Context. Mergers of two stellar origin black holes are a prime source of gravitational waves and are under intensive investigation. One crucial ingredient in their modeling has been neglected: pair-instability pulsation supernovae with associated severe mass loss may suppress the formation of massive black holes, decreasing black hole merger rates for the highest black hole masses. Aims. We demonstrate the effects of pair-instability pulsation supernovae on merger rate and mass using populations of double black hole binaries formed through the isolated binary classical evolution channel. Methods. The mass loss from pair-instability pulsation supernova is estimated based on existing hydrodynamical calculations. This mass loss is incorporated into the StarTrack population synthesis code. StarTrack is used to generate double black hole populations with and without pair-instability pulsation supernova mass loss. Results. The mass loss associated with pair-instability pulsation supernovae limits the Population I/II stellar-origin black hole mass to 50 M ⊙ , in tension with earlier predictions that the maximum black hole mass could be as high as 100 M ⊙ . In our model, neutron stars form with mass 1-2 M ⊙ , then we encounter the first mass gap at 2-5 M ⊙ with an absence of compact objects due to rapid supernova explosions, followed by the formation of black holes with mass 5-50 M ⊙ , with a second mass gap at 50-135 M ⊙ created by pairinstability pulsation supernovae and by pair-instability supernovae. Finally, black holes having masses above 135 M ⊙ may potentially form to arbitrarily high mass limited only by the extent of the initial mass function and the strength of stellar winds. Suppression of double black hole merger rates by pair-instability pulsation supernovae is negligible for our evolutionary channel. Our standard evolutionary model with the inclusion of pair-instability pulsation supernovae and pair-instability supernovae is fully consistent with the LIGO observations of black hole mergers: GW150914, GW151226, and LVT151012. The LIGO results are inconsistent with high ( 400 km s −1 ) BH natal kicks. We predict the detection of several, and up to as many as ∼ 60, BH-BH mergers with a total mass of 10-150 M ⊙ (most likely range: 20-80 M ⊙ ) in the forthcoming ∼ 60 effective days of the LIGO O2 observations, assuming the detectors reach the optimistic target O2 sensitivity. Conclusions.

In this paper, the second of a series, we study the stellar dynamical and evolutionary processes leading to the formation of compact binaries containing neutron stars (NSs) in dense globular clusters. For this study, 70 dense clusters were simulated independently, with a total stellar mass ∼2 × 107 M⊙, exceeding the total mass of all dense globular clusters in our Galaxy. We find that, in order to reproduce the empirically derived formation rate of low‐mass X‐ray binaries (LMXBs), we must assume that NSs can be formed via electron‐capture supernovae with typical natal kicks smaller than in core‐collapse supernovae. Our results explain the observed dependence of the number of LMXBs on ‘collision number’ as well as the large scatter observed between different globular clusters. We predict that the number of quiescent LMXBs in different clusters should not have a strong metallicity dependence. We compare the results obtained from our simulations with the observed population of millisecond pulsars (MSPs). We find that in our cluster model the following mass‐gaining events create populations of MSPs that do not match the observations (either they are inconsistent with the observed LMXB production rates, or the inferred binary periods or companion masses are not observed among radio bMSPs): (i) accretion during a common‐envelope event with a NS formed through electron‐capture supernovae (ECSNe), and (ii) mass transfer (MT) from a white dwarf donor. Some processes lead only to a mild recycling – physical collisions or MT in a post‐accretion‐induced collapse system. In addition, for MSPs, we distinguish low magnetic field (long‐lived) and high magnetic field (short‐lived) populations, where in the latter NSs are formed as a result of accretion‐induced collapse or merger‐induced collapse. With this distinction and by considering only those mass‐gaining events that appear to lead to NS recycling, we obtain good agreement of our models with the numbers and characteristics of observed MSPs in 47 Tuc and Terzan 5, as well as with the cumulative statistics for MSPs detected in globular clusters of different dynamical properties. We find that significant production of merging double NSs potentially detectable as short γ‐ray bursts occurs only in very dense, most likely core‐collapsed clusters.

It is firmly established that the stellar mass distribution is smooth, covering the range 0.1-100 M ⊙ . It is to be expected that the masses of the ensuing compact remnants correlate with the masses of their progenitor stars, and thus it is generally thought that the remnant masses should be smoothly distributed from the lightest white dwarfs to the heaviest black holes. However, this intuitive prediction is not borne out by observed data. In the rapidly growing population of remnants with observationally determined masses, a striking mass gap has emerged at the boundary between neutron stars and black holes. The heaviest neutron stars reach a maximum of two solar masses, while the lightest black holes are at least five solar masses. Over a decade after the discovery, the gap has become a significant challenge to our understanding of compact object formation. We offer new insights into the physical processes that bifurcate the formation of remnants into lower mass neutron stars and heavier black holes. Combining the results of stellar modeling with hydrodynamic simulations of supernovae, we both explain the existence of the gap, and also put stringent constraints on the inner workings of the supernova explosion mechanism. In particular, we show that core-collapse supernovae are launched within 100-200 milliseconds of the initial stellar collapse, implying that the explosions are driven by instabilities with a rapid (10-20 ms) growth time. Alternatively, if future observations fill in the gap, this will be an indication that these instabilities develop over a longer (> 200 milliseconds) timescale.

We study the evolution of binary stars in globular clusters using a new Monte Carlo approach combining a population synthesis code (startrack) and a simple treatment of dynamical interactions in the dense cluster core using a new tool for computing three‐ and four‐body interactions (fewbody). We find that the combination of stellar evolution and dynamical interactions (binary–single and binary–binary) leads to a rapid depletion of the binary population in the cluster core. The maximum binary fraction today in the core of a typical dense cluster such as 47 Tuc, assuming an initial binary fraction of 100 per cent, is only ∼ 5–10 per cent. We show that this is in good agreement with recent Hubble Space Telescope observations of close binaries in the core of 47 Tuc, provided that a realistic distribution of binary periods is used to interpret the results. Our findings also have important consequences for the dynamical modelling of globular clusters, suggesting that ‘realistic models’ should incorporate much larger initial binary fractions than has usually been the case in the past.

We present theoretical delay times and rates of thermonuclear explosions that are thought to produce Type Ia supernovae (SNe Ia), including the double‐detonation sub‐Chandrasekhar mass model, using the population synthesis binary evolution code startrack. If detonations of sub‐Chandrasekhar mass carbon–oxygen white dwarfs following a detonation in an accumulated layer of helium on the white dwarf’s surface (‘double‐detonation’ models) are able to produce thermonuclear explosions which are characteristically similar to those of SNe Ia, then these sub‐Chandrasekhar mass explosions may account for at least some substantial fraction of the observed SN Ia rate. Regardless of whether all double‐detonations look like ‘normal’ SNe Ia, in any case the explosions are expected to be bright and thus potentially detectable. Additionally, we find that the delay time distribution of double‐detonation sub‐Chandrasekhar mass SNe Ia can be divided into two distinct formation channels: the ‘prompt’ helium‐star channel with delay times <500 Myr (13 per cent of all sub‐Chandras), and the ‘delayed’ double white dwarf channel, with delay times ≳ 800 Myr spanning up to a Hubble time (87 per cent). These findings coincide with recent observationally derived delay time distributions which have revealed that a large number of SNe Ia are prompt with delay times <500 Myr, while a significant fraction also have delay times spanning ∼1 Gyr to a Hubble time.

Double white dwarfs are expected to be a source of confusion-limited noise for the future gravitational wave observatory LISA. In a specific frequency range, this 'foreground noise' is predicted to rise above the instrumental noise and hinder the detection of other types of signals, e.g., gravitational waves arising from stellar mass objects inspiraling into massive black holes. In many previous studies only detached populations of compact object binaries have been considered in estimating the LISA gravitational wave foreground signal. Here, we investigate the influence of compact object detached and Roche-Lobe Overflow Galactic binaries on the shape and strength of the LISA signal. Since > 99% of remnant binaries which have orbital periods within the LISA sensitivity range are white dwarf binaries, we consider only these binaries when calculating the LISA signal. We find that the contribution of RLOF binaries to the foreground noise is negligible at low frequencies, but becomes significant at higher frequencies, pushing the frequency at which the foreground noise drops below the instrumental noise to > 6 mHz. We find that it is important to consider the population of mass transferring binaries in order to obtain an accurate assessment of the foreground noise on the LISA data stream. However, we estimate that there still exists a sizeable number (∼ 11300) of Galactic double white dwarf binaries which will have a signal-to-noise ratio > 5, and thus will be potentially resolvable with LISA. We present the LISA gravitational wave signal from the Galactic population of white dwarf binaries, show the most important formation channels contributing to the LISA disc and bulge populations and discuss the implications of these new findings.9 For a circular binary, fgr [Hz] = 2/(orbital period) [s].

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The observed samples of supernovae (SNe) and double compact objects (DCOs) provide several critical constraints on population synthesis models. The parameters of these models must be carefully chosen to reproduce, among other factors, (1) the formation rates of double neutron star (NS-NS) binaries and white dwarfYneutron star (WD-NS) binaries, estimated from binary samples, and (2) the Type II and Ib/c SN rates. Even allowing for extremely conservative accounting of the uncertainties in observational and theoretical predictions, we find that only a few plausible population synthesis models (roughly 9%) are consistent with DCO and SN rates empirically determined from observations. As a proof of concept, we describe the information that can be extracted about population synthesis models given these observational tests, including surprisingly good agreement with the neutron star kick distributions inferred from pulsar proper-motion measurements. In the present study, we find that the current observational constraints favor kicks described by a single Maxwellian with a characteristic velocity of about 350 km s À1 (i.e., at maximum likelihood; kick velocities between 100 and 700 km s À1 remain within the 90% confidence interval of unimodal distributions), mass-loss fractions during nonconservative but stable mass transfer episodes of about 90%, and common envelope parameters of about 0.15Y0.5. Finally, we use the subset of astrophysically consistent models to predict the rates at which black holeYneutron star (BH-NS) and NS-NS binaries merge in the Milky Way and the nearby universe, assuming that Milky WayYlike galaxies dominate. Inevitably, the resulting probability distributions for merger rates depend on our assumed priors for the population model input parameters. In this study we adopt relatively conservative priors (flat) for all model parameters covering a rather wide range of values. However, as we gain confidence in our knowledge of these inputs, the range of merger rates consistent with our knowledge should shift and narrow.