Search

Dynamical interactions in dense star clusters are considered one of the most effective formation channels of binary black holes (BBHs). Here, we present direct N -body simulations of two different star cluster families: low-mass (∼500–800 M⊙) and relatively high-mass star clusters (≥5000 M⊙). We show that the formation channels of BBHs in low- and high-mass star clusters are extremely different and lead to two completely distinct populations of BBH mergers. Low-mass clusters host mainly low-mass BBHs born from binary evolution, while BBHs in high-mass clusters are relatively massive (chirp mass up to ∼100 M⊙) and driven by dynamical exchanges. Tidal disruption dramatically quenches the formation and dynamical evolution of BBHs in low-mass clusters on a very short time-scale (≲100 Myr), while BBHs in high-mass clusters undergo effective dynamical hardening until the end of our simulations (1.5 Gyr). In high-mass clusters, we find that 8 per cent of BBHs have primary mass in the pair-instability mass gap at metallicity Z  = 0.002, all of them born via stellar collisions, while only one BBH with primary mass in the mass gap forms in low-mass clusters. These differences are crucial for the interpretation of the formation channels of gravitational-wave sources. [ABSTRACT FROM AUTHOR]

Intermediate-mass black holes (IMBHs) in the mass range |$10^2\!-\!10^5\, \mathrm{M_{\odot }}$| bridge the gap between stellar black holes (BHs) and supermassive BHs. Here, we investigate the possibility that IMBHs form in young star clusters via runaway collisions and BH mergers. We analyse 104 simulations of dense young star clusters, featuring up-to-date stellar wind models and prescriptions for core collapse and (pulsational) pair instability. In our simulations, only nine IMBHs out of 218 form via binary BH mergers, with a mass ∼100–140 M⊙. This channel is strongly suppressed by the low escape velocity of our star clusters. In contrast, IMBHs with masses up to ∼438 M⊙ efficiently form via runaway stellar collisions, especially at low metallicity. Up to ∼0.2 per cent of all the simulated BHs are IMBHs, depending on progenitor's metallicity. The runaway formation channel is strongly suppressed in metal-rich (Z  = 0.02) star clusters, because of stellar winds. IMBHs are extremely efficient in pairing with other BHs: ∼70 per cent of them are members of a binary BH at the end of the simulations. However, we do not find any IMBH–BH merger. More massive star clusters are more efficient in forming IMBHs: ∼8 per cent (∼1 per cent) of the simulated clusters with initial mass 104–3 × 104 M⊙ (103–5 × 103 M⊙) host at least one IMBH. [ABSTRACT FROM AUTHOR]

Most of massive stars form in binary or higher order systems in clumpy, substructured clusters. In the very first phases of their life, these stars are expected to interact with the surrounding environment, before being released to the field when the cluster is tidally disrupted by the host galaxy. We present a set of N -body simulations to describe the evolution of young stellar clusters and their binary content in the first phases of their life. To do this, we have developed a method that generates realistic initial conditions for binary stars in star clusters from hydrodynamical simulations. We considered different evolutionary cases to quantify the impact of binary and stellar evolution. Also, we compared their evolution to that of King and fractal models with different length-scales. Our results indicate that the global expansion of the cluster from hydrodynamical simulations is initially balanced by the subclump motion and accelerates when a monolithic shape is reached, as in a post-core collapse evolution. Compared to the spherical initial conditions, the ratio of the 50 per cent to 10 per cent Lagrangian radius shows a very distinctive trend, explained by the formation of a hot core of massive stars triggered by the high initial degree of mass segregation. As for its binary population, each cluster shows a self-regulating behaviour by creating interacting binaries with binding energies of the order of its energy scales. Also, in the absence of original binaries, the dynamically formed binaries display a mass-dependent binary fraction, spontaneously reproducing the trend of the observed binary fraction. [ABSTRACT FROM AUTHOR]

Young star clusters are the most common birthplace of massive stars and are dynamically active environments. Here, we study the formation of black holes (BHs) and binary black holes (BBHs) in young star clusters, by means of 6000  N -body simulations coupled with binary population synthesis. We probe three different stellar metallicities (Z  = 0.02, 0.002, and 0.0002) and two initial-density regimes (density at the half-mass radius ρh ≥ 3.4 × 104 and ≥1.5 × 102 M⊙ pc−3 in dense and loose star clusters, respectively). Metal-poor clusters tend to form more massive BHs than metal-rich ones. We find ∼6, ∼2, and <1 per cent of BHs with mass m BH > 60 M⊙ at Z  = 0.0002, 0.002, and 0.02, respectively. In metal-poor clusters, we form intermediate-mass BHs with mass up to ∼320 M⊙. BBH mergers born via dynamical exchanges (exchanged BBHs) can be more massive than BBH mergers formed from binary evolution: the former (latter) reach total mass up to ∼140 M⊙ (∼80 M⊙). The most massive BBH merger in our simulations has primary mass ∼88 M⊙, inside the pair-instability mass gap, and a mass ratio of ∼0.5. Only BBHs born in young star clusters from metal-poor progenitors can match the masses of GW 170729, the most massive event in first and second observing run (O1 and O2), and those of GW 190412, the first unequal-mass merger. We estimate a local BBH merger rate density ∼110 and ∼55 Gpc−3 yr−1, if we assume that all stars form in loose and dense star clusters, respectively. [ABSTRACT FROM AUTHOR]

Searching for distinctive signatures, which characterize different formation channels of binary black holes (BBHs), is a crucial step towards the interpretation of current and future gravitational wave detections. Here, we investigate the demography of merging BBHs in young star clusters (SCs), which are the nursery of massive stars. We performed 4 × 103 N -body simulations of SCs with metallicity Z  = 0.002, initial binary fraction 0.4, and fractal initial conditions, to mimic the clumpiness of star-forming regions. Our simulations include a novel population-synthesis approach based on the code mobse. We find that SC dynamics does not affect the merger rate significantly, but leaves a strong fingerprint on the properties of merging BBHs. More than 50 per cent of merging BBHs in young SCs form by dynamical exchanges in the first few Myr. Dynamically formed merging BBHs are significantly heavier than merging BBHs in isolated binaries: merging BBHs with total mass up to ∼120 M⊙ form in young SCs, while the maximum total mass of merging BBHs in isolated binaries with the same metallicity is only ∼70 M⊙. Merging BBHs born via dynamical exchanges tend to have smaller mass ratios than BBHs in isolated binaries. Furthermore, SC dynamics speeds up the merger: the delay time between star formation and coalescence is significantly shorter in young SCs. In our simulations, massive systems such as GW170729 form only via dynamical exchanges. Finally ∼2 per cent of merging BBHs in young SCs have mass in the pair-instability mass gap (∼60–120 M⊙). This represents a unique fingerprint of merging BBHs in SCs. [ABSTRACT FROM AUTHOR]