In this context, the recent report1 of an approximately 70-solar-mass (M®) black hole in the galactic binary system LB-1 challenges conventional theories of massive-star evolution, stellar winds and ...core-collapse supernovae, thus requiring a more exotic scenario to explain the existence and properties of this system2,3. ...we used a barycentre method as well as a bisector method using an identical mask as that in Liu et al.1, to estimate the apparent radial-velocity shift resulting from the combined Ha profile, obtaining similar results using both methods. ...there is no evidence for a large mass ratio, and hence also no evidence for a large absolute mass of a black hole. Data availability The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. https://doi.org/10.1038/s41586-020-2216-x Received: 6 December 2019 Accepted: 27 February 2020 Published online: 29 April 2020 Acknowledgements We acknowledge support from the Fonds Wetenschappelijk Onderzoek (FWO, Research Foundation Flanders) under project IDs G0F8H6N, G0B3818N, 12ZY520N, G0H5416N and GST-D6267-I002519N, and from the Onderzoeksraad (Research Council), KU Leuven under project IDs C16/17/007, C16/18/005 and C14/17/082.
ABSTRACT
Upsilon Sagittarii is a hydrogen-deficient binary that has been suggested to be in its second stage of mass transfer, after the primary has expanded to become a helium supergiant following ...core helium exhaustion. A tentative identification of the faint companion in the ultraviolet led to mass estimates of both components that made the helium star in Upsilon Sagittarii a prototypical immediate progenitor of a type Ib/c supernova. However, no consistent model for the complex spectrum has been achieved, casting doubt on this interpretation. In this study, we provide for the first time a composite spectral model that fits the ultraviolet data, and clearly identifies the companion as a rapidly rotating, slowly moving $\approx 7\, \mathrm{M}_\odot$ B-type star, unlike previously suggested. The stripped helium supergiant is less luminous than previous estimates, and with an estimated mass of $\lt 1\, \mathrm{M}_\odot$ is ruled out as a core-collapse supernova progenitor. We provide a detailed binary evolution scenario that explains the temperature and luminosity of the two components as well as the very low gravity (log g ≈ 1) and extreme hydrogen deficiency of the primary (atmospheric mass fraction XH, 1 ≈ 0.001). The best-fitting model is an intermediate-mass primary ($M_\mathrm{ZAMS,1} \approx 5\, \mathrm{M}_\odot$) with an initial orbital period of a few days, and a secondary that appears to have gained a significant amount of mass despite its high rotation. We conclude that Upsilon Sagittarii is a key system for testing binary evolution processes, especially envelope stripping and mass accretion.
Context.
The recent gravitational wave measurements have demonstrated the existence of stellar mass black hole binaries. It is essential for our understanding of massive star evolution to identify ...the contribution of binary evolution to the formation of double black holes.
Aims.
A promising way to progress is investigating the progenitors of double black hole systems and comparing predictions with local massive star samples, such as the population in 30 Doradus in the Large Magellanic Cloud (LMC).
Methods.
With this purpose in mind, we analysed a large grid of detailed binary evolution models at LMC metallicity with initial primary masses between 10 and 40
M
⊙
, and identified the model systems that potentially evolve into a binary consisting of a black hole and a massive main-sequence star. We then derived the observable properties of such systems, as well as peculiarities of the OB star component.
Results.
We find that ∼3% of the LMC late-O and early-B stars in binaries are expected to possess a black hole companion when stars with a final helium core mass above 6.6
M
⊙
are assumed to form black holes. While the vast majority of them may be X-ray quiet, our models suggest that these black holes may be identified in spectroscopic binaries, either by large amplitude radial velocity variations (≳50 km s
−1
) and simultaneous nitrogen surface enrichment, or through a moderate radial velocity (≳10 km s
−1
) and simultaneous rapid rotation of the OB star. The predicted mass ratios are such that main-sequence companions can be excluded in most cases. A comparison to the observed OB+WR binaries in the LMC, Be and X-ray binaries, and known massive black hole binaries supports our conclusion.
Conclusions.
We expect spectroscopic observations to be able to test key assumptions in our models, with important implications for massive star evolution in general and for the formation of double black hole mergers in particular.
ABSTRACT
The Humphreys–Davidson (HD) limit empirically defines a region of high luminosities (log10(L/L⊙) ≳ 5.5) and low effective temperatures ($T_{\rm eff} \lesssim 20 \, {\rm kK}$) on the ...Hertzsprung–Russell diagram in which hardly any supergiant stars are observed. Attempts to explain this limit through instabilities arising in near- or super-Eddington winds have been largely unsuccessful. Using modern stellar evolution, we aim to re-examine the HD limit, investigating the impact of enhanced mixing on massive stars. We construct grids of stellar evolution models appropriate for the Small Magellanic Cloud (SMC) and Large Magellanic Cloud (LMC), as well as for the Galaxy, spanning various initial rotation rates and convective overshooting parameters. Significantly enhanced mixing apparently steers stellar evolution tracks away from the region of the HD limit. To quantify the excess of overluminous stars in stellar evolution simulations, we generate synthetic populations of massive stars and make detailed comparisons with catalogues of cool ($T_\mathrm{eff} \le 12.5\, \mathrm{kK}$) and luminous (log10(L/L⊙) ≥ 4.7) stars in the SMC and LMC. We find that adjustments to the mixing parameters can lead to agreement between the observed and simulated red supergiant populations, but for hotter supergiants the simulations always overpredict the number of very luminous (log10(L/L⊙) ≥ 5.4) stars compared to observations. The excess of luminous supergiants decreases for enhanced mixing, possibly hinting at an important role mixing has in explaining the HD limit. Still, the HD limit remains unexplained for hotter supergiants.
ABSTRACT
Gaia BH1, the first quiescent black hole (BH) detected from Gaia data, poses a challenge to most binary evolution models: its current mass ratio is ≈0.1, and its orbital period seems to be ...too long for a post-common envelope system and too short for a non-interacting binary system. Here, we explore the hypothesis that Gaia BH1 formed through dynamical interactions in a young star cluster (YSC). We study the properties of BH-main sequence (MS) binaries formed in YSCs with initial mass 3 × 102–3 × 104 M⊙ at solar metallicity, by means of 3.5 × 104 direct N-body simulations coupled with binary population synthesis. For comparison, we also run a sample of isolated binary stars with the same binary population synthesis code and initial conditions used in the dynamical models. We find that BH-MS systems that form via dynamical exchanges populate the region corresponding to the main orbital properties of Gaia BH1 (period, eccentricity, and masses). In contrast, none of our isolated binary systems match the orbital period and MS mass of Gaia BH1. Our best-matching Gaia BH1-like system forms via repeated dynamical exchanges and collisions involving the BH progenitor star, before it undergoes core collapse. YSCs are at least two orders of magnitude more efficient in forming Gaia BH1-like systems than isolated binary evolution.
Context.
The star cluster R136 inside the Large Magellanic Cloud hosts a rich population of massive stars, including the most massive stars known. The strong stellar winds of these very luminous ...stars impact their evolution and the surrounding environment. We currently lack detailed knowledge of the wind structure that is needed to quantify this impact.
Aims.
Our goal is to observationally constrain the stellar and wind properties of the massive stars in R136, in particular the wind-structure parameters related to wind clumping.
Methods.
We simultaneously analyse optical and ultraviolet spectroscopy of 53 O-type and three WNh-stars using the F
astwind
model atmosphere code and a genetic algorithm. The models account for optically thick clumps and effects related to porosity and velocity-porosity, as well as a non-void interclump medium.
Results.
We obtain stellar parameters, surface abundances, mass-loss rates, terminal velocities, and clumping characteristics and compare them to theoretical predictions and evolutionary models. The clumping properties include the density of the interclump medium and the velocity-porosity of the wind. For the first time, these characteristics are systematically measured for a wide range of effective temperatures and luminosities.
Conclusions.
We confirm a cluster age of 1.0–2.5 Myr and derived an initial stellar mass of ≥250
M
⊙
for the most massive star in our sample, R136a1. The winds of our sample stars are highly clumped, with an average clumping factor of
f
cl
= 29 ± 15. We find tentative trends in the wind-structure parameters as a function of the mass-loss rate, suggesting that the winds of stars with higher mass-loss rates are less clumped. We compare several theoretical predictions to the observed mass-loss rates and terminal velocities and find that none satisfactorily reproduce both quantities. The prescription of Krtička & Kubát (2018) matches the observed mass-loss rates best.
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 internal mixing processes. From a set of 32 Very Large Telescope/FLAMES visual and eight Hubble Space Telescope/Cosmic Origins Spectrograph ultraviolet spectra, we used spectral disentangling to separate the primary and secondary components. Using a genetic algorithm wrapped around the NLTE model atmosphere and the 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 of the system environment.
The recently reported observation of VFTS 243 is the first example of a massive black-hole binary system with negligible binary interaction following black-hole formation. The black-hole mass ...(≈10M_{⊙}) and near-circular orbit (e≈0.02) of VFTS 243 suggest that the progenitor star experienced complete collapse, with energy-momentum being lost predominantly through neutrinos. VFTS 243 enables us to constrain the natal kick and neutrino-emission asymmetry during black-hole formation. At 68% confidence level, the natal kick velocity (mass decrement) is ≲10 km/s (≲1.0M_{⊙}), with a full probability distribution that peaks when ≈0.3M_{⊙} were ejected, presumably in neutrinos, and the black hole experienced a natal kick of 4 km/s. The neutrino-emission asymmetry is ≲4%, with best fit values of ∼0-0.2%. Such a small neutrino natal kick accompanying black-hole formation is in agreement with theoretical predictions.
Magnetars are highly magnetized neutron stars, the formation mechanism of which is unknown. Hot helium-rich stars with spectra dominated by emission lines are known as Wolf-Rayet stars. We observed ...the binary system HD 45166 using spectropolarimetry and reanalyzed its orbit using archival data. We found that the system contains a Wolf-Rayet star with a mass of 2 solar masses and a magnetic field of 43 kilogauss. Stellar evolution calculations indicate that this component will explode as a supernova, and that its magnetic field is strong enough for the supernova to leave a magnetar remnant. We propose that the magnetized Wolf-Rayet star formed by the merger of two lower-mass helium stars.
Editor’s summary
Magnetars are neutron stars with extremely strong magnetic fields, the origin of which is debated. One possibility is amplification of a magnetic field in the core of the parent star, which produces the neutron star during a supernova explosion. However, such fields have not been observed in stars that are about to explode. Shenar
et al
. spectropolarimetry to identify a high magnetic field on a Wolf-Rayet star, the exposed helium core of a star that has lost its outer layers of hydrogen. The mass of the Wolf-Rayet is high enough that it will produce a neutron star in a supernova, and the field is sufficiently strong to generate a magnetar during core collapse. —Keith T. Smith
Observations and models of a Wolf-Rayet star indicate that it will produce a magnetar when it explodes as a supernova.
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