Stellar Evolution Research Articles

Abstract Recently discovered supermassive black holes with masses of ∼108 M⊙ at redshifts z ∼ 9–11 in active galactic nuclei (AGN) pose severe challenges to our understanding of supermassive black hole formation. One proposed channel are rapidly accreting supermassive PopIII stars (SMSs) that form in large primordial gas halos and grow up to <106 M⊙. They eventually collapse due to the general relativistic instability and could lead to supernova-like explosions. This releases massive and energetic ejecta that then interact with the halo medium via an optically thick shock. We develop a semi-analytic model to compute the shock properties, bolometric luminosity, emission spectrum and photometry over time. The initial data is informed by stellar evolution and general relativistic SMS collapse simulations. We find that SMS explosion light curves reach a brightness ∼1045-47 erg/s and last 10–200 years in the source frame – up to 250–3000 years with cosmic time dilation. This makes them quasi-persistent sources which vary indistinguishably to little red dots and AGN within 0.5–9 (1 + z) yrs. Bright SMS explosions are observable in long-wavelength JWST filters up to z ≤ 20 (24–26 mag) and pulsating SMSs up to z ≤ 15. EUCLID and the Roman space telescope (RST) can detect SMS explosions at z < 11–12. Their deep fields could constrain the SMS rate down to 10−11Mpc−3yr−1, which is much deeper than JWST bounds. Based on cosmological simulations and observed star formation rates, we expect to image up to several hundred SMS explosions with EUCLID and dozens with RST deep fields.

Carbon--oxygen (C--O) shell mergers in massive stars play a crucial role in nucleosynthesis and in the final stages of stellar evolution. These convective-reactive events significantly alter the internal structure of the star shortly before core collapse. We investigated how the enhanced production of light particles (especially protons) during a C--O shell merger, relative to classical oxygen shell burning, affects the energy balance and evolution of the convective shell. We derived the budget for direct and reverse nucleosynthesis flows across all relevant nuclear reactions from stellar evolution models, and we assessed the relative energy produced. We find that proton capture reactions on S P and Ar (SPAr) dominate the nuclear energy production in typical C--O shell mergers, as predicted by 1D stellar models. Their combined energy output is approximately 400 times greater than that of C and O fusion under the same conditions. Our results highlight the critical importance of including these proton-capture reactions in simulations of convective-reactive burning. This work suggests that excluding their contribution can lead to inaccurate modeling of the dynamics and nucleosynthesis in advanced stellar evolutionary phases. These results will need to be confirmed by new 1D stellar simulations and 3D hydrodynamics models.

Abstract The formation mechanisms of merging binary black holes (BBHs) observed by the LIGO-Virgo-KAGRA collaboration remain uncertain. Detectable eccentricity provides a powerful diagnostic for distinguishing between different formation channels, but resolving their eccentricity distributions requires the detection of a large number of eccentric mergers. Future gravitational wave detectors such as the Einstein Telescope and Cosmic Explorer will detect tens of thousands of BBH mergers out to redshifts z ≥ 10, making it critical to understand the redshift-dependent evolution of eccentricity distributions. We simulate this evolution for two key channels: dynamical assembly in globular clusters (GCs), which leads to rapid, eccentric mergers; and hierarchical triples in the field, where three-body dynamics can induce eccentricity in the inner binary. When considering all BBH mergers, the GC channel dominates overall, consistent with previous studies. However, when focusing on mergers with detectable eccentricity in next-generation detectors, we find that hierarchical triples dominate the eccentric merger rate at 0 ≤ z ≤ 4, with GC mergers becoming competitive at higher redshifts. Across all model variations, eccentric mergers in the local Universe (z ≲ 1) have significant contributions from field triples, challenging the common view that such systems primarily form in dense environments. We show that, regardless of cluster and stellar evolution uncertainties, hierarchical triples contribute at least 30 per cent of eccentric mergers across a large range of redshifts.

We present Cesam2k20, the latest version of the hydrostatic stellar evolution code CESAM originally developed by P. Morel and collaborators. Over the last three decades, it has undergone many improvements and has been extensively tested against other stellar evolution codes before being selected to compute the first-generation grid of stellar models for the PLATO mission. Among all the developments made thus far, Cesam2k20 now implements state-of-the-art models for the transport of chemical elements and angular momentum. It was recently made publicly available with an ecosystem of other codes interfaced with it: 1D and 2D oscillation codes ADIPLS and ACOR, optimisation program OSM, and Python utility package pycesam . This paper recalls the numerical peculiarities of Cesam2k20, namely, the use of a collocation method where the structure variables are decomposed as piecewise polynomials projected on a B-spline basis. Here, we review the options available for modelling the different physical processes. In particular, we illustrate the improvements made in the transport of chemical elements and angular momentum with a series of standard and non-standard solar models.

Both jets and ionized outflows in active galactic nuclei (AGNs) are thought to play important roles in affecting the star formation and evolution of host galaxies, but their relationship is still unclear. As a pilot study, we performed a detailed spectral analysis of a radio-loud (RL) AGN 3C 59 (z=0.1096) by systematically considering various factors that may affect the fitting results, thereby establishing a general spectral fitting strategy for subsequent research with a larger sample. AGN 3C 59 is a rare target for simultaneously studying jets and warm absorbers, which are one type of ionized outflow. Based on the multiwavelength data from near-infrared (NIR) to hard X-ray bands detected by the Dark Energy Spectroscopic Instrument, Galaxy Evolution Explorer, and XMM-Newton, we used the spex code to build broadband continuum models and perform photoionization modeling with the pion code to constrain the physical parameters of warm absorbers in 3C 59. We found two warm absorbers with ionization parameters of łog ξ/( erg cm s and $1.65± 0.11$; their outflowing velocities are v_ ̊m out km s and -228^ km s respectively. These warm absorbers are located between the outer torus and the narrow (emission-)line region, and their positive v_̊m out-ξ relation can be explained by the radiation pressure-driven mechanism. We found that the estimations of these physical properties are affected by the different spectral fitting strategies, such as the inclusion of NIR to ultraviolet data, the choice of energy range of spectrum, or the composition of the spectral energy distribution. Based on the same fitting strategy, this work presents a comparative study of the outflow-driven mechanism between a RL AGN (3C 59) and a radio-quiet AGN (NGC 3227), which suggests a similar driven mechanism of their warm absorber outflows and a negligible role of jets in this process.

The Lithium-Dip is a severe lithium depletion observed in mid-F (6200-6650 K) dwarfs, which has puzzled astronomers since it was discovered in 1986. Proposed mechanisms include effects related to rotation, magnetic fields, diffusion, gravity waves, and mass loss. Which, if any, of these is realistic remains unclear. Here we show that mixing due to shear induced by stellar angular momentum loss is the unique mechanism driving the lithium depletion. Each mechanism leaves a different signature in the subsurface lithium distribution. The deepening surface convection zones of subgiants of NGC 188 evolving out of the Lithium-Dip dredge up the subsurface material and thus reveal the signature of the responsible mechanism, rotation. Subgiants can also be used more generally, thereby improving fundamental understanding of stellar evolution. Rotational mixing may be the dominant lithium-depleting mechanism in a wide range of solar-type stars, including in the Sun. Our results may further reconcile the cosmological lithium discrepancy.

The origin of carbon in the Universe remains uncertain. It has been suggested that at the solar metallicity, binary-stripped massive stars -- stars that lost their envelope through a stable interaction with a companion -- produce twice as much carbon as their single-star counterparts. However, understanding the chemical evolution of galaxies over cosmic time requires examining stellar yields across a range of metallicities. Using the stellar evolution code MESA, we computed the carbon yields from wind mass loss and supernova explosions of single and binary-stripped stars across a wide range of initial masses ($10–46,M_⊙$), metallicities (Z = 0.0021, $0.0047$, $0.0142$), and initial orbital periods ($10–5000$ days). We find that metallicity is the dominant factor influencing the carbon yields of massive stars, outweighing the effects of binarity and orbital parameters. Since the chemical yields from massive binary stars are highly sensitive to metallicity, we caution that yields predicted at the solar metallicity should not be directly extrapolated to lower metallicities. At subsolar metallicities (Z=0.0021), weak stellar winds and inefficient binary stripping result in carbon yields from binary-stripped stars that closely resemble those of single stars. This suggests that binary-stripped massive stars cannot explain the presence of carbon-enhanced metal-poor stars or the carbon enrichment observed in high-redshift galaxies as probed by the James Webb Space Telescope. Our findings only cover the stripped stars in massive binaries. The impact of other paths of binary star evolution, in particular stellar mergers and accretors, remains largely unexplored; future study will be necessary for a full understanding of the role of massive binaries in nucleosynthesis.

Studies of the rotational velocities of intermediate-mass main-sequence stars are crucial for testing stellar evolution theory. They often rely on spectroscopic measurements of the projected rotation velocities, V_ eq These not only suffer from the unknown projection factor sin,i but tend to ignore additional line-profile broadening mechanisms aside from rotation, such as pulsations and turbulent motions near the stellar surface. This limits the accuracy of V_ eq distributions derived from V_ eq measurements. We use asteroseismic measurements to investigate the distribution of the equatorial rotation velocity V_ eq, its ratio with respect to the critical rotation velocity, V_ eq /V_ crit and the specific angular momentum, J/M, for several thousands of BAF-type stars, covering a mass range from 1.3,M_⊙ to 8.8,M_⊙ and almost the entire core-hydrogen burning phase. We rely on high-precision model-independent internal rotation frequencies, as well as on masses and radii from asteroseismology to deduce V_̊m eq, $V_ ̊m eq /V_ crit $, and J/M for 2937 gravity-mode pulsators in the Milky Way. The sample stars have rotation frequencies between almost zero and 33μHz, corresponding to rotation periods above 0.35,d. We find that intermediate-mass stars experience a break in their J/M occurring in the mass interval $ M_⊙$. We establish unimodal V_̊m eq and $V_ ̊m eq /v_ ̊m crit $ distributions for the mass range $,M_⊙ while stars with M∈2.5,8.8,M_⊙ reveal some structure in their distributions. We find that the near-core rotation slows down as stars evolve pointing to very efficient angular momentum transport. The kernel density estimators of the asteroseismic internal rotation frequency, equatorial rotation velocity, and specific angular momentum of this large sample of intermediate-mass field stars can conveniently be used for population synthesis studies and to fine-tune the theory of stellar rotation across the main sequence evolution.

The interaction of post-explosion supernova ejecta with the surrounding circumstellar medium creates emissions across the electromagnetic spectrum. Since the circumstellar medium is created by the mass lost from the progenitor star, it carries tell-tale signatures of the progenitor. Consequently, observations and modeling of radiation produced by the interaction in various types of supernovae have provided valuable insights into their progenitors. Detailed studies have shown that the interaction in supernovae begins and sustains over various timescales and lengthscales, with differing mass-loss rates in distinct sub-classes. This reveals diverse progenitor histories for these stellar explosions. This review paper summarizes various supernova subtypes, linking them to stellar death pathways, and presents an updated supernova classification diagram. We then present a multi-wavelength study of circumstellar interaction in different supernova classes. We also present unpublished X-ray as well as radio observations of a type IIn supernova, SN 2010jl, which allow us to extend its circumstellar interaction studies to about 7 years post-explosion. The new data indicates that the extreme mass-loss rate (∼0.1 M⊙ yr−1) in SN 2010jl, reported by Chandra et al. commenced within the last 300 years before the explosion. We summarize the current status of the field and argue that via detailed studies of the circumstellar interaction, a.k.a. “Time Machine” technique, one of the big mysteries of stellar evolution, i.e., mapping supernovae progenitors to their explosive outcomes can be solved.

Context. Luminous blue variables (LBVs) are an elusive stage of the post-main sequence evolution of massive stars. Wray 15−0751 is one of the few LBVs known to belong to a cluster, FSR 1570, to which we can constrain the distance and kinematics thanks to Gaia, providing additional valuable information of the star. Aims. We jointly consider the Gaia astrometric information on Wray 15−0751 and its cluster, plus its spectral classification and that of the brightest cluster members, to assess the evolutionary path leading to the current stage of the star and infer some properties of the likely binary system that originated it. Methods. In addition to the astrometry from Gaia DR3 we present classification spectroscopy in the visible of Wray 15−0751 and the 18 brightest cluster members of FSR 1570, plus new photometric and spectroscopic observations of Wray 15−0751, which we analyzed jointly with published evolutionary tracks. Results. Astrometry confirms the membership of Wray 15−0751 in its host cluster, without displaying any evidence of a kick velocity. The current cluster turnoff at late O types implies an age of 10.2 +6.3 −3.2 Myr. Together with the results of existing far-infrared observations pointing to it having recently undergone the red supergiant phase, we conclude that Wray 15−0751 is most likely a blue straggler now in the blue loop, resulting from a complete merger of two members of a close binary system with similar initial masses, not much in excess of ∼20 solar masses each.

Envelope stripping, whether through single-star wind mass loss or binary mass transfer, is a key evolutionary pathway for the formation of classical Wolf-Rayet stars and lower mass stripped helium (He) stars. However, to study the evolution of these objects into black holes, neutron stars, and stripped-envelope supernovae, we require appropriate input models for the core-He burning phase without relying on the uncertain evolution into this evolved phase. Reliable mass-luminosity relations (MLRs) for He stars are needed for stellar wind and evolution studies, but the MLRs currently available in the literature either refer to fully stripped or chemically homogeneous stars, neither of which reflect the important and recently also observationally confirmed stage of partial stripping. We alleviate this drawback by computing sets of MESA synthetic structure models with partially stripped chemical profiles, consisting of a pure-He core and a hydrogen (H)-depleted envelope with an H/He chemical gradient left behind from the receding convective core during the main sequence. As the H-profile slope increases from 0 (full chemical homogeneity) to ∞ (pure-He stars) in our synthetic models, we find the luminosity to initially increase before eventually decreasing. The maximum luminosity for a given mass is reached for an intermediate H-profile slope, corresponding to a partially stripped structure, exceeding even the values documented for pure-He stars; this is primarily due to the H shell disproportionately dominating the total luminosity budget. We also provide convenient mass-luminosity fit relations to predict the minimum, maximum, and pure-He luminosities for a given mass (and vice versa), while accounting for structures achievable through partial stripping. We have also explored the impact of the higher luminosity on the wind properties of partially stripped configurations using hydrodynamically consistent atmosphere models.

Supernovae (SNe) are among the most energetic and transient events in the universe, offering crucial insights into stellar evolution, nucleosynthesis, and cosmic expansion. Optical observations have historically played a central role in the discovery, classification, and physical interpretation of SNe. In this review, we summarize recent progress in the optical study of SNe, with a focus on advancements in time-domain surveys and photometric and spectroscopic follow-up strategies. High-cadence optical monitoring is pivotal in capturing the diverse behaviors of SNe, from early-time emission to late-phase decline. Leveraging data from ARIES telescopes and national/international collaborations, we systematically investigate various SN types, including Type Iax, IIP/L, IIb, IIn/Ibn and Ib/c events. Our analysis includes light curve evolution and spectral diagnostics, providing insights into early emission signatures (e.g., shock breakout), progenitor systems, explosion mechanisms, and circumstellar medium (CSM) interactions. Through detailed case studies, we demonstrate the importance of both early-time and nebular-phase observations in constraining progenitor and CSM properties. This comprehensive approach underscores the importance of coordinated global efforts in time-domain astronomy to deepen our understanding of SN diversity. We conclude by discussing the challenges and opportunities for future optical studies in the era of wide-field observatories such as the Vera C. Rubin Observatory (hereafter Rubin), with an emphasis on detection strategies, automation, and rapid-response capabilities.

Galactic double white dwarfs will be prominent gravitational wave sources for the Laser Interferometer Space Antenna (LISA). While previous studies have primarily focused on formation scenarios in which binaries form and evolve in isolation, we present the first detailed study of the role of triple stellar evolution in forming the population of LISA double white dwarfs. We used the multiple stellar evolution code ( MSE ) to model the stellar evolution, binary interactions, and the dynamics of triple star systems and then used a Milky Way-like galaxy from the TNG50 simulations to construct a representative sample of LISA double white dwarfs. In our simulations, about $7 Galactic double white dwarfs in the LISA frequency bandwidth originate from triple systems, whereas ∼4 are in isolated binary stars. The properties of double white dwarfs formed in triples closely resemble those formed from isolated binaries, but we also find a small number of systems, ∼ O (10), that reach extreme eccentricities $(>0.9)$, a feature unique to the dynamical formation channels. Our population produces ∼ O (10^4) individually resolved double white dwarfs (from triple and binary channels) and an unresolved stochastic foreground below the level of the LISA instrumental noise. About $57,%$ of the double white dwarfs from triple systems retain a bound third star when entering the LISA frequency bandwidth. However, we expect the tertiary stars to be too distant to have a detectable imprint in the gravitational wave signal of the inner binary.

Abstract The James Webb Space Telescope has ushered in an era of abundant high-redshift observations of young stellar populations characterized by strong emission lines, motivating us to integrate nebular emission into the new Maraston stellar population model which incorporates the latest Geneva stellar evolutionary tracks for massive stars with rotation. We use the photoionization code Cloudy to obtain the emergent nebular continuum and line emission for a range of modelling parameters, then compare our results to observations on various emission line diagnostic diagrams. We carry out a detailed comparison with several other models in the literature assuming different input physics, including modified prescriptions for stellar evolution and the inclusion of binary stars, and find close agreement in the H$\rm \beta$, H$\rm \alpha$, [N II]λ6583, and [S II]λ6716, 6731 luminosities between the models. However, we find significant differences in lines with high ionization energies, such as He IIλ1640 and [O III]λ5007, due to large variations in the hard ionizing photon production rates. The models differ by a maximum of $\Delta \hat{Q}_{\rm [O III]\lambda 5007} \rm \approx 10^{44} \,\, s^{-1} \, {\rm M}_{\odot }^{-1}$, where these differences are mostly caused by the assumed stellar rotation and effective temperatures for the Wolf Rayet phase. Interestingly, rotation and uncorrected effective temperatures in our single star population models alone generate [O III] ionizing photon production rates higher than models including binary stars with ages between 1 to 6 Myr. These differences highlight the dependence of derived properties from SED fitting on the assumed model, as well as the sensitivity of predictions from cosmological simulations.

Stellar rotation is a fundamental parameter in studies of the formation and evolution of stars. However, large homogeneous catalogues of rotational velocities derived from high-resolution stellar spectra are still lacking. The main objective of this work is to determine the line-broadening parameter ( which is a proxy for the stellar rotational velocity, in a large sample of FGKM stars based on their ESO/FEROS spectra. All these stars were previously parameterised by the AMBRE Project. The line-broadening parameter was estimated by cross-correlating the FEROS spectra with binary masks, specifically chosen on the basis of the AMBRE stellar parameters. This methodology also relies on a specific calibration of a coupling constant between the rotational velocity and the width of the cross-correlation function. This fundamental step was performed by adopting the AMBRE grid of synthetic spectra. The derived were then validated using data from the literature, ground-based spectroscopic surveys, and Gaia/RVS. After analysing more than 5,000 FEROS spectra (including repeated spectra for several stars), we obtained the line-broadening coefficients for 2,584 stars covering the FGKM spectral types, any stellar gravity, and metallicities between the metal-poor up to sub-solar regimes. The mean relative uncertainty of this sample was found to be smaller than 8%. As expected, most stars were found to be slow rotators (below a few ̌units), in particular, cool dwarfs and giants. However, several hot dwarfs and high-luminosity stars with high- rates were identified, most of them not previously classified as fast rotators and/or affected by large macro-turbulent effects before the present work. The measured rotational broadening values are of high-quality and verified on the basis of literature comparisons. We publicly provide this catalogue of line-broadening parameters, including stellar atmospheric and quality parameters, for the analysed AMBRE/FEROS sources.

The Hyades cluster is key for the study of the rotational, activity, and chemical evolution of solar-like low-mass stars. We present quantitative surface-activity information for a sequence of 21 Hyades dwarf stars with effective temperatures 6160 K to 3780 K (all cooler than the red edge of the Li dip), rotation periods 5,d to 16,d, and normalized Rossby numbers ( between 0.14 to 0.54 with respect to the Sun (Ro(Sun)=1). High-resolution Stokes-V spectra and a least-squares deconvolution of thousands of spectral lines per spectrum were employed to measure the longitudinal surface magnetic field. We obtained the velocities, lithium abundances, metallicity, and chromospheric Ca ii infrared-triplet (IRT) fluxes from Stokes-I spectra. The average metallicity, +0.186±0.045 (rms), for our stars with T_ ̊m eff ≥ 4200,K agrees well with the metallicity in the recent literature. The lithium abundances A(Li) range from 95-times solar (A(Li)≈ +3.0) on the warm end of the sample to $1/25$ solar (A(Li)≈ -0.4) on the cool end. We confirm the tight relation of A(Li) with T_̊m eff and extend it to K--M stars with even lower Li abundances than previously measurable. A formal relation with rotational period and velocity in the sense of a higher Li abundance for faster rotators is present. Targets that rotate faster than v i of 6 ̨ms (P_ ̊m rot ≈ 8,d) appear to be Li saturated at A(Li)≈3.0,dex. The Ca ii IRT fluxes for our sample indicate (logarithmic) chromospheric radiative losses R^ IRT in the range -4.0 to -4.9 in units of the bolometric flux. These radiative losses are also related to T_̊m eff, P_̊m rot, and v i, but opposite to A(Li), in an inverse sense with higher radiative losses for the slower, that is, cooler rotators. Longitudinal magnetic field strengths were measured in the range zero to -100,G and $+150$,G with phase-averaged disk-integrated unsigned values łangle |B|̊angle of 15.4±3.6(rms),G for targets warmer than ≈5000,K and 91±61(rms),G for targets cooler than this. These unsigned field strengths are related to P_̊m rot, v i, and but in a dual-slope fashion. The short-period bona fide single M-target RSP,348 was found to be a double-lined spectroscopic binary with a classification dM3e+dM5e. We conclude that the dependence on Rossby number of the surface activity tracers A(Li), R^ IRT, and łangle |B|̊angle on our Hyades dwarf sequence primarily originates from convective motions, expressed by its turnover time, and only to a smaller and sometimes inverse extent from surface rotation and its related additional mixing.

PSR J1928+1815, the first recycled pulsar-helium (He) star binary discovered by the Five-hundred-meter Aperture Spherical radio Telescope, consists of a 10.55 ms pulsar and a companion star with mass 1-1.6,M_ in a 0.15-day orbit. Theoretical studies suggest that this system originated from a neutron star (NS) intermediate-mass or high-mass X-ray binary that underwent common envelope (CE) evolution, leading to the successful ejection of the giant envelope. The traditional view is that hypercritical accretion during the CE phase may have recycled the NS. However, the specific mechanism responsible for accelerating its spin period remains uncertain due to the complex processes involved in CE evolution. In this study, we investigate the influence of Roche lobe overflow (RLO) accretion that takes place prior to the CE phase on the spin evolution of NSs. Our primary objective is to clarify how this process affects the spin characteristics of pulsars. We utilized the stellar evolution code MESA and the binary population synthesis code BSE to model the formation and evolution of NS-He star binaries. We calculated the distributions of the orbital period, He star mass, NS spin period, and magnetic field for NS + He star systems in the Galaxy. Our results indicate that RLO accretion preceding the CE phase could spin up NSs to millisecond periods through super-Eddington accretion. Considering a range of CE efficiencies α_̊m CE from 0.3 to 3, we estimate the birthrate (total number) of NS + He star systems in our Galaxy to be 9.0times 10^ yr^-1 (626 systems) to 1.9times 10^ yr^-1 (2684 systems).

Abstract Star clusters are interesting laboratories to study star formation, single and binary stellar evolution, and stellar dynamics. We have used the exquisite data from Gaia’s Data Release 3 (DR3) to study 21 relatively rich and nearby open clusters with member numbers ( N cl ) > 500. We have developed a nonparametric method to identify cluster members. Our method works well for clusters located in both sparse and crowded environments and hence can be applied to a wide variety of star clusters. Since the member classification scheme does not make any assumptions on the expected distributions of potential cluster members, our method can identify members associated with clusters that are oddly shaped or have complex internal spatial or kinematic structures. In addition, since the membership determination does not depend on the proximity to any well-defined sequences on the color–magnitude diagram, this method easily identifies straggler members. Furthermore, for each of these clusters, we estimate essential cluster properties including age, metallicity, distance, and reddening using detailed Markov Chain Monte Carlo parameter estimation. We report the full posteriors for these important cluster properties for all clusters in our study.

Abstract GW231123 is a binary black hole (BBH) merger whose primary component lies within or above the pair-instability mass gap, while the secondary component falls within this gap. The standard theory of stellar evolution is significantly challenged by this event. We investigate the formation of candidate progenitors of GW231123 in Population III (Pop III) star clusters. We find that they could form through stellar mergers, BBH mergers, and mixed mergers. The mass distribution of these candidate progenitors covers the component masses of GW231123. Under our model assumptions, their predicted merger rate density spans the range of 0.001−0.26 Gpc −3 yr −1 , encompassing that of GW231123. These findings suggest that GW231123 may originate from Pop III star clusters. Furthermore, such candidate progenitors are expected to be detectable by future gravitational-wave detectors LISA/Taiji/TianQin/DECIGO/Cosmic Explorer/Einstein Telescope, which would provide valuable insights into the formation scenarios of events like GW231123.

Unexpected Stellar Chemistry as a Marker of Atypical Stellar Evolution?