arXiv subjects

Erika Korb

Publications and source records attributed to Erika Korb.

6 recordsLinked to original sources

arXiv2025

Optically thick winds of very massive stars suppress intermediate-mass black hole formation

Intermediate-mass black holes (IMBHs) are the link between stellar-mass and supermassive black holes. Gravitational waves have started unveiling a population of IMBHs in the $\sim 100-300 \, \mathrm{M_{\odot}}$ range. Here, we investigate the formation of IMBHs from non-rotating very massive stars (VMSs, $>100\,{} \mathrm{M_{\odot}}$). We calculate new VMS models that account for the transition from optically thin to optically thick winds, and study how this enhanced mass loss affects IMBH formation and the black hole mass function at intermediate and high metallicity ($Z=10^{-4}-0.02$). We show that optically thick winds suppress the formation of IMBHs from direct VMS collapse at metallicities $Z>0.001$, one order of magnitude lower than predicted by previous models. Our models indicate that the stellar progenitors of GW231123 must have had a metallicity $Z<0.002$, if the primary black hole formed via direct VMS collapse.

astro-ph.HE↗

arXiv2025

Metal-poor single Wolf-Rayet stars: The interplay of optically thick winds and rotation

The Small Magellanic Cloud (SMC) hosts 12 known Wolf-Rayet (WR) stars, seven of which are apparently single. Their formation is a challenge for current stellar evolution models because line-driven winds are generally assumed to be quenched at a metallicity of Z < 0.004. Here, we present a set of mesa models of single stars with zero-age main sequence masses of 20 - 80 Msun considering different initial rotation speeds ({\Omega} = 0 - 0.7 {\Omega}_c), metallicities (Z = 0.002 - 0.0045), and wind mass-loss models (optically thin and thick winds). We show that if we account for optically thick winds, fast rotating ({\Omega} = 0.6 {\Omega}_c) single metal-poor O-type stars (with M > 20 Msun) shed their envelope and become WR stars even at the low metallicity of the SMC. The luminosity, effective temperature, evolutionary timescale, surface abundance, and rotational velocity of our simulated WR stars are compatible to the WRs observed in the SMC. We speculate that this scenario can also alleviate the excess of giant stars across the Humphreys-Davidson limit. Our results have key implications for black hole masses, (pair instability) supernova explosions, and other observable signatures.

astro-ph.SR↗

arXiv2025

Impact of accretion-induced chemically homogeneous evolution on stellar and compact binary populations

In binary star systems, mass transfer can spin up the accretor, possibly leading to efficient chemical mixing and chemically quasi-homogeneous evolution (CHE). Here, we explore the effects of accretion-induced CHE on both stellar populations and their compact binary remnants with the state-of-the-art population synthesis code SEVN. We find that CHE efficiently enhances the formation of Wolf-Rayet stars (WRs) from secondary stars, which are spun-up by accretion, while simultaneously preventing their evolution into red supergiant stars (RSGs). Including CHE in our models increases the fraction of WRs in our stellar sample by nearly a factor of $\approx3$ at low metallicity ($Z=0.001$). WRs formed through CHE are, on average, more massive and luminous than those formed without CHE. Most WRs formed via CHE end their life as black holes. As a direct consequence, the CHE mechanism enhances the formation of binary black holes (BBHs) and black hole-neutron star (BHNS) systems, while simultaneously quenching the production of binary neutron stars (BNSs). However, CHE significantly quenches the merger rate of BBHs, BHNSs and BNSs at low metallicity ($Z\leq{}0.004$), because most binary compact objects formed via CHE have large orbital periods. For instance, the number of BBH and BHNS mergers decreases by one order of magnitude at $Z=0.004$ in the CHE model compared to the standard scenario. Finally, we find that secondary stars experiencing CHE frequently produce the most massive compact object in the binary system. In BHNSs, this implies that the black hole progenitor is the secondary star. Conversely, BBHs formed through accretion-induced CHE likely have asymmetric black hole components, but only a negligible fraction of these asymmetric systems ultimately merge within an Hubble time.

astro-ph.HE↗

arXiv2024

Wolf-Rayet -- compact object binaries as progenitors of binary compact objects

Binaries with a Wolf-Rayet star and a compact object (WR-COs), either a black hole (BH) or a neutron star (NS), have been proposed as possible progenitors for the binary compact object mergers (BCOs) observed with the gravitational wave (GW) detectors. In this work, we use the open-source population synthesis code SEVN to investigate the role of WR-COs as BCO progenitors. We consider an initial population of $5 \times 10^6$ binaries and we evolve it across 96 combinations of metallicities, common envelope efficiencies, core-collapse supernova models and natal kick distributions. We find that WR-COs are the progenitors of most BCOs, especially at high and intermediate metallicity. At $Z=0.02,\,{}0.014,$ and $0.0014$, more than $\gtrsim 99 \%$ of all the BCOs in our simulations evolved as WR-COs. At $Z = 0.00014$, inefficient binary-stripping lowers the fraction of BCOs with WR-CO progenitors to $\approx 83-95 \%$. Despite their key role in BCO production, only $\approx 5-30 \%$ of WR-COs end their life as BCOs. We find that Cyg X-3, the only WR-CO candidate observed in the Milky Way, is a promising BCO progenitor, especially if it hosts a BH. In our simulations, about $\approx 70-100 \%$ of the Cyg X-3 - like systems in the WR-BH configuration (BH mass $ \leq 10 \rm ~ M_\odot$) are BCO progenitors, in agreement with the literature. Future observations of WR-COs similar to Cyg X-3 may be the Rosetta stone to interpret the formation of BCOs.

astro-ph.HE↗

arXiv2024

The boring history of Gaia BH3 from isolated binary evolution

Gaia BH3 is the first observed dormant black hole (BH) with a mass of $\approx{30}$ M$_\odot$ and represents the first confirmation that such massive BHs are associated with metal-poor stars. Here, we explore the isolated binary formation channel for Gaia BH3 focusing on the old and metal-poor stellar population of the Milky Way halo. We use the MIST stellar models and our open-source population synthesis code SEVN to evolve $5.6 \times 10^8$ binaries exploring 20 sets of parameters. We find that systems like Gaia BH3 form preferentially from binaries initially composed of a massive star ($40-60$ M$_\odot$) and a low mass companion ($<1$ M$_\odot$) in a wide ($P>10^3$ days) and eccentric orbit ($e>0.6$). Such progenitor binary stars do not undergo any Roche-lobe overflow episode during their entire evolution, so that the final orbital properties of the BH-star system are determined at the core collapse of the primary star. Low natal kicks ($\lesssim$ 10~km/s) significantly favour the formation of Gaia BH3-like systems, but high velocity kicks up to $\approx 220$ km/s are also allowed. We estimate the formation efficiency for Gaia BH3-like systems in old ($t>10$ Gyr) and metal-poor ($Z<0.01$) populations to be $\sim 4 \times 10^{-8}$ M$_\odot^{-1}$ (for our fiducial model), representing $\sim 3\%$ of the whole simulated BH-star population. We expect up to $\approx 4000$ BH-star systems in the Galactic halo formed through isolated evolution, of which $\approx 100$ are compatible with Gaia BH3-like. Given the density profile of the Galactic halo we do not expect more than one at the observed distance of Gaia BH3. Our models show that, even if it was born inside a stellar cluster, Gaia BH3 is compatible with a primordial binary star that escaped from its parent cluster without experiencing significant dynamical interactions.

astro-ph.GA↗

arXiv2022

Compact object mergers: exploring uncertainties from stellar and binary evolution with SEVN

Population-synthesis codes are an unique tool to explore the parameter space of massive binary star evolution and binary compact object (BCO) formation. Most population-synthesis codes are based on the same stellar evolution model, limiting our ability to explore the main uncertainties. Here, we present the new version of the code SEVN, which overcomes this issue by interpolating the main stellar properties from a set of pre-computed evolutionary tracks. We describe the new interpolation and adaptive time-step algorithms of SEVN, and the main upgrades on single and binary evolution. With SEVN, we evolved $1.2\times10^9$ binaries in the metallicity range $0.0001\leq Z \leq 0.03$, exploring a number of models for electron-capture, core-collapse and pair-instability supernovae, different assumptions for common envelope, stability of mass transfer, quasi-homogeneous evolution and stellar tides. We find that stellar evolution has a dramatic impact on the formation of single and binary compact objects. Just by slightly changing the overshooting parameter ($\lambda_{\rm ov}=0.4,0.5$) and the pair-instability model, the maximum mass of a black hole can vary from $\approx{60}$ to $\approx{100}\ \mathrm{M}_\odot$. Furthermore, the formation channels of BCOs and the merger efficiency we obtain with SEVN show significant differences with respect to the results of other population-synthesis codes, even when the same binary-evolution parameters are used. For example, the main traditional formation channel of BCOs is strongly suppressed in our models: at high metallicity ($Z\gtrsim{0.01}$) only $<20$% of the merging binary black holes and binary neutron stars form via this channel, while other authors found fractions $>70$%. The local BCO merger rate density of our fiducial models is consistent with the most recent estimates by the LIGO--Virgo--KAGRA collaboration.

astro-ph.HE↗