Tassos Fragos

Extremely massive stars, with masses of 10 ³ –10 ⁴ M _⊙ , have been proposed as potential sources of the chemical enrichment associated with multiple populations in globular clusters. Early in the main sequence, these stars reach central temperatures of 65–75 MK, enabling nucleosynthesis through the CNO, NeNa, and MgAl cycles and producing abundance patterns similar to those observed in the so-called cluster’s second population. Among the leading formation channels for extremely massive stars are stellar mergers. We followed the formation of a ∼6700 M _⊙ star via 253 stellar mergers drawn from a direct N -body star cluster simulation with properties similar to the densest star clusters observed at redshift z ∼ 10. We evolved each star as a 1D stellar model and constructed the merger products from mixing and shock-heating prescriptions calibrated to 3D simulations of parabolic collisions. We find that entropy-based mixing efficiently transports nuclear-processed material from the convective core to the envelope, while merger-driven heating promotes chemical homogenization. As a result, the stellar surface becomes enriched with H-burning products, producing abundance patterns consistent with those observed in multiple stellar populations in globular clusters and supporting the role of extremely massive stars formed through mergers as potential sources of early chemical enrichment in proto-globular cluster-like conditions.

Context. The recently discovered “little red dots” identified by the James Webb Space Telescope represent a population of compact high-redshift sources whose observed properties have motivated models involving black holes embedded within optically thick gaseous envelopes. Aims. To investigate the rest-frame optical emission of little red dots, we modeled the formation and evolution of quasi-stars, i.e., stellar envelopes supported by the accretion luminosity onto a central black hole, originating from rapidly accreting proto-stars reaching the supermassive star regime (> 10 ⁴ M _⊙ ) and undergoing general relativistic instability. Methods. We computed stellar evolution models with net mass gain rates of 0.01, 0.1, and 1 M _⊙ /yr and metallicities of Z = 0–0.01. For the mass gain rates ≥0.1 M _⊙ /yr, stars remain nearly fully convective with T _eff ∼ 4000–9000 K. Results. The general relativistic instability leading to central black hole formation occurs at M _★ ∼ 3.5 × 10 ⁴ M _⊙ (6.8 × 10 ⁴ M _⊙ ) for Ṁ _acc = 0.1 M _⊙ /yr (1 M _⊙ /yr) at luminosities of L ∼ 10 ⁹ L _⊙ . The maximum lifetime of quasi-stars is estimated to be 10 ⁷ –10 ⁸ yr, under the assumption that the central black hole is capable of sustaining the surrounding envelope in a state of hydrostatic equilibrium until the envelope has been fully accreted (i.e., M _BH, max / M _QS = 1). This estimate is ∼100–1000 times longer than the quasi-star progenitor’s lifetime. In an environment that allows for rapid accretion independent of metallicity, the formation, evolution, and properties of quasi-stars are found to also be independent of metallicity. Conclusions. Comparing the luminosities of our models with those of little red dots at z < 4.5 ( L _bol ∼ 10 ^9.5 –10 ^11.5 L _⊙ ) yields quasi-star masses of 10 ^4.5 –10 ^6.5 M _⊙ . The observed minimum luminosity of ∼10 ^9.5 L _⊙ implies accretion rates ≳0.1 M _⊙ /yr for little red dot progenitors. Our models offer a framework supporting quasi-stars as the source of little red dot optical emission and provide insights into their lifetimes, composition, and progenitor environment as well as into their minimum and maximum observed luminosities.

Metallicity can play a significant role in massive binary evolution through its impact on the opacity within stellar interiors and wind-driven mass loss. In this work, we investigate how the double neutron star (DNS) delay time distribution (DTD) is shaped by the metallicity-dependent evolution of the helium star−NS progenitor system. Drawing from insights rooted in single and binary star physics, we argue that at a given metallicity, the stellar radius during the helium main sequence sets a lower limit on the size of the DNS orbit at birth. We then perform population synthesis with the detailed binary evolution code POSYDON to illustrate the resulting DTD across a range of metallicities. Our results indicate that, independent of the common envelope efficiency and reasonable natal kicks, the majority of DNS mergers across metallicities typically occur no earlier than ≃40 Myr after star formation and peak strongly between 80 and 250 Myr. Roughly 15% of DNSs merge within 80 Myr, which may explain r -process enrichment in environments with brief star formation histories, while ≳20% merge on delay times >1 Gyr, providing an explanation for short gamma-ray bursts in old, metal-poor galaxies. The shape of the DTD can be complex, with a metallicity-dependent split in the dominant formation channel imprinting a characteristic double-peaked structure. Although ideally oriented natal kicks can produce very short merging DNS, we find that the required kick magnitudes are inconsistent with observations. Our work has implications for assessing the contribution of DNS mergers to r -process enrichment and gamma-ray bursts/kilonovae transients across cosmic time.

Context. In isolated binary evolution, binary black hole (BBH) mergers are generally formed through stable mass transfer (SMT) or common envelope evolution. In recent years, the SMT channel has received significant attention due to detailed binary models showing increased mass transfer stability compared to previous studies. Aims. In this work, we perform a full zero-age-main-sequence to compact object merger analysis using detailed binary models at eight metallicities between 10 ⁻⁴ Z _⊙ and 2 Z _⊙ to self-consistently model the population properties of BBH mergers in the SMT channel, determined their progenitor initial conditional, and investigate the binary physics governing their formation and metallicity dependence. Methods. We used the population synthesis code POSYDON that incorporates detailed single-star and binary model grids to determine the population of BBH mergers from SMT. Using its extended grids of MESA binary models, we determined the essential physics in the formation of BBH mergers. Results. The SMT channel produces BBH mergers predominantly from systems with P _ZAMS ≤ 10 days. In these systems, both the initial mass transfer between two stars and the subsequent interaction between the remaining star and the first-born BH take place while the respective donor star is on the main sequence (Case A). We find a limited contribution from wider Case B or C systems. Without a natal kick, the SMT channel does not produce BBH mergers above Z > 0.2 Z _⊙ due to orbital widening from stellar wind mass loss. The primary BH mass distribution shows a strong dependence on metallicity, while the mass ratio prefers unity independent of metallicity due to mass ratio reversal. Additionally, the χ _eff distributions contain peaks at χ _eff = 0 and ∼0.15, of which the former disappears at high metallicities. A mass-scaled natal kick leave this subpopulation unchanged but introduce a low-mass, unequal mass ratio subpopulation that merges within the Hubble time due to their eccentricity.

Mass transfer between nondegenerate stars is a fundamental but still poorly understood process in binary evolution. The commonly used rotationally limited accretion prescription in detailed binary evolution simulations that account for stellar rotation generally yields low accretion efficiencies that are difficult to reconcile with several observational constraints. We present a physically motivated mass-accretion prescription in which accretion or decretion disks regulate the angular momentum transported to the accretor, thereby allowing for continued accretion at near-critical rotation. The accretion efficiency can be calculated from the conservation of the mass and the angular momentum of the disk. Analytical estimates show that the accretion efficiency depends on stellar rotation and mass ratio for direct-impact accretion, and additionally on stellar radius and orbital separation in the disk accretion regime. The overall mass-weighted accretion efficiencies are close to the values expected near the threshold rotation rate, where the accreted specific angular momentum declines sharply. Applying this model to binary evolution simulations, we find that rotationally limited accretion systematically underestimates Be-star masses in Be+subdwarf O/B-type star (sdOB) systems, whereas the disk–star coupling model can produce more-massive Be stars that are consistent with observations. The final binary component masses depend not only on accretion efficiency but also the core-envelope mass ratio, which itself depends sensitively on the assumed overshooting. We find that our new disk–star coupling model with reduced overshooting yields component masses for Be+sdOB systems that are in closer agreement with observations.

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.

Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that ET will have on domains as broad and diverse as fundamental physics, cosmology, early Universe, astrophysics of compact objects, physics of matter in extreme conditions, and dynamics of stellar collapse. We discuss how the study of extreme astrophysical events will be enhanced by multi-messenger observations. We highlight the ET synergies with ground-based and space-borne GW observatories, including multi-band investigations of the same sources, improved parameter estimation, and complementary information on astrophysical or cosmological mechanisms obtained combining observations from different frequency bands. We present advancements in waveform modeling dedicated to third-generation observatories, along with open tools developed within the ET Collaboration for assessing the scientific potentials of different detector configurations. We finally discuss the data analysis challenges posed by third-generation observatories, which will enable access to large populations of sources and provide unprecedented precision.

Mass transfer between non-degenerate stars is a fundamental but still poorly understood process in binary evolution. The commonly used rotationally limited accretion prescription in detailed binary evolution simulations that account for stellar rotation generally yields low accretion efficiencies that are difficult to reconcile with several observational constraints. We present a physically-motivated mass-accretion prescription in which accretion or decretion disks regulate the angular momentum transported to the accretor, thereby allowing for continued accretion at near-critical rotation. The accretion efficiency can be calculated from the conservation of the mass and the angular momentum of the disk. Analytical estimates show that the accretion efficiency depends on stellar rotation and mass ratio for direct impact accretion, and additionally on stellar radius and orbital separation in the disk accretion regime. The overall mass-weighted accretion efficiencies are close to the values expected near the threshold rotation rate, where the accreted specific angular momentum declines sharply. Applying this model to binary evolution simulations, we find that rotationally limited accretion systematically underestimates Be-star masses in Be+subdwarf O/B-type star (sdOB) systems, whereas the disk-star coupling model can produce more massive Be stars that are consistent with observations. The final binary component masses depend not only on accretion efficiency but also core-envelope mass ratio, which itself depends sensitively on the assumed overshooting. We find that our new disk-star coupling model with reduced overshooting yields component masses for Be+sdOB systems that are in closer agreement with observations.

With over two dozen detections in the Milky Way, double neutron stars (DNSs) provide a unique window into massive binary evolution. We use the POSYDON binary population synthesis code to model DNS populations and compare them to the observed Galactic sample. By tracing their origins to underlying single and binary star physics, we place constraints on the detailed evolutionary stages leading to DNS formation. Our study reveals a bifurcation within the well-known common envelope (CE) formation channel for DNSs, which naturally explains an observed split in the orbital periods of the Galactic systems. The two subchannels are defined by whether the donor star has a helium core (Case B mass transfer) or a carbon-oxygen core (Case C) at the onset of the CE, with only the helium core systems eventually merging due to gravitational-wave-modulated orbital decay. We find that across different treatments of the CE phase, the formation of DNSs through both subchannels requires either a generous core definition of ≃30% H-fraction or a high CE ejection efficiency of α _CE ≳ 1.2. By testing different supernova kick velocity models, we find that galactic DNSs are best reproduced using a prescription that favors low velocity kicks (≲50 km s ⁻¹ ), in agreement with previous studies. Furthermore, our models indicate that merging DNSs are born from a stripped progenitor with a median pre-supernova envelope mass ∼0.2 M _⊙ . Our results highlight the value of detailed evolutionary models for improving our understanding of exotic binary star formation.

Whether considering rare astrophysical events on cosmological scales or unresolved stellar populations, accurate models must account for the integrated contribution from the entire history of star formation upon which that population is built. Here, we describe the second version of POSYDON, an open-source binary population synthesis code based on extensive grids of detailed binary evolution models computed using the MESA code, which follows both stars’ structures as a binary system evolves through its complete evolution from the zero-age main sequence, through multiple phases of mass transfer and supernovae, to their death as compact objects. To generate synthetic binary populations, POSYDON uses advanced methods to interpolate between our large, densely spaced grids of simulated binaries. In our updated version of POSYDON, we account for the evolution of stellar binaries across a cosmological range of metallicities, extending from 10⁻⁴ to 2 Z _⊙, including grids specifically focused on the Small and Large Magellanic Clouds (0.2 and 0.45 Z _⊙). In addition to describing our model grids and detailing our methodology, we outline several improvements to POSYDON. These include the incorporation of single stars in stellar populations, a treatment for stellar mergers, and a careful modeling of “reverse-mass transferring” binaries in which a once-accreting star later becomes a donor star. Our simulations are focused on binaries with at least one high-mass component, such as those that host neutron stars and black holes, and we provide postprocessing methods to account for the cosmological evolution of metallicity and star formation as well as rate calculations for transient events.