Massive Binary Stars Research Articles

Detection Rate of Galaxy Cluster-lensed Stellar Binary Black Hole Mergers by the Third-generation Gravitational-wave Detectors

Gravitational waves (GWs) from stellar binary black hole (sBBH) mergers can be strongly gravitational lensed by intervening galaxies/galaxy clusters. Only a few works have investigated cluster-lensed sBBH mergers by adopting oversimplified models, while galaxy-lensed ones have been intensively studied. In this paper, we estimate the detection rate of cluster-lensed sBBH mergers with third-generation GW detectors and its dependence on the lens models. We adopt detailed modeling of galaxy cluster lenses by using the mock clusters in the Synthetic Sky Catalog for Dark Energy Science with LSST (cosmoDC2) and/or approximations of the pseudo-Jaffe profile or an eccentric Navarro–Frenk–White dark matter halo plus a bright central galaxy with singular isothermal sphere profile. Considering that the formation of sBBH mergers is dominated by the evolution of the massive binary star (EMBS) channel, we find that the detection rate of cluster-lensed sBBHs is ∼5–84 yr−1, depending on the adopted lens model and uncertainty in the merger rate density, which is about ∼13−2.0+28 yr−1 if adopting relatively more realistic galaxy clusters with central main and member galaxies in the cosmoDC2 catalog, close to the estimated detection rate of sBBH mergers lensed by galaxies. In addition, we also consider the case in which the production of sBBH mergers is dominated by the dynamical interactions in dense stellar systems. We find that the detection rate of cluster-lensed sBBHs from the dynamical channel is about 1.5 times larger than that from the EMBS channel, and the redshift distribution of the former peaks at a higher redshift (∼3) compared with that from the latter (∼2).

Evolutionary States and Triplicity of Four Massive Semidetached Binaries with Long-term Decreasing Orbital Periods in the LMC

The massive semidetached binary with a long-term decreasing orbital period may involve a rapid mass-transfer phase in Case A, and thus, they are good astrophysical laboratories for investigating the evolution of massive binary stars. In this work, by using the long-term observational light curves from the Optical Gravitational Lensing Experiment project and other data in the low-metallicity Large Magellanic Cloud, four semidetached massive binaries with long-term decreases in the orbital periods are detected from 165 EB-type close binaries. It is found that the more massive component in S07798 is filling its Roche lobe, where the period decrease is caused by mass transfer from the primary to the secondary. However, the other three (S03065, S12631, S16873) are semidetached binaries with a lobe-filling secondary where the mass transfer between the components should cause the period to increase if the angular momentum is conservative. The long-term period decreases in these three systems may be caused by angular momentum loss. Additionally, the orbital periods of three systems (S03065, S07798, S16873) are detected to show cyclic variation with periods shorter than 11 yr, which can be plausibly explained by the presence of close-in third bodies in these massive binaries. Based on all of these results, it is suggested that the detected four semidetached binaries almost have multiplicity. The companion stars are crucial for the origin and evolution of these massive close binaries.

Stable case BB/BC mass transfer to form GW190425-like massive binary neutron star mergers

Context. On April 25, 2019, the LIGO-Virgo Collaboration discovered a gravitational-wave (GW) signal from a binary neutron star (BNS) merger, that is, GW190425. Due to the inferred large total mass, the origin of GW190425 remains unclear. Aims. Assuming GW190425 originated from the standard isolated binary evolution channel, its immediate progenitor is considered to be a close binary system, consisting of a He-rich star and a NS just after the common envelope phase. We aim to study the formation of GW190425 in a solar-like environment by using the detailed binary evolution code MESA. Methods. We perform detailed stellar structure and binary evolution calculations that take into account mass loss, internal differential rotation, and tidal interactions between a He-rich star and a NS companion. We explore the parameter space of the initial binary properties, including initial NS and He-rich masses and initial orbital period. Results. We find that the immediate post-common-envelope progenitor system, consisting of a primary ∼2.0 M⊙ (∼1.7 M⊙) NS and a secondary He-rich star with an initial mass of ∼3.0 − 5.5 M⊙ (∼5.5 − 6.0 M⊙) in a close binary with an initial period of ∼0.08 − 0.5 days (∼0.08 − 0.4 days), that experiences stable Case BB/BC mass transfer (MT) during binary evolution, can reproduce the formation of GW190425-like BNS events. Our studies reveal that the secondary He-rich star of the GW190425’s progenitor before its core collapse can be efficiently spun up through tidal interaction, finally remaining as a NS with rotational energy even reaching ∼1052 erg, which is always much higher than the neutrino-driven energy of the supernova (SN) explosion. If the newborn secondary NS is a magnetar, we expect that GW190425 can be the remnant of a magnetar-driven SN, namely a magnetar-driven ultra-stripped SN, a superluminous SN, or a broad-line Type Ic SN. Conclusions. Our results show that GW190425 could be formed through the isolated binary evolution, which involves a stable Case BB/BC MT just after the common envelope phase. On top of that, we show the He-rich star can be tidally spun up, potentially forming a spinning magnetized NS (magnetar) during the second SN explosion.

Investigating the Cosmological Rate of Compact Object Mergers from Isolated Massive Binary Stars

Gravitational-wave (GW) detectors are observing compact object mergers from increasingly far distances, revealing the redshift evolution of the binary black hole (BBH)—and soon the black hole–neutron star (BHNS) and binary neutron star (BNS)—merger rate. To help interpret these observations, we investigate the expected redshift evolution of the compact object merger rate from the isolated binary evolution channel. We present a publicly available catalog of compact object mergers and their accompanying cosmological merger rates from population synthesis simulations conducted with the COMPAS software. To explore the impact of uncertainties in stellar and binary evolution, our simulations use two-parameter grids of binary evolution models that vary the common-envelope efficiency with mass transfer accretion efficiency and supernova (SN) remnant mass prescription with SN natal kick velocity, respectively. We quantify the redshift evolution of our simulated merger rates using the local (z ∼ 0) rate, the redshift at which the merger rate peaks, and the normalized differential rates (as a proxy for slope). We find that although the local rates span a range of ∼103 across our model variations, their redshift evolutions are remarkably similar for BBHs, BHNSs, and BNSs, with differentials typically within a factor 3 and peaks of z ≈ 1.2–2.4 across models. Furthermore, several trends in our simulated rates are correlated with the model parameters we explore. We conclude that future observations of the redshift evolution of the compact object merger rate can help constrain binary models for stellar evolution and GW formation channels.

The Evolution of Massive Binary Stars

Massive stars play a major role in the evolution of their host galaxies and serve as important probes of the distant Universe. It has been established that the majority of massive stars reside in close binaries and interact with their companion stars during their lifetimes. Such interactions drastically alter their life cycles and complicate our understanding of their evolution, but are also responsible for the production of interesting and exotic interaction products. ▪Extensive observation campaigns with well-understood detection sensitivities have enabled the conversion of observed properties into intrinsic characteristics, facilitating a direct comparison to theory.▪Studies of large samples of massive stars in our Galaxy and the Magellanic Clouds have unveiled new types of interaction products, providing critical constraints on the mass transfer phase and the formation of compact objects.▪The direct detection of gravitational waves has revolutionized the study of stellar mass compact objects, providing a new window to study massive star evolution. Their formation processes are, however, still unclear. The known sample of compact object mergers will increase by orders of magnitude in the coming decade, which is vastly outgrowing the number of stellar-mass compact objects detected through electromagnetic radiation.

X-Shooting ULLYSES: Massive stars at low metallicity. V. Effect of metallicity on surface abundances of O stars

Massive stars rotate faster, on average, than lower mass stars. Stellar rotation triggers hydrodynamical instabilities which transport angular momentum and chemical species from the core to the surface. Models of high-mass stars that include these processes predict that chemical mixing is stronger at lower metallicity. We aim to test this prediction by comparing the surface abundances of massive stars at different metallicities. We performed a spectroscopic analysis of single O stars in the Magellanic Clouds (MCs) based on the ULLYSES and XshootU surveys. We determined the fundamental parameters and helium, carbon, nitrogen, and oxygen surface abundances of 17 LMC and 17 SMC non-supergiant O6-9.5 stars. We complemented these determinations by literature results for additional MCs and also Galactic stars to increase the sample size and metallicity coverage. We investigated the differences in the surface chemical enrichment at different metallicities and compared them with predictions of three sets of evolutionary models. Surface abundances are consistent with CNO-cycle nucleosynthesis. The maximum surface nitrogen enrichment is stronger in MC stars than in Galactic stars. Nitrogen enrichment is also observed in stars with higher surface gravities in the SMC than in the Galaxy. This trend is predicted by models that incorporate chemical transport caused by stellar rotation. The distributions of projected rotational velocities in our samples are likely biased towards slow rotators. A metallicity dependence of surface abundances is demonstrated. The analysis of larger samples with an unbiased distribution of projected rotational velocities is required to better constrain the treatment of chemical mixing and angular momentum transport in massive single and binary stars.

Molecular Bubble and Outflow in S Mon Revealed by Multiband Data Sets

We identify a molecular bubble, and study the star formation and its feedback in the S Mon region, using multiple molecular lines, young stellar objects (YSOs), and infrared data. We revisit the distance to S Mon, ∼722 ± 9 pc, using Gaia Data Release 3 parallaxes of the associated Class II YSOs. The bubble may be mainly driven by a massive binary system (namely 15 Mon), the primary of which is an O7V-type star. An outflow is detected in the shell of the bubble, suggesting ongoing star formation activities in the vicinity of the bubble. The total wind energy of the massive binary star is 3 orders of magnitude higher than the sum of the observed turbulent energy in the molecular gas and the kinetic energy of the bubble, indicating that stellar winds help to maintain the turbulence in the S Mon region and drive the bubble. We conclude that the stellar winds of massive stars have an impact on their surrounding environment.

On the origin of outward migration of Population III stars

ABSTRACT Outward migration of massive binary stars or black holes in their circumbinary disc is often observed in simulations and it is key to the formation of wide black hole binaries. Using numerical simulations of Population III (Pop III) star formation, we study the angular momentum of Pop III binaries and the torques between stars and gas discs to understand the origin of outward migration and high ellipticity. The outward migration of protostars is produced by gravitational torques exerted on them by their circumstellar minidiscs. The minidiscs, on the other hand, migrate outward mainly by gaining angular momentum by accreting gas from the circumbinary disc. The angular momentum transfer is most efficient for rapidly accreting equal-mass binaries, and weaker when the secondary mass is small or the massive companion evaporates the gas disc via radiative feedback. We conclude that outward migration and the formation of wide equal-mass massive binaries is common in metal-free/metal-poor star formation, mainly driven by their large accretion rates. We expect that the lower gas temperature and accretion rates in metal-enriched circumstellar discs would lead more often to inward migration and closer binary separations. We also observe inward migration for smaller mass Pop III protostars/fragments, leading to the rapid merging of sink particles and likely the formation of close binary black holes that, however, reach separations below the resolution of our simulations. We discuss the implications that Pop III separations and ellipticity may have on the interpretation that gravitational wave signals from merging intermediate-mass black holes come from Pop III remnants.

How Low Can You Go: Exploring the Extreme Mass Ratio of Massive Binary Stars

Massive stars burn bright and die young, and yet play a vital role in the evolution of our Universe. Massive stars are primarily found with companions in binary or multiple systems, but observational bias has obstructed the detection of extreme mass ratio binaries. With the advancement of high spatial resolution instrumentation, we are now poised to explore the lower end of the binary mass ratio, consisting of sub-solar, pre-main sequence stars. Here I present an analysis of seven massive stars in the young, active star-forming region M17 using data from the Spectro-Polarimetric High contrast Exoplanet REsearch (SPHERE) instrument on the Very Large Telescope. A radial velocity study of M17 by Ramirez–Tannus et al. (2017, DOI:10.1051/0004-6361/201629503) found fewer binary systems than expected. Our study is complementary to the previous work in the region to search for wide companions to massive stars in order to fill in our picture of massive star formation and the effect of multiple systems on the dynamical history of the region. SPHERE is uniquely capable of probing the lower end of the binary mass ratio due to its ground-breaking extreme adaptive optics and coronagraphic capabilities which allow us to achieve greater contrast ratios than traditional adaptive optics. We utilized the high spatial resolution imaging techniques of SPHERE in order to resolve binary systems with contrast ratios of up to 10 mag in the infrared and detect stellar companions to massive stars in the subsolar mass ranges. We present simultaneous dual-band imaging and integral-field spectroscopy of massive stars ranging from O4V–O9V. From a preliminary inspection, we found potential companions for all seven target stars. We measured flux contrast, position angles, and angular separation of our systems. Utilizing the Vortex Image Processing pipeline, we applied the negative fake companion technique in order to characterize the companions with differential magnitudes. Using this analysis, we will determine if the detected companions are foreground or field objects and estimate their mass ratio.

A CHIME/FRB Study of Burst Rate and Morphological Evolution of the Periodically Repeating FRB 20180916B

FRB 20180916B is a repeating fast radio burst (FRB) with a 16.3 day periodicity in its activity. In this study, we present morphological properties of 60 FRB 20180916B bursts detected by CHIME/FRB between 2018 August and 2021 December. We recorded raw voltage data for 45 of these bursts, enabling microseconds time resolution in some cases. We studied variation of spectro-temporal properties with time and activity phase. We find that the variation in dispersion measure (DM) is ≲1 pc cm−3 and that there is burst-to-burst variation in scattering time estimates ranging from ∼0.16 to over 2 ms, with no discernible trend with activity phase for either property. Furthermore, we find no DM and scattering variability corresponding to the recent change in rotation measure from the source, which has implications for the immediate environment of the source. We find that FRB 20180916B has thus far shown no epochs of heightened activity as have been seen in other active repeaters by CHIME/FRB, with its burst count consistent with originating from a Poissonian process. We also observe no change in the value of the activity period over the duration of our observations and set a 1σ upper limit of 1.5 × 10−4 day day−1 on the absolute period derivative. Finally, we discuss constraints on progenitor models yielded by our results, noting that our upper limits on changes in scattering and DM as a function of phase do not support models invoking a massive binary companion star as the origin of the 16.3 day periodicity.

Unusually Powerful Flare Phenomenon of the Water Maser in W51 and the Possibility of Detecting Gravitational Radiation from It

Because of detailed monitoring of the 22.2 GHz water maser, carried out from 2021 October to 2023 May, a very powerful flare phenomenon was detected in the galactic object W51 near the radial velocity of 60 km s−1 with the amplitude of 140 kJy. A phenomenon of this magnitude was unprecedented in the entire history of observations of the source. Eleven short flares were recorded. The exponential increase and decrease in the flare flux density while reducing in their spectral line widths indicated that water masers were in an unsaturated state during the flares. All flares were located at the top of the less powerful Flare 0 with the amplitude of 13.5 kJy and the spectral line half-width of 3.0 km s−1. Such a wide line of the water maser, as well as the amplitude, of the flare phenomenon are so far unique discoveries. The water maser of Flare 0 may have been saturated and created a significant input flux density for other flares of this phenomenon. The extremely high density of maser spots in a cluster led to their partial overlap on the observer’s line of sight. This also confirmed the hypothesis about the need for a significant length of the path along which the generation of maser radiation occurs. New parameters of water masers and the most important physical conclusions have been obtained. The possibility of detecting gravitational waves from massive binary stars at the stage of evolution close to merging is considered for the case of W51 Main.

The B-type Binaries Characterisation Programme – II. VFTS 291: a stripped star from a recent mass transfer phase

ABSTRACT Recent studies of massive binaries with putative black hole companions have uncovered a phase of binary evolution that has not been observed before, featuring a bloated stripped star that very recently ceased transferring mass to a main-sequence companion. In this study, we focus on the candidate system VFTS 291, a binary with an orbital period of 108 d and a high semi-amplitude velocity (K1 = 93.7 ± 0.2 km s−1). Through our analysis of the disentangled spectra of the two components, together with dynamical and evolutionary arguments, we identify a narrow-lined star of ∼1.5–2.5 $\, \mathrm{M}_\odot$ dominating the spectrum, and an early B-type main-sequence companion of 13.2 ± 1.5 $\, \mathrm{M}_\odot$. The low mass of the narrow-lined star, and the high mass ratio, suggest that VFTS 291 is a post-mass-transfer system, with the narrow-lined star being bloated and stripped of its hydrogen-rich envelope, sharing many similarities with other recently discovered stripped stars. Our finding is supported by our detailed binary evolution models, which indicate that the system can be well explained by an initial configuration consisting of an 8.1 $\, \mathrm{M}_\odot$ primary with an 8 $\, \mathrm{M}_\odot$ companion in a 7 d orbital period. While some open questions remain, particularly concerning the surface helium enrichment of the stripped star and the rotational velocity of the companion, we expect that high-resolution spectroscopy may help reconcile our estimates with theory. Our study highlights the importance of multi-epoch spectroscopic surveys to identify and characterize binary interaction products, and provides important insights into the evolution of massive binary stars.

Event Rate of Strongly Lensed Gravitational Waves of Stellar Binary Black Hole Mergers Produced by Dynamical Interactions

Gravitational waves emitted from stellar binary black hole (sBBH) mergers can be gravitationally lensed by intervening galaxies and detected by future ground-based detectors. A great amount of effort has been put into the estimation of the detection rate of lensed sBBH originating from the evolution of massive binary stars (EMBS channel). However, sBBHs produced by the dynamical interaction in dense clusters (dynamical channel) may also be dominant in our universe and their intrinsic distribution of physical properties can be significantly different from those produced by massive stars, especially mass and redshift distribution. In this paper, we investigate the event rate of lensed sBBHs produced via the dynamical channel by Monte Carlo simulations and the number is yr−1 for the Einstein telescope and yr−1 for Cosmic Explorer, of which the median is about ∼2 times the rate of sBBHs originating from the EMBS channel (calibrated by the local merger rate density estimated for the dynamical and the EMBS channel, i.e., and , respectively). Therefore, one may constrain the fraction of both the EMBS and dynamical channels through the comparison of the predicted and observed number of lensed sBBH events statistically.

Detection of six massive contact binaries with tertiary component candidates in the Small Magellanic Cloud

Abstract To study massive binaries in different evolution stages or environments, we use the Small Magellanic Cloud (SMC) as our target because the metallicity in the SMC is much lower than that in our Milky Way. The period change of early-type close binary systems in the SMC was studied based on OGLE collections. Six of these systems are found to have periodic period changes. Since all of them are of early type, the light-traveltime effect probably created by these massive binaries with third bodies is used to explain such a phenomenon. We use the Wilson–Devinney code (WD method) to analyze their I-band photometric light curves. The results show the six third bodies as having orbital periods from 6.41–24.65 yr and minimum masses from 0.31–4.11 M⊙. Among all six systems, three have a negative $\dot{P}$, which means that their periods keep decreasing. In addition, from the WD result, we find there are three deep-contact binaries, one intermediate-contact binary, and two shallow-contact binaries. The fraction of companions in massive contact binaries is quite high based on this sample, which may demonstrate the notion of high multiplicity in massive binary stars. This might mean that additional components may play an important role in the evolution of massive close binaries.

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

ABSTRACT 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 × 109 binaries in the metallicity range 0.0001 ≤ Z ≤ 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 BCOs. Just by slightly changing the overshooting parameter (λov = 0.4, 0.5) and the pair-instability model, the maximum mass of a black hole can vary from ≈60 to ≈100 M⊙. 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 ≳ 0.01) only <20 per cent of the merging binary black holes and binary neutron stars form via this channel, while other authors found fractions >70 per cent.

Merger Conditions of Population III Protostar Binaries

Massive close binary stars with extremely small separations have been observed, and they are possible progenitors of gravitational-wave sources. The evolution of massive binaries in the protostellar accretion stage is key to understanding their formation process. We, therefore, investigate how close the protostars, consisting of a high-density core and a vast low-density envelope, can approach each other but not coalesce. To investigate the coalescence conditions, we conduct smoothed particle hydrodynamics simulations following the evolution of equal-mass binaries with different initial separations. Since Population (Pop) I and III protostars have similar interior structures, we adopt a specific Pop III model with the mass and radius of 7.75 M ⊙ and 61.1 R ⊙ obtained by the stellar evolution calculations. Our results show that the binary separation decreases due to the transport of the orbital angular momentum to spin angular momentum. If the initial separation is less than about 80% of the sum of the protostellar radius, the binary coalesces in a time shorter than the tidal lock timescale. The mass loss up to the merging is ≲3%. After coalescence, the star rotates rapidly, and its interior structure is independent of the initial separation. We conclude that there must be some orbital shrinking mechanism after the protostars contract to enter the zero-age main-sequence stage.

A low-metallicity massive contact binary undergoing slow Case A mass transfer: A detailed spectroscopic and orbital analysis of SSN 7 in NGC 346 in the SMC

Context. Most massive stars are believed to be born in close binary systems where they can exchange mass, which impacts the evolution of both binary components. Their evolution is of great interest in the search for the progenitors of gravitational waves. However, there are unknowns in the physics of mass transfer as observational examples are rare, especially at low metallicity. Nearby low-metallicity environments are particularly interesting hunting grounds for interacting systems as they act as the closest proxy for the early universe where we can resolve individual stars. Aims. Using multi-epoch spectroscopic data, we complete a consistent spectral and orbital analysis of the early-type massive binary SSN 7 hosting a ON3 If*+O5.5 V((f)) star. Using these detailed results, we constrain an evolutionary scenario that can help us to understand binary evolution in low metallicity. Methods. We were able to derive reliable radial velocities of the two components from the multi-epoch data, which were used to constrain the orbital parameters. The spectroscopic data covers the UV, optical, and near-IR, allowing a consistent analysis with the stellar atmosphere code, PoWR. Given the stellar and orbital parameters, we interpreted the results using binary evolutionary models. Results. The two stars in the system have comparable luminosities of log(L1/L⊙) = 5.75 and log(L2/L⊙) = 5.78 for the primary and secondary, respectively, but have different temperatures (T1 = 43.6 kK and T2 = 38.7 kK). The primary (32 M⊙) is less massive than the secondary (55 M⊙), suggesting mass exchange. The mass estimates are confirmed by the orbital analysis. The revisited orbital period is 3 d. Our evolutionary models also predict mass exchange. Currently, the system is a contact binary undergoing a slow Case A phase, making it the most massive Algol-like system yet discovered. Conclusions. Following the initial mass function, massive stars are rare, and to find them in an Algol-like configuration is even more unlikely. To date, no comparable system to SSN 7 has been found, making it a unique object to study the efficiency of mass transfer in massive star binaries. This example increases our understanding of massive star binary evolution and the formation of gravitational wave progenitors.

General relativistic simulations of collapsing binary neutron star mergers with Monte Carlo neutrino transport

Recent gravitational wave observations of neutron-star-neutron-star and neutron-star-black-hole binaries appear to indicate that massive neutron stars may not be too uncommon in merging systems. These discoveries have led to an increased interest in the simulation of merging compact binaries involving massive stars. In this paper, we present a first set of evolution of massive neutron star binaries using Monte Carlo radiation transport for the evolution of neutrinos. We study a range of systems, from nearly symmetric binaries that collapse to a black hole before forming a disk or ejecting material, to more asymmetric binaries in which tidal disruption of the lower mass star leads to the production of more interesting postmerger remnants. For the latter type of systems, we additionally study the impact of viscosity on the properties of the outflows, and compare our results to two recent simulations of identical binaries performed with the whiskythc code. We find agreement on the black hole properties, disk mass, and mass and velocity of the outflows within expected numerical uncertainties, and some minor but noticeable differences in the evolution of the electron fraction when using a subgrid viscosity model, with viscosity playing a more minor role in our simulations. The method used to account for r-process heating in the determination of the outflow properties appears to have a larger impact on our result than those differences between numerical codes. We also use the simulation with the most ejected material to verify that our newly implemented Lagrangian tracers provide a reasonable sampling of the matter outflows as they leave the computational grid. We note that, given the lack of production of hot outflows in these mergers, the main role of neutrinos in these systems is to set the composition of the postmerger remnant. One of the main potential uses of our simulations is, thus, as improved initial conditions for longer evolutions of such remnants.

Expectations for fast radio bursts in neutron star–massive star binaries

Context. Recent observations of a small sample of repeating fast radio bursts (FRBs) have revealed a periodicity in their bursting activity that suggests a binary origin for the modulation. Aims. We set out to explore the scenario where a subset of repeating FRBs originates in binary systems that host a highly energetic neutron star and a massive companion star, akin to γ-ray binaries and young high-mass X-ray binaries. Methods. In this scenario, we infer observables, compare them with current observational constraints, and make predictions for future observations. Firstly, we specifically focused on the host galaxy properties and binary formation rates. Subsequently, we investigated the expected evolution of the rotation and dispersion measure in this scenario, the predicted birth site offsets, and the origin of the persistent radio emission observed in a subset of these systems. Results. The host galaxies for repeating FRBs favour the formation of neutron star–massive star binary systems, but any conclusive evidence will require future discoveries and localisations of FRBs. The birth rate of high-mass X-ray binaries, used as a proxy for all considered binaries, significantly exceeds the estimated rate of FRBs, which can be explained if only a small subset of these systems produce FRBs. We show that, under simple assumptions, we can reproduce the dispersion measure and rotation measure evolution that is seen in a subset of repeating FRBs. We also discuss the possibility of detecting a persistent radio source associated with the FRB due to an intra-binary shock between the companion star wind and either the pulsar wind or giant magnetar flares. The observed long-term luminosity stability of the persistent radio sources is most consistent with a giant flare-powered scenario. However, this explanation is highly dependent on the magnetic field properties of the neutron star. Conclusions. With these explorations, we provide a framework to discuss future FRB observations in the context of neutron star–massive star binary scenarios. We conclude that more localisations and observations of repeaters will be necessary to conclusively determine or rule out a connection between (repeating) FRBs and such binaries.

Massive pre-stellar cores in radiation-magneto-turbulent simulations of molecular clouds

ABSTRACT We simulate the formation and collapse of pre-stellar cores at few-au resolution in a set of radiation-magnetohydrodynamic simulations of giant molecular clouds (GMCs) using the grid-based code RAMSES-RT. We adopt, for the first time to our best knowledge, realistic initial/boundary conditions by zooming in on to individual massive pre-stellar cores within the GMC. We identify two distinct modes of fragmentation: ‘quasi-spherical’ and ‘filamentary’. In both modes, the fragments eventually become embedded in a quasi-steady accretion disc or toroid with radii ∼500–5000 au and opening angles H/R ∼ 0.5 − 1. The discs/toroids are Toomre stable but the accreted pre-existing fragments are found orbiting the outer disc, appearing as disc fragmentation. Each core converts nearly 100 per cent of the gas mass into a few massive stars forming near the disc centre. Large and massive discs around high-mass stars are supported by magnetic pressure in the outer disc, at radii >200–1000 au, and turbulent pressure in the inner disc. The most massive core accretes several times more mass than its initial mass, forming a cluster of 8 massive (proto)stars enshrouded by a toroid, suggesting a competitive accretion scenario for the formation of stars above ∼30 M⊙. We also find that the H ii regions produced by a single massive star remain trapped in the dense circumstellar discs for a few hundred kiloyears, while the dynamic motions of massive stars in wide binaries or multiple systems displace the stars from the densest parts of the disc, allowing UV radiation to escape producing steady or pulsating bipolar H ii regions.