arXiv subjects

Dorottya Szécsi

Publications and source records attributed to Dorottya Szécsi.

At least 19 recordsLinked to original sources

arXiv2026

Study of the Ultra-Long GRB 220627A: Exploring a Possible Blue Supergiant Collapsar Origin

Ultra-long gamma-ray bursts (ul-GRBs) challenge standard collapsar models because their extreme durations require central engines active for $\sim10^{3}-10^{4}$ s. While the prompt spectral evolution of typical GRBs is difficult to resolve due to their short durations, ul-GRBs provide a unique opportunity to study spectral evolution on extended timescales. We present a time-resolved spectroscopic study of GRB 220627A, one of the most luminous ul-GRBs known ($E_{\rm iso}\sim10^{54}$ erg, $T_{90}>1000$ s), consisting of two bright emission episodes separated by $\sim600$ s quiescent interval. Using a novel HOP algorithm-based segmentation method, we identify optimal time intervals for time-resolved spectroscopy. A Comptonized power-law (CPL) model well describes the prompt emission ($\sim 10$ keV-40 MeV) across all timescales. Fermi-GBM spectroscopy reveals a transition from a hard, slow-cooling synchrotron spectrum ($α_{\rm I}=-0.70$) during episode-I to a softer, fast-cooling synchrotron spectrum ($α_{\rm II}=-1.29$) during episode-II. This evolution closely resembles that of the magnetar-candidate GRB 091024A, highlighting a potential progenitor degeneracy based solely on spectral evolution. However, the extreme isotropic-equivalent energy exceeds the maximum energy available from magnetar spin-down, strongly favouring a blue supergiant (BSG) collapsar origin. The prolonged activity and multi-episode structure are naturally explained by sustained accretion through an extended stellar envelope. Our results demonstrate that ul-GRBs can display magnetar-like spectral evolution while being powered by BSG collapsars. GRB 220627A highlights the need for combining time-resolved spectroscopy with energetic constraints to distinguish between progenitor scenarios and understand the physical origin of ul-GRBs.

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arXiv2026

Spectral characteristics of fast rotating metal-poor massive stars

Low-metallicity massive stars are assumed to be progenitors of certain supernovae, gamma-ray bursts, and gravitational wave emitting mergers. These exotic phenomena contribute to their host galaxies through strong ionizing radiation and mechanical feedback. Here we investigate a certain type of very metal-poor (0.02 Zsun) hot massive single stars that rotate fast and evolve chemically homogeneously. Combining state-of-the-art theories of stellar evolution and stellar atmospheres modelling we predict synthetic spectra of these stars corresponding to different masses and evolutionary phases. The predicted spectra in early evolutionary phases is classified mainly as an early-O type giant or supergiant while in later evolutionary phases most of our model spectra are assigned to the WO-type spectral class. The Hubble Space Telescope's (HST) Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) program will enable us to compare our predicted spectra with observations of stars of similar nature (e.g., metal-poor stars in Sextant A).

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arXiv2025

Low-metallicity massive single stars with rotation. III. Source of ionization and C-IV emission in I Zw 18

Chemically homogeneously evolving stars have been proposed to account for several exotic phenomena, including gravitational-wave emissions and gamma-ray bursts. Here we study whether these stars can explain the metal-poor dwarf galaxy I Zwicky 18. We apply our synthetic spectral models from Paper II to (i) establish a classification sequence for these hot stars, (ii) predict the photonionizing flux and the strength of emission lines from a IZw18-like stellar population, and (iii) compare our predictions to available observations of this galaxy. Adding two new models computed with PoWR, we report (i) these stars to follow a unique sequence of classes: O->WN->WO (i.e. without ever being WC). From our population synthesis with standard assumptions, we predict that (ii) the source of the UV C-IV and other emission bumps is a couple dozen WO-type Wolf-Rayet stars (not WC as previously assumed) which are the result of chem. hom. evolution, while these, combined with the rest of the O-star population, account for the He-II ionizing flux and spectral hardness. Contrasting our results against published optical and UV data and accounting for different aperture sizes and spatial regions probed by the observations, we find that (iii) our models are highly consistent with them. Since our "massive Pop II stars" might just as well exist in early star-forming regions, our findings have implications for upcoming JWST surveys; and given that our results apply for binary populations too as long as the same fraction (10%) of the systems evolves chem. homogeneously, we conclude that the stellar progenitors of gravitational waves may very well exist today in IZw18.

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arXiv2024

Star clusters forming in a low metallicity starburst -- rapid self-enrichment by (very) massive stars

Stellar winds of massive ($\gtrsim9\,\mathrm{M_\odot}$) and very massive ($\gtrsim100\,\mathrm{M_\odot}$) stars may play an important role in the metal-enrichment during the formation of star clusters. With novel high-resolution hydrodynamical \textsc{griffin}-project simulations, we investigate the rapid recycling of stellar wind-material during the formation of massive star clusters up to $M_\mathrm{cluster}\sim2\times10^5\,\mathrm{M_\odot}$ in a low-metallicity dwarf galaxy starburst. The simulation realises new stars from a stellar initial mass function (IMF) between $0.08\,\mathrm{M_\odot}$ and $\sim400\,\mathrm{M_\odot}$ and follows stellar winds, radiation and supernova-feedback of single massive stars with evolution tracks. Star clusters form on timescales less than $\sim5$ Myr, and their supernova-material is very inefficiently recycled. Stellar wind-material, however, is trapped in massive clusters resulting in the formation of stars self-enriched in Na, Al, and N within only a few Myr. Wind-enriched (second population) stars can be centrally concentrated in the most massive clusters ($\gtrsim10^4\,\mathrm{M_\odot}$) and the locked wind-material increases approximately as $M_\mathrm{cluster}^{2}$. These trends resemble the characteristics of observed second population stars in globular clusters. We fit scaling relations to the log-normal distributed wind-mass fractions and extrapolate to possible globular cluster progenitors of $M_\mathrm{cluster}=10^7\,\mathrm{M_\odot}$ to investigate whether a dominant second population could form. This can only happen if the IMF is well sampled, single massive stars produce at least a factor of a few more enriched winds e.g. through a top-heavy IMF, and a significant fraction of the first population (unenriched) stars is lost during cluster evolution.

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arXiv2023

Formation of star clusters and enrichment by massive stars in simulations of low-metallicity galaxies with a fully sampled initial stellar mass function

We present new GRIFFIN project hydrodynamical simulations that model the formation of galactic star cluster populations in low-metallicity ($Z=0.00021$) dwarf galaxies, including radiation, supernova and stellar wind feedback of individual massive stars. In the simulations, stars are sampled from the stellar initial mass function (IMF) down to the hydrogen burning limit of $0.08$ M$_\odot$. Mass conservation is enforced within a radius of $1$ pc for the formation of massive stars. We find that massive stars are preferentially found in star clusters and follow a correlation set at birth between the highest initial stellar mass and the star cluster mass that differs from pure stochastic IMF sampling. With a fully sampled IMF, star clusters lose mass in the galactic tidal field according to mass-loss rates observed in nearby galaxies. Of the released stellar feedback, $60\%$ of the supernova material and up to $35\%$ of the wind material reside either in the hot interstellar medium (ISM) or in gaseous, metal enriched outflows. While stellar winds (instantaneously) and supernovae (delayed) start enriching the ISM right after the first massive stars form, the formation of supernova-enriched stars and star clusters is significantly delayed (by $>50$ Myr) compared to the formation of stars and star clusters enriched by stellar winds. Overall, supernova ejecta dominate the enrichment by mass, while the number of enriched stars is determined by continuous stellar winds. These results present a concept for the formation of chemically distinct populations of stars in bound star clusters, reminiscent of multiple populations in globular clusters.

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arXiv2023

Bringing Stellar Evolution & Feedback Together: Summary of proposals from the Lorentz Center Workshop, 2022

Stars strongly impact their environment, and shape structures on all scales throughout the universe, in a process known as ``feedback''. Due to the complexity of both stellar evolution and the physics of larger astrophysical structures, there remain many unanswered questions about how feedback operates, and what we can learn about stars by studying their imprint on the wider universe. In this white paper, we summarize discussions from the Lorentz Center meeting `Bringing Stellar Evolution and Feedback Together' in April 2022, and identify key areas where further dialogue can bring about radical changes in how we view the relationship between stars and the universe they live in.

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arXiv2022

A systematic study of super-Eddington layers in the envelopes of massive stars

The proximity to the Eddington luminosity has been attributed as the cause of several observed effects in massive stars. Computationally, if the luminosity carried through radiation exceeds the local Eddington luminosity in the low-density envelopes of massive stars, it can result in numerical difficulties, inhibiting further computation of stellar models. This problem is exacerbated by the fact that very few massive stars are observed beyond the Humphreys-Davidson limit, the same region in the Hertzsprung-Russell diagram where the aforementioned numerical issues relating to the Eddington luminosity occur in stellar models. One-dimensional stellar evolution codes have to use pragmatic solutions to evolve massive stars through this computationally difficult phase. In this work, we quantify the impact of these solutions on the evolutionary properties of massive stars. We used the stellar evolution code MESA with commonly used input parameters for massive stellar models to compute the evolution of stars in the initial mass range of 10-110 M$_\odot$ at one-tenth of solar metallicity. We find that numerical difficulties in stellar models with initial masses greater than or equal to 30 M$_\odot$ cause these models to fail before the end of core helium burning. Recomputing these models using the same physical inputs but three different pragmatic solutions to treat the numerical instability, we find that the maximum radial expansion achieved by stars can vary by up to 2000 R$_\odot$, while the remnant mass of the stars can vary by up to 14 M$_\odot$ between the sets. These differences can have implications on studies such as binary population synthesis.

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arXiv2022

Dust Grain Growth & Dusty Supernovae in Low-Metallicity Molecular Clouds

We present 3-D hydrodynamical models of the evolution of superbubbles powered by stellar winds and supernovae from young coeval massive star clusters within low metallicity ($Z = 0.02$Z$_{\odot}$), clumpy molecular clouds. We explore the initial stages of the superbubble evolution, including the occurrence of pair-instability and core-collapse supernovae. Our aim is to study the occurrence of dust grain growth within orbiting dusty clumps, and in the superbubble's swept-up supershell. We also aim to address the survival of dust grains produced by sequential supernovae. The model accounts for the star cluster gravitational potential and self-gravity of the parent cloud. It also considers radiative cooling (including that induced by dust) and a state-of-the-art population synthesis model for the coeval cluster. As shown before, a superbubble embedded into a clumpy medium becomes highly distorted, expanding mostly due to the hot gas streaming through low density channels. Our results indicate that in the case of massive ($\sim10^7$M$_{\odot}$) molecular clouds, hosting a super star cluster ($\sim5.6\times10^5$M$_{\odot}$), grain growth increments the dust mass at a rate $\sim4.8\times10^{-5}$M$_{\odot}$ yr$^{-1}$ during the first $2.5$Myr of the superbubble's evolution, while the net contribution of pair-instability and core-collapse supernovae to the superbubble's dust budget is $\sim1200$M$_{\odot} (M_{SC}/5.6\times10^{5}$M$_{\odot})$, where $M_{SC}$ is the stellar mass of the starburst. Therefore, dust grain growth and dust injection by supernovae lead to create, without invoking a top-heavy initial mass function, massive amounts of dust within low-metallicity star-forming molecular clouds, in accordance with the large dust mass present in galaxies soon after the onset of cosmic reionization.

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arXiv2022

Explaining the differences in massive star models from various simulations

The evolution of massive stars is the basis of several astrophysical investigations, from predicting gravitational-wave event rates to studying star-formation and stellar populations in clusters. However, uncertainties in massive star evolution present a significant challenge when accounting for these models' behaviour in stellar population studies. In this work, we present a comparison between five published sets of stellar models from the BPASS, BoOST, Geneva, MIST, and PARSEC simulations at near-solar metallicity. The different sets of stellar models have been computed using slightly different physical inputs in terms of mass-loss rates and internal mixing properties. Moreover, these models also employ various pragmatic methods to overcome the numerical difficulties that arise due to the presence of density inversions in the outer layers of stars more massive than 40 M$_\odot$. These density inversions result from the combination of inefficient convection in the low-density envelopes of massive stars and the excess of radiative luminosity to the Eddington luminosity. We find that the ionizing radiation released by the stellar populations can change by up to 18 percent, the maximum radial expansion of a star can differ between 100-1600 R$_\odot$, and the mass of the stellar remnant can vary up to 20 M$_\odot$ between the five sets of simulations. We conclude that any attempts to explain observations that rely on the use of models of stars more massive than 40 M$_\odot$ should be made with caution.

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arXiv2022

X-ray emission from star cluster winds in starburst galaxies

Inspired by the excess soft X-ray emission recently detected in Green Pea galaxies, we model the soft X-ray emission (0.5 - 2.0 keV) of hot gas from star cluster winds. By combining individual star clusters, we estimate the soft X-ray emission expected from the typically unresolved diffuse hot gas in starburst galaxies, devoid of competing emission from e.g., AGN or other unresolved point sources. We use stellar models of sub-solar metallicities (0.02 $Z_{\odot}$ and 0.4 $Z_{\odot}$), and take into account supernova explosions for massive stars. For lower metallicities, we find that stellar winds do not contribute significantly ($\lesssim 3$ % of the mechanical energy) to the observed soft X-ray emission of normal star forming galaxies. For higher metallicities and possibly also for larger proportions of massive star clusters in the simulated starburst galaxies, we reproduce well the observed correlation between star formation rate and X-ray luminosity previously reported in the literature. However, we find that no combination of model assumptions is capable of reproducing the substantial soft X-ray emission observed from Green Pea galaxies, indicating that other emission mechanisms (i.e. unusually large quantities of High-/Low-Mass X-ray Binaries, Ultra-Luminous X-ray sources, a modified initial mass function, Intermediate-Mass Black Holes, or AGN) are more likely to be responsible for the X-ray excess.

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arXiv2021

Bonn Optimized Stellar Tracks (BoOST). Simulated Populations of Massive and Very Massive Stars for Astrophysical Applications

Massive and very massive stars can play important roles in stellar populations by ejecting strong stellar winds and exploding in energetic phenomena. It is therefore imperative that their behavior be properly accounted for in synthetic model populations. We present nine grids of stellar evolutionary model sequences, together with finely resolved interpolated sequences and synthetic populations, of stars with 9-500 Msun and with metallicities ranging from Galactic metallicity down to 1/250 Zsun. The stellar models were computed with the Bonn evolutionary code with consistent physical ingredients, and covering core hydrogen- and core helium-burning phases. The interpolation and population synthesis were performed with our newly developed routine 'synStars'. Eight of the grids represent slowly rotating massive stars with a normal or classical evolutionary path, while one grid represents fast-rotating, chemically homogeneously evolving models. The grids contain data on stellar wind properties such as estimated wind velocity and kinetic energy of the wind, as well as common stellar parameters such as mass, radius, surface temperature, luminosity, mass-loss rate, and surface abundances of 34 isotopes. We also provide estimates of the helium and carbon-oxygen core mass for calculating the mass of stellar remnants. The Bonn Optimized Stellar Tracks (BoOST) project is published as simple tables that include stellar models, interpolated tracks, and synthetic populations. Covering the broadest mass and metallicity range of any published massive star evolutionary model sets to date, BoOST is ideal for further scientific applications such as star formation studies in both low- and high-redshift galaxies.

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arXiv2021

Detailed evolutionary models of massive contact binaries: I. Model grids and synthetic populations for the Magellanic Clouds

The majority of close massive binary stars with initial periods of a few days experience a contact phase, in which both stars overflow their Roche lobes simultaneously. We perform the first dedicated study of the evolution of massive contact binaries and provide a comprehensive prediction of their observed properties. We compute 2790 detailed binary models for the Large and Small Magellanic Clouds each, assuming a conservative mass transfer. The initial parameter space for both grids span total masses from 20 to 80$\,\textrm{M}_{\odot}$, orbital periods of 0.6 to 2 days and mass ratios of 0.6 to 1.0. We find that models that remain in contact over nuclear timescales evolve towards equal masses, echoing the mass ratios of their observed counterparts. Ultimately, the fate of our nuclear-timescale models is to merge on the main sequence. Our predicted period-mass ratio distributions of O-type contact binaries are similar for both galaxies, and we expect 10 such systems together in both Magellanic Clouds. While we can largely reproduce the observed distribution, we over-estimate the population of equal-mass contact binaries. This situation is somewhat remedied if we also account for binaries that are approaching contact. Our theoretical distributions work particularly well for contact binaries with periods $<$2 days and total masses $\lessapprox45\,\textrm{M}_{\odot}$. We expect stellar winds, non-conservative mass transfer and envelope inflation to have played a role in the formation of the more massive and longer-period contact binaries.

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arXiv2020

The fates of massive stars: exploring uncertainties in stellar evolution with METISSE

In the era of advanced electromagnetic and gravitational wave detectors, it has become increasingly important to effectively combine and study the impact of stellar evolution on binaries and dynamical systems of stars. Systematic studies dedicated to exploring uncertain parameters in stellar evolution are required to account for the recent observations of the stellar populations. We present a new approach to the commonly used Single-Star Evolution (SSE) fitting formulae, one that is more adaptable: Method of Interpolation for Single Star Evolution (METISSE). It makes use of interpolation between sets of pre-computed stellar tracks to approximate evolution parameters for a population of stars. We have used METISSE with detailed stellar tracks computed by the Modules for Experiments in Stellar Astrophysics (MESA), Bonn Evolutionary Code (BEC) and Cambridge STARS code. METISSE better reproduces stellar tracks computed using the STARS code compared to SSE, and is on average three times faster. Using stellar tracks computed with MESA and BEC, we apply METISSE to explore the differences in the remnant masses, the maximum radial expansion, and the main-sequence lifetime of massive stars. We find that different physical ingredients used in the evolution of stars, such as the treatment of radiation dominated envelopes, can impact their evolutionary outcome. For stars in the mass range 9 to 100 M$_\odot$, the predictions of remnant masses can vary by up to 20 M$_\odot$, while the maximum radial expansion achieved by a star can differ by an order of magnitude between different stellar models.

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arXiv2019

The impact of pair-instability mass loss on the binary black hole mass distribution

A population of binary black hole mergers has now been observed in gravitational waves by Advanced LIGO and Virgo. The masses of these black holes appear to show evidence for a pile-up between $30$--$45$ $M_\odot$ and a cut-off above $\sim 45$ $M_\odot$. One possible explanation for such a pile-up and subsequent cut-off are pulsational pair-instability supernovae (PPISNe) and pair-instability supernovae (PISNe) in massive stars. We investigate the plausibility of this explanation in the context of isolated massive binaries. We study a population of massive binaries using the rapid population synthesis software COMPAS, incorporating models for PPISNe and PISNe. Our models predict a maximum black hole mass of $40$ $M_\odot$. We expect $\sim 10$\% of all binary black hole mergers at redshift z = 0 will include at least one component that went through a PPISN (with mass $30$--$40$ $M_\odot$), constituting $\sim 20$--$50$\% of binary black hole mergers observed during the first two observing runs of Advanced LIGO and Virgo. Empirical models based on fitting the gravitational-wave mass measurements to a combination of a power law and a Gaussian find a fraction too large to be associated with PPISNe in our models. The rates of PPISNe and PISNe track the low metallicity star formation rate, increasing out to redshift $z = 2$. These predictions may be tested both with future gravitational-wave observations and with observations of superluminous supernovae.

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arXiv2019

The effect of the metallicity-specific star formation history on double compact object mergers

We investigate the impact of uncertainty in the metallicity-specific star formation rate over cosmic time on predictions of the rates and masses of double compact object mergers observable through gravitational waves. We find that this uncertainty can change the predicted detectable merger rate by more than an order of magnitude, comparable to contributions from uncertain physical assumptions regarding binary evolution, such as mass transfer efficiency or supernova kicks. We statistically compare the results produced by the COMPAS population synthesis suite against a catalog of gravitational-wave detections from the first two Advanced LIGO and Virgo observing runs. We find that the rate and chirp mass of observed binary black hole mergers can be well matched under our default evolutionary model with a star formation metallicity spread of $0.39$ dex around a mean metallicity $\left $ that scales with redshift $z$ as $\left =0.035 \times 10^{-0.23 z}$, assuming a star formation rate of $0.01 \times (1+z)^{2.77} / (1+((1+z)/2.9)^{4.7}) \, \rm{M}_\odot$ Mpc$^{-3}$ yr$^{-1}$. Intriguingly, this default model predicts that 80\% of the approximately one binary black hole merger per day that will be detectable at design sensitivity will have formed through isolated binary evolution with only dynamically stable mass transfer, i.e., without experiencing a common-envelope event.

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arXiv2018

Role of supergiants in the formation of globular clusters

Multiple stellar populations are observed in almost all globular-clusters, but the origin of this phenomenon is still debated. We investigate the role cool supergiants may have played. To do this, we combine two investigative methods: state-of-the-art massive stellar evolution and calculations of the hydrodynamic structure of the cluster-gas. This approach allows us to study how star-formation in young massive clusters depends on the energy- and mass-input of the first-generation of stars, while predicting the chemical composition of the second-generation. We find that the presence of massive (9-500 M$_{\odot}$) metal-poor supergiants in the young cluster leads to a star-formation episode within the first 4 Myr of the cluster's lifetime, that is, before the first core-collapse supernovae explode or the gas is expelled. The stellar winds accumulate in the cluster center, forming the second-generation there. Its composition is predicted to show variations in O & Na abundances, consistently with observations. The abundance of helium is, similarly to other scenarios involving massive stars, higher than what is referred from observations. Supposing dynamical removal of stars from the outskirts of the cluster, or applying a top-heavy initial-mass-function, we can predict a number ratio of the second-generation as high as 20-80%. The effect of metallicity is shown to be important, as the most luminous supergiants are only predicted at low-metallicity, thus limiting $-$ but not excluding $-$ the extent of a polluted second-generation at high-metallicity. These massive stars becoming black-holes suggests globular-clusters hosting gravitational-wave progenitors. Our scenario predicts a correlation between the mass of the cluster and the extent of the multiple population phenomenon.

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arXiv2018

On the formation history of Galactic double neutron stars

Double neutron stars (DNSs) have been observed as Galactic radio pulsars, and the recent discovery of gravitational waves from the DNS merger GW170817 adds to the known DNS population. We perform rapid population synthesis of massive binary stars and discuss model predictions, including formation rates, mass distributions, and delay time distributions. We vary assumptions and parameters of physical processes such as mass transfer stability criteria, supernova kick distributions, remnant mass distributions and common-envelope energetics. We compute the likelihood of observing the orbital period-eccentricity distribution of the Galactic DNS population under each of our population synthesis models, allowing us to quantitatively compare the models. We find that mass transfer from a stripped post-helium-burning secondary (case BB) onto a neutron star is most likely dynamically stable. We also find that a natal kick distribution composed of both low (Maxwellian $σ=30\rm~km~s^{-1}$) and high ($σ=265\rm~km~s^{-1}$) components is preferred over a single high-kick component. We find that the observed DNS mass distribution can place strong constraints on model assumptions.

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arXiv2017

Supergiants and their shells in young globular clusters

Anomalous surface abundances are observed in a fraction of the low-mass stars of Galactic globular clusters, that may originate from hot-hydrogen-burning products ejected by a previous generation of massive stars. We present and investigate a scenario in which the second generation of polluted low-mass stars can form in shells around cool supergiant stars within a young globular cluster. Simulations of low-Z massive stars (M$_{\rm i}\sim$ 150$-$600 M$_{\odot}$) show that both core-hydrogen-burning cool supergiants and hot ionizing stellar sources are expected to be present simulaneously in young globular clusters. Under these conditions, photoionization-confined shells form around the supergiants. We find that the shell is gravitationally unstable on a timescale that is shorter than the lifetime of the supergiant, and the Bonnor-Ebert mass of the overdense regions is low enough to allow star formation. Since the low-mass stellar generation formed in this shell is made up of the material lost from the supergiant, its composition necessarily reflects the composition of the supergiant wind. We show that the wind contains hot-hydrogen-burning products, and that the shell-stars therefore have very similar abundance anomalies that are observed in the second generation stars of globular clusters. Considering the mass-budget required for the second generation star-formation, we offer two solutions. Either a top-heavy initial mass function is needed with an index of $-$1.71..$-$2.07. Alternatively, we suggest the shell-stars to have a truncated mass distribution, and solve the mass budget problem by justifiably accounting for only a fraction of the first generation. Even without forming a photoionizaton-confined shell, the cool supergiant stars predicted at low-Z could contribute to the pollution of the interstellar medium of the cluster from which the second generation was born.

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