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Numerical simulations of the formation of binary black holes provide a framework within which to interpret the recent detection of the first gravitational-wave source and to predict the properties of subsequent binary-black-hole gravitational-wave events; the calculations predict detections of about 1,000 black-hole mergers per year once gravitational-wave observatories reach full sensitivity. A roadmap for black-hole merger detection Krzysztof Belczynski et al . present numerical simulations of the formation of binary black holes that provide a framework for interpreting the recent detection of the first gravitational-wave source (known as GW150914) — a merger of two massive black holes. Their models imply that these events take place in an environment where the metallicity is less than 10 per cent of that of the Sun, and that the progenitors are stars with initial masses of 40–100 solar masses that interact through mass transfer and a common-envelope phase. The calculations predict detections of about a thousand black-hole mergers per year once gravitational-wave observatories reach full sensitivity. The merger of two massive (about 30 solar masses) black holes has been detected in gravitational waves 1 . This discovery validates recent predictions 2 , 3 , 4 that massive binary black holes would constitute the first detection. Previous calculations, however, have not sampled the relevant binary-black-hole progenitors—massive, low-metallicity binary stars—with sufficient accuracy nor included sufficiently realistic physics to enable robust predictions to better than several orders of magnitude 5 , 6 , 7 , 8 , 9 , 10 . Here we report high-precision numerical simulations of the formation of binary black holes via the evolution of isolated binary stars, providing a framework within which to interpret the first gravitational-wave source, GW150914, and to predict the properties of subsequent binary-black-hole gravitational-wave events. Our models imply that these events form in an environment in which the metallicity is less than ten per cent of solar metallicity, and involve stars with initial masses of 40–100 solar masses that interact through mass transfer and a common-envelope phase. These progenitor stars probably formed either about 2 billion years or, with a smaller probability, 11 billion years after the Big Bang. Most binary black holes form without supernova explosions, and their spins are nearly unchanged since birth, but do not have to be parallel. The classical field formation of binary black holes we propose, with low natal kicks (the velocity of the black hole at birth) and restricted common-envelope evolution, produces approximately 40 times more binary-black-holes mergers than do dynamical formation channels involving globular clusters 11 ; our predicted detection rate of these mergers is comparable to that from homogeneous evolution channels 12 , 13 , 14 , 15 . Our calculations predict detections of about 1,000 black-hole mergers per year with total masses of 20–80 solar masses once second-generation ground-based gravitational-wave observatories reach full sensitivity.

All stellar-mass black holes have hitherto been identified by X-rays emitted from gas that is accreting onto the black hole from a companion star. These systems are all binaries with a black-hole mass that is less than 30 times that of the Sun 1 – 4 . Theory predicts, however, that X-ray-emitting systems form a minority of the total population of star–black-hole binaries 5 , 6 . When the black hole is not accreting gas, it can be found through radial-velocity measurements of the motion of the companion star. Here we report radial-velocity measurements taken over two years of the Galactic B-type star, LB-1. We find that the motion of the B star and an accompanying Hα emission line require the presence of a dark companion with a mass of 68 − 13 + 11 solar masses, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. Gravitational-wave experiments have detected black holes of similar mass, but the formation of such massive ones in a high-metallicity environment would be extremely challenging within current stellar evolution theories. Radial-velocity measurements of a Galactic B-type star show a dark companion that seems to be a black hole of about 68 solar masses, in a widely spaced binary system.

Modeling the evolution of the elements in the Milky Way is a multidisciplinary and challenging task. In addition to simulating the ∽ 13 billion years evolution of our Galaxy, chemical evolution simulations must keep track of the elements synthesized and ejected from every astrophysical site of interest (e.g., supernova, compact binary merger). The elemental abundances of such ejecta, which are a fundamental input for chemical evolution codes, are usually taken from theoretical nucleosynthesis calculations performed by the nuclear astrophysics community. Therefore, almost all chemical evolution predictions rely on the nuclear physics behind those calculations. In this proceedings article, we highlight the impact of nuclear physics uncertainties on galactic chemical evolution predictions. We demonstrate that nuclear physics and galactic evolution uncertainties both have a significant impact on interpreting the origin of neutron-capture elements in our Solar System. Those results serve as a motivation to create and maintain collaborations between the fields of nuclear astrophysics and galaxy evolution.

There are 214 X-ray point-sources (\\(L_{\\rm X}>10^{35} \\mathrm{erg/s}\\)) identified as X-ray binaries (XRBs) in the nearby spiral galaxy M83. Since XRBs are powered by accretion onto a neutron star or a black hole from a companion/donor star these systems are promising progenitors of merging double compact objects (DCOs): black hole - black hole (BH-BH), black hole - neutron star (BH-NS), or neutron star - neutron star (NS-NS) systems. The connection (i.e. XRBs evolving into DCOs) may provide some hints to the yet unanswered question: what is the origin of the LIGO/Virgo/KAGRA mergers? Available observations do not allow to determine what will be the final fate of the XRBs observed in M83. Yet, we can use evolutionary model of isolated binaries to reproduce the population of XRBs in M83 by matching model XRBs numbers/types/luminosities to observations. Knowing the detailed properties of M83 model XRBs (donor/accretor masses, their evolutionary ages and orbits) we follow their evolution to the death of donor stars to check whether any merging DCOs are formed. Although all merging DCOs in our isolated binary evolution model go through the XRB phase (defined as reaching X-ray luminosity from RLOF/wind accretion onto NS/BH above \\(10^{35}\\) erg/s), only very few XRBs evolve to form merging (in Hubble time) DCOs. For M83 with its solar-like metallicity stars and continiuous star-formation we find that only \\(\\sim 1-2\\%\\) of model XRBs evolve into merging DCOs depending on the adopted evolutionary physics. This is caused by (i) merger of donor star with compact object during common envelope phase, (ii) binary disruption at the supernova explosion of donor star, (iii) formation of a DCO on a wide orbit (merger time longer than Hubble time).

Stellar mass black hole binaries have individual masses between 10-80 solar masses. These systems may emit gravitational waves at frequencies detectable at Megaparsec distances by space-based gravitational wave observatories. In a previous study, we determined the selection effects of observing these systems with detectors similar to the Laser Interferometer Space Antenna by using a generated population of binary black holes that covered a reasonable parameter space and calculating their signal-to-noise ratio. We further our study by populating the galaxies in our nearby (less than 30 Mpc) universe with binary black hole systems drawn from a distribution found in the Synthetic Universe to ultimately investigate the likely event rate of detectable binaries from galaxies in the nearby universe.

The LIGO/Virgo collaboration has reported 50 black hole-black hole (BH-BH) mergers and 8 candidates recovered from digging deeper into the detectors noise. The majority of these mergers have low effective spins pointing toward low BH spins and efficient angular momentum transport in massive stars as proposed by several models (e.g., the Tayler-Spruit dynamo). However, out of these 58 mergers, 7 are consistent with having high effective spin parameter (chi_eff>0.3). Additionally, 2 events seem to have high effective spins sourced from the spin of the primary (more massive) BH. These particular observations could be used to discriminate between the isolated binary and dynamical formation channels. It might seem that high BH spins point to a dynamical origin if angular momentum transport in stars is efficient and forms low-spinning BHs. In such a case dynamical formation is required to produce second and third generations of BH-BH mergers with typically high-spinning BHs. Here we show, however, that isolated binary BH-BH formation also naturally reproduces such highly spinning BHs. Our models start with efficient angular momentum transport in massive stars that is needed to reproduce the majority of BH-BH mergers with low effective spins. Later, some of the binaries are subject to a tidal spin-up allowing the formation of a moderate fraction (~10%) of BH-BH mergers with high effective spins (chi_eff>=0.4-0.5). In addition, isolated binary evolution can produce a small fraction of BH-BH mergers with almost maximally spinning primary BHs. Therefore, the formation scenario of these atypical BH-BH mergers remains to be found.

It is speculated that a merger of two massive stellar-origin BHs in a dense stellar environment may lead to the formation of a massive BH in the pair-instability mass gap (50-135 Msun). Such a merger-formed BH is expected to typically have a high spin (a=0.7). If such a massive BH acquires another BH it may lead to another merger detectable by LIGO/Virgo in gravitational waves. Acquiring a companion may be hindered by gravitational-wave kick/recoil, which accompanies the first merger and may quickly remove the massive BH from its parent globular or nuclear cluster. We test whether it is possible for a massive merger-formed BH in the pair-instability gap to be retained in its parent cluster and have low spin. Such a BH would be indistinguishable from a primordial BH. We employed results from numerical relativity calculations of black hole mergers to explore the range of gravitational-wave recoil velocities for various combinations of merging BH masses and spins. We compared merger-formed massive BH speeds with typical escape velocities from globular and nuclear clusters. We show that a globular cluster is highly unlikely to form and retain a 100 Msun BH if the spin of the BH is low (a<0.3) as such BHs acquire high recoil speeds (>200 km/s) that exceed typical escape speeds from globular clusters (50 km/s). However, a very low-spinning (a=0.1) and massive (100 Msun) BH could be formed and retained in a galactic nuclear star cluster. Even though such massive merger-formed BHs with such low spins acquire high speeds during formation (400 km/s), they may avoid ejection since massive nuclear clusters have high escape velocities (300-500 km/s). A future detection of a massive BH in the pair-instability mass gap with low spin would therefore not be proof of the existence of primordial BHs, which are sometimes claimed to have low spins and arbitrarily high masses.

Properties of the to-date-observed binary black hole (BBH) merger events suggest a preference towards spin-orbit aligned mergers. Naturally, this has caused widespread interest and speculations regrading implications on various merger formation channels. Here we show that (i) not only the BBH-merger population from isolated binaries, but also (ii) BBH population formed in young massive clusters (YMC) would possess an asymmetry in favour of aligned mergers, in the distribution of the events' effective spin parameter (\\(\\chi_{\\rm eff}\\)). In our analysis, we utilize BBH-merger outcomes from state-of-the-art N-body evolutionary models of YMCs and isolated binary population synthesis. We incorporate, for the first time in such an analysis, misalignments due to both natal kicks and dynamical encounters. The YMC \\(\\chi_{\\rm eff}\\) distribution has a mean (an anti-aligned merger fraction) of \\(\\langle\\chi_{\\rm eff}\\rangle\\leq0.04\\) (\\(f_X-\\approx40\\%\\)), which is smaller (larger) than but consistent with the observed asymmetry of \\(\\langle\\chi_{\\rm eff}\\rangle\\approx0.06\\) (\\(f_X-\\approx28\\%\\)) as obtained from the population analysis by the LIGO-Virgo-KAGRA collaboration. In contrast, isolated binaries alone tend to produce a much stronger asymmetry; for the tested physical models, \\(\\langle\\chi_{\\rm eff}\\rangle\\approx0.25\\) and \\(f_X-\\lesssim7\\%\\). Although the YMC \\(\\chi_{\\rm eff}\\) distribution is more similar to the observed counterpart, none of the channels correctly reproduce the observed distribution. Our results suggest that further extensive model explorations for both isolated-binary and dynamical channels as well as better observational constraints are necessary to understand the physics of 'the symmetry breaking' of the BBH-merger population.

LIGO/Virgo Collaboration reported the detection of the most massive black hole - black hole (BH-BH) merger up to date with component masses of 85 Msun and 66 Msun (GW190521). Motivated by recent observations of massive stars in the 30 Doradus cluster in the Large Magellanic Cloud (>200 Msun; e.g. R136a) and employing newly estimated uncertainties on pulsational pair-instability mass-loss (that allow for possibility of forming BHs with mass up to 90Msun) we show that it is trivial to form such massive BH-BH mergers through the classical isolated binary evolution (with no assistance from either dynamical interactions or exotica). A binary consisting of two massive (180+150 Msun) Population II stars (Z=0.0001) evolves through a stable Roche lobe overflow and common envelope episode. Both exposed stellar cores undergo direct core-collapse and form massive BHs while avoiding pair-instability pulsation mass-loss or total disruption. LIGO/Virgo observations show that the merger rate density of light BH-BH mergers (both components: <50 Msun) is of the order of 10-100 Gpc^-3 yr^-1, while GW190521 indicates that the rate of heavier mergers is 0.02-0.43 Gpc^-3 yr^-1. Our model (with standard assumptions about input physics) but extended to include 200 Msun stars and allowing for the possibility of stellar cores collapsing to 90 Msun BHs produces the following rates: 63 Gpc^-3 yr^-1 for light BH-BH mergers and 0.04 Gpc^-3 yr^-1 for heavy BH-BH mergers. We do not claim that GW190521 was formed by an isolated binary, but it appears that such a possibility can not be excluded.