosmology. Here we review the features of a cosmological model of SMBH formation from Pop III.1 seeds, i.e., remnants of metal-free stars forming in locally-isolated minihalos, where energy injection from dark matter particle annihilation alters the structure of the protostar allowing growth to supermassive scales (Banik et al. 2019; Singh et al. 2023; Cammelli et al. 2024). The Pop III.1 model explains the paucity of intermediate-mass black holes (IMBHs) via a characteristic SMBH seed mass of $\sim10^5\:M_\odot$ that is set by the baryonic content of minihalos. Ionization feedback from supermassive Pop III.1 stars sets the cosmic number density of SMBHs to be $n_{\rm SMBH}\lesssim 0.2\:{\rm Mpc}^{-3}$. The model then predicts that all SMBHs form by $z\sim20$ with a spatial distribution that is initially unclustered. SMBHs at high redshifts $z\gtrsim7$ should all be single objects, with SMBH binaries and higher order multiples emerging only at lower redshifts. We also discuss the implications of this model for SMBH host galaxy properties, occupation fractions, gravitational wave emission, cosmic reionization, and the nature of dark matter. These predictions are compared to latest observational results, especially from HST, JWST and pulsar timing array observations.

astro-ph.GA↗

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.

astro-ph.SR↗