arXiv subjects

Norbert Langer

Publications and source records attributed to Norbert Langer.

At least 73 records · Page 4Linked to original sources

arXiv2020

Supernovae Ib and Ic from the explosion of helium stars

Much difficulty has so far prevented the emergence of a consistent scenario for the origin of Type Ib and Ic supernovae (SNe). Here, we follow a heuristic approach by examining the fate of helium stars in the mass range 4 to 12Msun, which presumably form in interacting binaries. The helium stars are evolved using stellar wind mass loss rates that agree with observations, and which reproduce the observed luminosity range of galactic WR stars, leading to stellar masses at core collapse in the range 3-5.5Msun. We then explode these models adopting an explosion energy proportional to the ejecta mass, roughly consistent with theoretical predictions. We impose a fixed 56Ni mass and strong mixing. The SN radiation from 3 to 100d is computed self-consistently starting from the input stellar models using the time-dependent non-local thermodynamic equilibrium radiative-transfer code CMFGEN. By design, our fiducial models yield similar light curves, with a rise time of ~20d and a peak luminosity of ~10^42.2erg/s, in line with representative SNe Ibc. The less massive progenitors retain a He-rich envelope and reproduce the color, line widths, and line strengths of a representative sample of SNe Ib, while stellar winds remove most of the helium in more massive progenitors, whose spectra match typical SNe Ic in detail. The transition between the predicted Ib-like and Ic-like spectra is continuous, but it is sharp, such that the resulting models essentially form a dichotomy. Further models computed with varying explosion energy, 56Ni mass, and long-term power injection from the remnant show that a moderate variation of these parameters can reproduce much of the diversity of SNe Ibc. We conclude that stars stripped by a binary companion can account for the vast majority of ordinary SNe Ib and Ic, and that stellar wind mass loss is the key to remove the helium envelope in SN Ic progenitors. [abridged]

astro-ph.SR↗

arXiv2020

Internal circulation in tidally locked massive binary stars -- Consequences for double black hole formation

Steady-state circulation currents are predicted in tidally deformed binary stars, which are believed to be progenitors of double black-hole merger events. This work aims to quantitatively characterise the steady-state circulation currents in components of a tidally locked binary system and to explore the effects of such currents on numerical models. Previous results describing the circulation in a single rotating star and a binary star are used to deduce a new prescription for the internal circulation in tidally locked binaries. We explore the effect of this prescription numerically for binary systems with primary masses between 25 and 100 solar masses. When comparing circulation velocities in the radial direction for the single rotating star and binary star, it is found that the average circulation velocity in the binary star may be described as an enhancement to the circulation velocity in a single rotating star. This velocity enhancement is a simple function depending on the masses of the binary components and amounts to a factor of approximately two when the components have equal masses. It is found that the ehancement causes the formation of double helium stars through efficient mixing to occur for systems with higher initial orbital periods, lower primary masses and lower mass ratios, compared to the standard circulation scenario. Taking into account appropriate distributions for primary mass, initial period and mass ratio, models with enhanced mixing predict 2.4 times more double helium stars being produced in the parameter space than models without. We conclude that the effects of companion-induced circulation have strong implications for the formation of close binary black holes. Not only do the predicted detection rates increase but double black-hole systems with mass ratios as low as 0.8 may be formed when companion-induced circulation is taken into account.

astro-ph.SR↗

arXiv2020

Type Ia supernovae from non-accreting progenitors

Type Ia supernovae (SNe Ia) are manifestations of stars deficient of hydrogen and helium disrupting in a thermonuclear runaway. While explosions of carbon-oxygen white dwarfs are thought to account for the majority of events, part of the observed diversity may be due to varied progenitor channels. We demonstrate that helium stars with masses between $\sim$1.8 and 2.5 M$_{\odot}$ may evolve into highly degenerate, near-Chandrasekhar mass cores with helium-free envelopes that subsequently ignite carbon and oxygen explosively at densities $\sim(1.8-5.9)\times 10^{9}$g cm$^{-3}$. This happens either due to core growth from shell burning (when the core has a hybrid CO/NeO composition), or following ignition of residual carbon triggered by exothermic electron captures on $^{24}$Mg (for a NeOMg-dominated composition). We argue that the resulting thermonuclear runaways is likely to prevent core collapse, leading to the complete disruption of the star. The available nuclear energy at the onset of explosive oxygen burning suffices to create ejecta with a kinetic energy of $\sim$10$^{51}$ erg, as in typical SNe Ia. Conversely, if these runaways result in partial disruptions, the corresponding transients would resemble SN Iax events similar to SN 2002cx. If helium stars in this mass range indeed explode as SNe Ia, then the frequency of events would be comparable to the observed SN Ib/c rates, thereby sufficing to account for the majority of SNe Ia in star-forming galaxies.

astro-ph.SR↗

arXiv2020

Effects of close binary evolution on the main-sequence morphology of young star clusters

Star clusters are the building blocks of galaxies. They are composed of stars of nearly equal age and chemical composition, allowing us to use them as chronometers and as testbeds for gauging stellar evolution. It has become clear recently that massive stars are formed preferentially in close binaries, in which mass transfer will drastically change the evolution of the stars. This is expected to leave a significant imprint in the distribution of cluster stars in the Hertzsprung-Russell diagram. Our results, based on a dense model grid of more than 50,000 detailed binary-evolution calculations, indeed show several distinct, coeval main-sequence (MS) components, most notably an extended MS turnoff region, and a group of near-critical rotating stars that is spread over a large luminosity range on the red side of the classical MS. We comprehensively demonstrate the time evolution of the features in an animation, and we derive analytic expressions to describe these features. We find quantitative agreement with results based on recent photometric and spectroscopic observations. We conclude that while other factors may also be at play, binary evolution has a major impact on the MS morphology of young star clusters.

astro-ph.SR↗

arXiv2019

The Single Star Path to Be Stars

Be stars are rapidly rotating B main sequence stars, which show line emission due to an outflowing disc. By studying the evolution of rotating single star models, we can assess their contribution to the observed Be star populations. We identify the main effects which are responsible for single stars to approach critical rotation as functions of initial mass and metallicity, and predict the properties of populations of rotating single stars. We perform population synthesis with single star models of initial masses ranging between 3 and 30 solar masses, initial equatorial rotation velocities between 0 and 600 kms$^{-1}$ at compositions representing the Milky Way, Large and Small Magellanic Clouds. These models include efficient core-envelope coupling mediated by internal magnetic fields and correspond to the maximum efficiency of Be star production. We predict Be star fractions and the positions of fast rotating stars in the colour-magnitude diagram. We identify stellar wind mass-loss and the convective core mass fraction as the key parameters which determine the time dependance of the stellar rotation rates. Using empirical distributions of initial rotational velocities,our single star models can reproduce the trends observed in Be star fractions with mass and metallicity. However,they fail to produce a significant number of stars rotating very close to critical. We also find that rapidly rotating Be stars in the Magellanic Clouds should have significant surface nitrogen enrichments, which may be in conflict with abundance determinations of Be stars. Single star evolution may explain the high number of Be stars if 70 to 80% of critical rotationwould be sufficient to produce the Be phenomenon. However even in this case, the unexplained presence of many Be stars far below the cluster turn-off indicates the importance of the binary channel for Be star production.

astro-ph.SR↗

arXiv2019

Clues on the Origin and Evolution of Massive Contact Binaries: Atmosphere Analysis of VFTS 352

The massive O4.5 V + O5.5 V binary VFTS 352 in the Tarantula nebula is one of the shortest-period and most massive overcontact binaries known. Recent theoretical studies indicate that some of these systems could ultimately lead to the formation of gravitational waves via black hole binary mergers through the chemically homogeneous evolution pathway. By analyzing ultraviolet-optical phase-resolved spectroscopic data, we aim to constrain atmospheric and wind properties that could be later used to confront theoretical predictions from binary evolution. In particular, surface abundances are powerful diagnostics of the evolutionary status, mass transfer and the internal mixing processes. From a set of 32 VLT/FLAMES visual and 8 HST/COS ultraviolet spectra, we used spectral disentangling to separate the primary and secondary components. Using a genetic algorithm wrapped around the NLTE model atmosphere and spectral synthesis code FASTWIND, we perform an 11-parameter optimization to derive the atmospheric and wind parameters of both components, including the surface abundances of He, C, N, O and Si. We find that both components are hotter than expected compared to single-star evolutionary models indicating that additional mixing processes may be at play. However the derived chemical abundances do not show significant indications of mixing when adopting baseline values typical for the system environment.

astro-ph.SR↗

arXiv2019

Constraining mixing in massive stars in the Small Magellanic Cloud

Context. The evolution of massive stars is strongly influenced by internal mixing processes such as semiconvection, convective core overshooting, and rotationally induced mixing. None of these is currently well constrained. Aims. We investigate models for massive stars in the Small Magellanic Cloud (SMC). We aim to constrain the various mixing efficiencies by comparing model results to observations. Methods. We use the stellar evolution code MESA to compute more than 60 grids of detailed evolutionary models for stars with initial masses of 9 to 100 Msol, in each grid assuming different combinations of mixing efficiencies. Results. We find that for most of the combinations of the mixing efficiencies, models in a wide mass range spend core-helium burning either only as blue supergiants, or only as red supergiants. The latter case corresponds to models that maintain a shallow slope of the hydrogen/helium (H/He) gradient separating the core and the envelope of the models. Only a small part of the mixing parameter space leads to models that produce a significant number of blue and red supergiants, which both exist abundantly in the SMC. Interestingly, these models contain steep H/He gradients, as is required to understand the hot, hydrogen-rich Wolf-Rayet stars in the SMC. We find that unless it is very fast, rotation has a limited effect on the H/He profiles in our models. Conclusions. While we use specific implementations of the considered mixing processes, they comprehensively probe the two first order structural parameters: the core mass and the H/He gradient in the core-envelope interface. Our results imply that, in massive stars, the H/He gradients above the helium cores become very steep. Our model grids can be tested with future observational surveys of the massive stars in the SMC, and thereby help to considerably reduce the uncertainties in models of massive star evolution.

astro-ph.SR↗

arXiv2018

Progenitors of gravitational wave mergers: Binary evolution with the stellar grid-based code ComBinE

The first gravitational wave detections of mergers between black holes and neutron stars represent a remarkable new regime of high-energy transient astrophysics. The signals observed with LIGO-Virgo detectors come from mergers of extreme physical objects which are the end products of stellar evolution in close binary systems. To better understand their origin and merger rates, we have performed binary population syntheses at different metallicities using the new grid-based binary population synthesis code ComBinE. Starting from newborn pairs of stars, we follow their evolution including mass loss, mass transfer and accretion, common envelopes and supernova explosions. We apply the binding energies of common envelopes based on dense grids of detailed stellar structure models, make use of improved investigations of the subsequent Case BB Roche-lobe overflow and scale supernova kicks according to the stripping of the exploding stars. We demonstrate that all the double black hole mergers, GW150914, LVT151012, GW151226, GW170104, GW170608 and GW170814, as well as the double neutron star merger GW170817, are accounted for in our models in the appropriate metallicity regime. Our binary interaction parameters are calibrated to match the accurately determined properties of Galactic double neutron star systems, and we discuss their masses and types of supernova origin. Using our default values for the input physics parameters, we find a double neutron star merger rate of about 3.0 Myr^-1 for Milky-Way equivalent galaxies. Our upper limit to the merger-rate density of double neutron stars is R=400 yr^-1 Gpc^-3 in the local Universe (z=0).

astro-ph.SR↗

arXiv2018

IR nebulae around bright massive stars as indicators for binary interactions

Recent studies show that more than 70% of massive stars do not evolve as effectively single stars, but as members of interacting binary systems. The evolution of these stars is thus strongly altered compared to similar but isolated objects. We investigate the occurrence of parsec-scale mid-infrared nebulae around early-type stars. If they exist over a wide range of stellar properties, one possible overarching explanation is non-conservative mass transfer in binary interactions, or stellar mergers. For ~3850 stars (all OBA stars in the Bright Star Catalogue [BSC], Be stars, BeXRBs, and Be+sdO systems), we visually inspect WISE 22 $μ$m images. Based on nebular shape and relative position, we distinguish five categories: offset bow shocks structurally aligned with the stellar space velocity, unaligned offset bow shocks, and centered, unresolved, and not classified nebulae. In the BSC, we find that 28%, 13%, and 0.4% of all O, B, and A stars, respectively, possess associated infrared (IR) nebulae. Additionally, 34/234 Be stars, 4/72 BeXRBs, and 3/17 Be+sdO systems are associated with IR nebulae. Aligned or unaligned bow shocks result from high relative velocities between star and interstellar medium (ISM) that are dominated by the star or the ISM, respectively. About 13% of the centered nebulae could be bow shocks seen head- or tail-on. For the rest, the data disfavor explanations as remains of parental disks, supernova remnants of a previous companion, and dust production in stellar winds. The existence of centered nebulae also at high Galactic latitudes strongly limits the global risk of coincidental alignments with condensations in the ISM. Mass loss during binary evolution seems a viable mechanism for the formation of at least some of these nebulae. In total, about 29% of the IR nebulae (2% of all OBA stars in the BSC) may find their explanation in the context of binary evolution.

astro-ph.SR↗

arXiv2018

Related progenitor models for long-duration gamma ray bursts and Type Ic superluminous supernovae

We model the late evolution and mass loss history of rapidly rotating Wolf-Rayet stars in the mass range $5\,\rm{M}_{\odot}\dots 100\,\rm{M}_{\odot}$. We find that quasi-chemically homogeneously evolving single stars computed with enhanced mixing retain very little or no helium and are compatible with Type\,Ic supernovae. The more efficient removal of core angular momentum and the expected smaller compact object mass in our lower mass models lead to core spins in the range suggested for magnetar driven superluminous supernovae. Our more massive models retain larger specific core angular momenta, expected for long-duration gamma-ray bursts in the collapsar scenario. Due to the absence of a significant He envelope, the rapidly increasing neutrino emission after core helium exhaustion leads to an accelerated contraction of the whole star, inducing a strong spin-up, and centrifugally driven mass loss at rates of up to $10^{-2}\,\rm{M}_{\odot}~\rm{yr^{-1}}$ in the last years to decades before core collapse. Since the angular momentum transport in our lower mass models enhances the envelope spin-up, they show the largest relative amounts of centrifugally enforced mass loss, i.e., up to 25\% of the expected ejecta mass. Our most massive models evolve into the pulsational pair-instability regime. We would thus expect signatures of interaction with a C/O-rich circumstellar medium for Type~Ic superluminous supernovae with ejecta masses below $\sim 10\,\rm{M}_{\odot}$ and for the most massive engine-driven explosions with ejecta masses above $\sim 30\,\rm{M}_{\odot}$. Signs of such interaction should be observable at early epochs of the supernova explosion, and may be related to bumps observed in the light curves of superluminous supernovae, or to the massive circumstellar CO-shell proposed for Type~Ic superluminous supernova Gaia16apd.

astro-ph.SR↗

arXiv2018

Spectroscopic and physical parameters of Galactic O-type stars. III. Mass discrepancy and rotational mixing

Massive stars play a key role in the evolution of the Universe. Our goal is to compare observed and predicted properties of single Galactic O stars to identify and constrain uncertain physical parameters and processes in stellar evolution and atmosphere models. We used a sample of 53 objects with spectral types from O3 to O9.7. For 30 of these, we determined the main photospheric and wind parameters, using optical spectroscopy and applying the FASTWIND code. For the remaining objects, literature data, obtained by means of the CMFGEN code, were used instead. The properties of our sample were compared to published predictions based on two grids evolution models that include rotationally induced mixing. Within each luminosity class, we find a close correlation of N surface abundance and luminosity, and a stronger N enrichment in more massive and evolved O stars. Additionally, a correlation of the surface nitrogen and helium abundances is observed. The large number of nitrogen-enriched stars above ~30 solar masses argues for rotationally induced mixing as the most likely explanation. However, none of the considered models can match the observed trends correctly, especially in the high mass regime. We confirm mass discrepancy for objects in the low mass O-star regime. We conclude that the rotationally induced mixing of helium to the stellar surface is too strong in some of the models. We also suggest that present inadequacies of the models to represent the N enrichment in more massive stars with relatively slow rotation might be related to problematic efficiencies of rotational mixing. We are left with a picture in which invoking binarity and magnetic fields is required to achieve a more complete agreement of the observed surface properties of a population of massive main- sequence stars with corresponding evolutionary models.

astro-ph.SR↗

arXiv2018

Subsonic structure and optically thick winds from Wolf--Rayet stars

Wolf-Rayet star's winds can be so dense and so optically thick that the photosphere appears in the highly supersonic part of the outflow, veiling the underlying subsonic part of the star, and leaving the initial acceleration of the wind inaccessible to observations. We investigate the conditions and the structure of the subsonic part of the outflow of Galactic WR stars, in particular of the WNE subclass; our focus is on the conditions at the sonic point. We compute 1D hydrodynamic stellar structure models for massive helium stars adopting outer boundaries at the sonic point. We find that the outflows of our models are accelerated to supersonic velocities by the radiative force from opacity bumps either at temperatures of the order of 200kK by the Fe opacity bump or of the order of 50kK by the HeII opacity bump. For a given mass-loss rate, the conditions in the subsonic part of the outflow are independent from the detailed physical conditions in the supersonic part. The close proximity to the Eddington limit at the sonic point allows us to construct a Sonic HR diagram, relating the sonic point temperature to the L/M ratio and the stellar mass-loss rate, thereby constraining the sonic point conditions, the subsonic structure, and the stellar wind mass-loss rates from observations. The minimum mass-loss rate necessary to have the flow accelerated to supersonic velocities by the Fe opacity bump is derived. A comparison of the observed parameters of Galactic WNE stars to this minimum mass-loss rate indicates that their winds are launched to supersonic velocities by the radiation pressure arising from the Fe-bump. Conversely, models which do not show transonic flows from the Fe opacity bump form inflated envelopes. We derive an analytic criterion for the appearance of envelope inflation in the subphotospheric layers.

astro-ph.SR↗

arXiv2018

Wolf-Rayet stars in the Small Magellanic Cloud as testbed for massive star evolution

The majority of Wolf-Rayet (WR) stars represent the stripped cores of evolved massive stars who lost most of their hydrogen envelope. In low metallicity environments, such as the Small Magellanic Cloud (SMC), stellar winds are weaker and binary interaction is expected to dominate WR-star formation. However, the WR binary fraction appears to be ~40% at any metallicity. We use the recently determined physical properties of the twelve known SMC WR stars to explore their possible formation channels through comparisons with grids of SMC models, simulated with the detailed stellar evolution code MESA. These include models of rapidly rotating single stars, which experience (partial) chemically homogeneous evolution (CHE). We find that CHE is not able to account for the majority of the SMC WR stars. However, the apparently single WN star SMC AB12 and the double WR system SMC AB5 (HD 5980) appear consistent with this channel. We also analyze core helium burning stellar models assuming constant hydrogen gradients in their envelopes. We find a dichotomy in the envelope hydrogen gradients required to explain the observed temperatures of the SMC WR stars. Shallow gradients are found for the WR stars with O star companions, consistent with binary models where mass transfer occurs early, which is in agreement with their binary properties. On the other hand, much steeper hydrogen gradients are inferred for the group of hot apparently single WR stars. Since the hydrogen profiles in post main sequence models of massive stars become steeper with time, we conclude that these stars have likely been stripped by a companion during a phase of common envelope evolution. The companions, either main sequence stars or compact objects, are expected to still be present. A corresponding search might identify the first immediate double black hole binary progenitor with masses as high as those detected in GW150914.

astro-ph.SR↗

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.

astro-ph.SR↗

arXiv2017

Bridging the Gap: From Massive Stars to Supernovae

Almost since the beginning, massive stars and their resultant supernovae have played a crucial role in the Universe. These objects produce tremendous amounts of energy and new, heavy elements that enrich galaxies, encourage new stars to form and sculpt the shapes of galaxies we see today. The end of millions of years of massive star evolution and the beginning of hundreds or thousands of years of supernova evolution are separated by a matter of a few seconds, in which some of the most extreme physics found in the Universe causes the explosive and terminal disruption of the star. Key questions remain unanswered in both the studies of how massive stars evolve and the behaviour of supernovae, and it appears the solutions may not lie on just one side of the explosion or the other or in just the domain of the stellar evolution or the supernova astrophysics communities. The need to view massive star evolution and supernovae as continuous phases in a single narrative motivated the Theo Murphy international scientific meeting "Bridging the gap: from massive stars to supernovae" at Chicheley Hall in June 2016, with the specific purpose to simultaneously address the scientific connections between theoretical and observational studies of massive stars and their supernovae, through engaging astronomers from both communities.

astro-ph.SR↗

arXiv2017

Chemical abundances of fast-rotating massive stars. II. Interpretation and comparison with evolutionary models

Aims: Past observations of fast-rotating massive stars exhibiting normal nitrogen abundances at their surface have raised questions about the rotational mixing paradigm. We revisit this question thanks to a spectroscopic analysis of a sample of bright fast-rotating OB stars, with the goal of quantifying the efficiency of rotational mixing at high rotation rates. Method: Our sample consists of 40 fast rotators on the main sequence, with spectral types comprised between B0.5 and O4. We compare the abundances of some key element indicators of mixing (He, CNO) with the predictions of evolutionary models for single objects and for stars in interacting binary systems. Results: The properties of half of the sample stars can be reproduced by single evolutionary models, even in the case of probable or confirmed binaries that can therefore be true single stars in a pre-interaction configuration. The main problem for the rest of the sample is a mismatch for the [N/O] abundance ratio (we confirm the existence of fast rotators with a lack of nitrogen enrichment) and/or a high helium abundance that cannot be accounted for by models. Modifying the diffusion coefficient implemented in single- star models does not solve the problem as it cannot simultaneously reproduce the helium abundances and [N/O] abundance ratios of our targets. Since part of them actually are binaries, we also compared their chemical properties with predictions for post-mass transfer systems. We found that these models can explain the abundances measured for a majority of our targets, including some of the most helium-enriched, but fail to reproduce them in other cases. Our study thus reveals that some physical ingredients are still missing in current models.

astro-ph.SR↗

arXiv2017

Novel modelling of ultra-compact X-ray binary evolution - stable mass transfer from white dwarfs to neutron stars

Tight binaries of helium white dwarfs (He WDs) orbiting millisecond pulsars (MSPs) will eventually "merge" due to gravitational damping of the orbit. The outcome has been predicted to be the production of long-lived ultra-compact X-ray binaries (UCXBs), in which the WD transfers material to the accreting neutron star (NS). Here we present complete numerical computations, for the first time, of such stable mass transfer from a He WD to a NS. We have calculated a number of complete binary stellar evolution tracks, starting from pre-LMXB systems, and evolved these to detached MSP+WD systems and further on to UCXBs. The minimum orbital period is found to be as short as 5.6 minutes. We followed the subsequent widening of the systems until the donor stars become planets with a mass of ~0.005 Msun after roughly a Hubble time. Our models are able to explain the properties of observed UCXBs with high helium abundances and we can identify these sources on the ascending or descending branch in a diagram displaying mass-transfer rate vs. orbital period.

astro-ph.SR↗

arXiv2017

Ultra-luminous X-ray sources and neutron-star-black-hole mergers from very massive close binaries at low metallicity

Gravitational waves from the binary black hole (BH) merger GW150914 may enlighten our understanding of ultra-luminous X-ray sources (ULXs), as BHs>30Msun can reach luminosities>4x10^39 erg s^-1 without exceeding their Eddington limit. It is then important to study variations of evolutionary channels for merging BHs, which might instead form accreting BHs and become ULXs. It was recently shown that massive binaries with mass ratios close to unity and tight orbits can undergo efficient rotational mixing and evolve chemically homogeneously, resulting in a compact BH binary. We study similar systems by computing ~120000 detailed binary models with the MESA code covering a wide range of initial parameters. For initial mass ratios M2/M1~0.1-0.4, primaries >40Msun can evolve chemically homogeneously, remaining compact and forming a BH without undergoing Roche-lobe overflow. The secondary then expands and transfers mass to the BH, initiating a ULX phase. We predict that ~1 out of 10^4 massive stars evolves this way, and that in the local universe 0.13 ULXs per Msun yr^-1 of star-formation rate are observable, with a strong preference for low-metallicities. At metallicities log Z>-3, BH masses in ULXs are limited to 60Msun due to the occurrence of pair-instability supernovae which leave no remnant, resulting in an X-ray luminosity cut-off. At lower metallicities, very massive stars can avoid exploding as pair-instability supernovae and instead form BHs with masses above 130Msun, producing a gap in the ULX luminosity distribution. After the ULX phase, neutron-star-BH binaries that merge in less than a Hubble time are produced with a formation rate <0.2 Gpc^-3 yr^-1. We expect that upcoming X-ray observatories will test these predictions, which together with additional gravitational wave detections will provide strict constraints on the origin of the most massive BHs that can be produced by stars.

astro-ph.HE↗