arXiv subjects

Sara Rastello

Publications and source records attributed to Sara Rastello.

At least 19 recordsLinked to original sources

arXiv2026

$N$-body modelling of the ED-2 stream progenitor shows Gaia BH3's formation involved dynamical interactions

Context. The Gaia collaboration announced the discovery of a binary of a massive black hole (33 M$_\odot$) with a low-mass giant star (Gaia BH3) in the ED-2 stellar stream. The properties of this binary, as well as its position in the stream, challenge a formation scenario invoking only isolated binary evolution. Aims. We aim to quantify the importance of cluster dynamics in the formation of Gaia BH3 in the progenitor cluster of the ED-2 stream. Methods. We perform detailed N-body simulations of the progenitor cluster of the ED-2 stream, including the effects of single and binary stellar evolution. We compare these simulations to observations of the ED-2 stream and the properties of Gaia BH3. Results. We determine that Gaia BH3 most likely formed as an exchange binary which underwent multiple strong dynamical interactions. We highlight the importance of cluster dynamics in assembling Gaia BH3, and disfavour a formation scenario where it evolved unperturbed by dynamical interactions. Conclusions. The role of dynamics should be considered when interpreting the properties of the population of star-black hole binaries found in the next Gaia Data Release.

astro-ph.GA↗

arXiv2026

BE Lyncis is not a Black Hole Binary: Lessons From Gaia and Hipparcos Astrometry

BE Lyncis (BE Lyn) is a well-studied high-amplitude $δ$ Scuti variable star (HADS). Recently, Niu et al. (2026) analyzed a 39-year baseline of times of maximum light of BE Lyn, reporting that it is the most eccentric binary known ($e \approx 0.9989$) and hosts the nearest black hole (BH) known to date. We analyze Hipparcos and Gaia astrometry of BE Lyn, predicting what the observed proper motion anomaly (PMA) over the 25 year baseline between the two missions would be were the companion really a $\gtrsim 17.5\,M_{\odot}$ BH. We find that the predicted PMA is at least an order of magnitude larger than the observed value of $\approx 1.7 \pm 0.8$ mas yr$^{-1}$, regardless of the assumed orientation of the orbit. We predict the expected Gaia DR3 RUWE for different orientations of the putative BH binary, finding that it ranges from $\approx 2.5$-$4.0$, much larger than the reported value of $1.073$. The observed value is instead consistent with a low-mass secondary or a single star. We find that BE Lyncis would have received a 7-parameter acceleration solution if it were a BH binary, in contradiction with its absence from the Gaia DR3 non-single star catalogs. Finally, we show that the reported orbit is impossible because the luminous star would overflow its Roche lobe at periastron, irrespective of inclination. We recommend caution in interpreting light-travel time effect (LTTE) models that require very high eccentricities, face-on inclinations, or large companion masses. The observed pulsation timing variations are most likely simply a result of red noise or pulsation phase evolution.

astro-ph.SR↗

arXiv2026

Hide and Seek with Gaia. Detectability of Predicted Thin-Disc Metal-Rich RR Lyrae Binaries in Gaia DR3 and DR4

RR Lyrae stars (RRLs) are classical tracers of old stellar populations, yet growing evidence suggests the presence of a metal-rich ([Fe/H]>-0.5), intermediate-age (2-7 Gyr) sub-population in the Milky Way disc. Binary evolution, particularly stable mass transfer, has been proposed as a viable formation channel, predicting that most metal-rich, intermediate-age (<9 Gyr) RRLs should reside in binaries with orbital periods of ~900-2000 days. However, no genuine RRL binaries have been robustly identified, including in the Gaia DR3 astrometric binary catalogues, despite Gaia being sensitive to the predicted orbital-period range. We investigate whether the lack of detections in Gaia DR3 reflects an intrinsically low binary fraction or instead arises from observational biases. We analyse a carefully selected sample of 100 Gaia DR3 RRLs designed to trace the metal-rich population with thin-disc kinematics and compare them with predictions from binary evolution models. We generate realistic Gaia observation mocks, including variability-induced astrometric biases, and assess the detectability of binaries and the posterior constraints on the hidden binary fraction using astrometric quality indicators, such as RUWE, and a robust Bayesian inference. While current uncertainties prevent a definitive rejection of a high fraction of hidden binaries, our results reveal tensions between existing binary evolution predictions and the Gaia DR3 non-detections. This suggests either the presence of unaccounted systematics in the modelling of Gaia observations or the need to revise assumptions in binary evolution models. We predict that Gaia DR4 will significantly improve the binary detectability and provide powerful new constraints on the post-interaction binary populations.

astro-ph.SR↗

arXiv2026

Teen TITANS simulations -- I. Inefficient intermediate-mass black hole seeding via stellar collisions in young massive clusters

Young massive clusters (YMCs) provide favorable environments for frequent stellar collisions, potentially leading to the formation of very massive stars (VMSs) and seeds of intermediate-mass black holes (IMBHs). We investigate the role of repeated stellar collisions in YMCs using TITANS, a new suite of 18 direct $N$-body simulations. Our models span cluster masses $10^5 - 10^6\,\rm M_\odot$, half-mass densities $ρ_{\rm h}=100 - 10^5\,\rm M_\odot\,pc^{-3}$, and include high primordial binary fractions, consistent with observations of massive stars in young clusters. Overall, our simulations assume cluster properties that are typical of YMCs in the low-redshift Universe. We find that repeated stellar collisions are efficient only in the densest clusters with short relaxation times and are absent in systems with $ρ_{\rm h}<500\,\rm M_\odot\,pc^{-3}$ and $t_{\rm rh}>1.3\,\rm Gyr$. Rapid mass segregation allows massive stars to sink to the cluster center, merge, and undergo subsequent collisions, even in clusters with long core-collapse times. However, collision chains are typically triggered by primordial binary mergers and usually involve only two collisions. In our simulations, only three VMSs form through repeated collisions and reach $m_*>330\,\rm M_\odot$, while most VMSs have $m_*<300\,\rm M_\odot$ and form through primordial binary mergers. None constitute viable IMBH seeds, as their helium cores fall in the (pulsational) pair-instability regime. We form five IMBHs from stellar collisions involving stars at different evolutionary stages, while the dominant channel is the merger of stellar-mass black holes, producing twelve IMBHs. For properties typical of local YMCs, stellar collision chains are therefore inefficient in producing IMBHs more massive than $140\,\rm M_\odot$, as most collisionally formed VMSs attain masses that fall in the pair-instability regime.

astro-ph.GA↗

arXiv2025

The formation of Gaia BH3

The Gaia collaboration announced the discovery of a massive black hole (BH) with a low-mass giant star companion, Gaia BH3, located in the ED-2 stellar stream. The properties of Gaia BH3 bridge the gap between known Milky Way BHs and extragalactic BHs found with gravitational waves (GWs). We aim to determine the most likely formation scenario for Gaia BH3 in the progenitor cluster of the ED-2 stream. We perform $N$-body simulations of that progenitor cluster and find that, most likely, Gaia BH3 formed from a stellar binary that formed during cluster formation, which then underwent multiple dynamical interactions that significantly altered its properties, including exchanging the companion star. We highlight the importance of cluster dynamics and discard a formation scenario where it evolved in quasi-isolation.

astro-ph.GA↗

arXiv2025

Star-Black Hole Interactions in Young Star Clusters

Close encounters between stars and black holes can trigger micro-tidal disruption events (micro-TDEs) in dense young star clusters (YSCs). Using direct N-body simulations with PETAR, we found that most micro-TDEs arise from few-body multiple encounters. The inferred rate is approximately 350-450 Gpc$^{-3}$ yr$^{-1}$. Micro-TDEs could be detected both by upcoming surveys such as LSST, expected to observe roughly 10-100 events per year, and by their gravitational-wave (GW) signals peaking in the deci-Hertz band, detectable with future instruments such as LGWA and DECIGO.

astro-ph.HE↗

arXiv2025

Micro-Tidal Disruption Events in Young Star Clusters

Dense young star clusters (YSCs) are ideal environments for dynamical interactions between stars and stellar mass black holes (BHs). In such dense environments, stars can undergo close encounters with BHs and fall within their tidal radius, resulting in micro-tidal disruption events (micro-TDEs), transient phenomena with potential multi-messenger signatures. We performed a suite of direct N-body simulations using the PETAR code, to which we implemented new prescriptions for modeling micro-TDEs. We constructed a set of realistic YSC models including primordial binaries, based on the observed Milky Way population. Our simulations incorporate stellar and binary evolution, supernova kicks, and stellar winds using the BSE code, and account for the Galactic tidal field via the GALPY library. We identify three dynamical channels for micro-TDE production: single star-single BH encounters, binary-mediated interactions (including supernova-kick triggers), and interactions involving higher-order multiple systems such as hierarchical triples and quadruples, as well as chaotic few-body interactions. Multiple encounters are the most efficient production channel, which dominates the total rate: 350-450 Gpc$^{-3}$ yr$^{-1}$. Micro-TDEs from YSCs are expected to be detectable by upcoming surveys, particularly the Legacy Survey of Space and Time, with detection rates potentially up to hundreds per year. The gravitational wave (GW) signals expected from micro-TDEs peak in the deci-Hertz band, making them accessible to future instruments such as the Lunar Gravitational Wave Antenna and the Deci-Hertz Interferometer Gravitational-wave Observatory. Micro-TDEs emerge as promising multi-messenger sources, potentially offering unique insights into star cluster dynamics, stellar collisions, and the population of dormant stellar-mass BHs, through both electromagnetic and GW observations.

astro-ph.HE↗

arXiv2024

The boring history of Gaia BH3 from isolated binary evolution

Gaia BH3 is the first observed dormant black hole (BH) with a mass of $\approx{30}$ M$_\odot$ and represents the first confirmation that such massive BHs are associated with metal-poor stars. Here, we explore the isolated binary formation channel for Gaia BH3 focusing on the old and metal-poor stellar population of the Milky Way halo. We use the MIST stellar models and our open-source population synthesis code SEVN to evolve $5.6 \times 10^8$ binaries exploring 20 sets of parameters. We find that systems like Gaia BH3 form preferentially from binaries initially composed of a massive star ($40-60$ M$_\odot$) and a low mass companion ($<1$ M$_\odot$) in a wide ($P>10^3$ days) and eccentric orbit ($e>0.6$). Such progenitor binary stars do not undergo any Roche-lobe overflow episode during their entire evolution, so that the final orbital properties of the BH-star system are determined at the core collapse of the primary star. Low natal kicks ($\lesssim$ 10~km/s) significantly favour the formation of Gaia BH3-like systems, but high velocity kicks up to $\approx 220$ km/s are also allowed. We estimate the formation efficiency for Gaia BH3-like systems in old ($t>10$ Gyr) and metal-poor ($Z<0.01$) populations to be $\sim 4 \times 10^{-8}$ M$_\odot^{-1}$ (for our fiducial model), representing $\sim 3\%$ of the whole simulated BH-star population. We expect up to $\approx 4000$ BH-star systems in the Galactic halo formed through isolated evolution, of which $\approx 100$ are compatible with Gaia BH3-like. Given the density profile of the Galactic halo we do not expect more than one at the observed distance of Gaia BH3. Our models show that, even if it was born inside a stellar cluster, Gaia BH3 is compatible with a primordial binary star that escaped from its parent cluster without experiencing significant dynamical interactions.

astro-ph.GA↗

arXiv2024

Dynamical formation of Gaia BH3 in the progenitor globular cluster of the ED-2 stream

Context. The star-black hole (S-BH) binary known as Gaia BH3, discovered by the Gaia Collaboration is chemically and kinematically associated with the metal-poor ED-2 stream in the Milky Way halo. Aims. We explore the possibility that Gaia BH3 was assembled dynamically in the progenitor globular cluster (GC) of the ED-2 stream. Methods. We used a public suite of star-by-star dynamical Monte Carlo models to identify S-BH binaries in GCs with different initial masses and (half-mass) radii. Results. We show that a likely progenitor of the ED-2 stream was a relatively low-mass ($\lesssim10^5M_\odot$) GC with an initial half-mass radius of ~4 pc. Such a GC can dynamically retain a large fraction of its BH population and dissolve on the orbit of ED-2. From the suite of models we find that GCs produce ~ 3 - 30 S-BH binaries, approximately independent of initial GC mass and inversely correlated with initial cluster radius. Scaling the results to the Milky Way GC population, we find that ~75% of the S-BH binaries formed in GCs are ejected from their host GC, all in the early phases of evolution ($\lesssim1$ Gyr); these are expected to no longer be close to streams. The ~25% of S-BH binaries retained until dissolution are expected to form part of streams, such that for an initial mass of the progenitor of ED-2 of a few $10^4M_\odot$, we expect ~2-3 S-BH to end up in the stream. GC models with metallicities similar to Gaia BH3 ($\lesssim1\%$ solar) include S-BH binaries with similar BH masses ($\gtrsim30M_\odot$), orbital periods, and eccentricities. Conclusions. We predict the Galactic halo contains of order $10^5$ S-BH binaries that formed dynamically in GCs, a fraction of which may readily be detected in Gaia DR4. The detection of these sources provides valuable tests of BH dynamics in clusters and the possible role in formation of gravitational wave sources.

astro-ph.GA↗

arXiv2023

Dynamical formation of $Gaia$ BH1 in a young star cluster

$Gaia$ BH1, the first quiescent black hole (BH) detected from $Gaia$ data, poses a challenge to most binary evolution models: its current mass ratio is $\approx{0.1}$, 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 $3\times{}10^2-3\times{}10^4$ M$_\odot$ at solar metallicity, by means of $3.5\times{}10^4$ 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 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 matches 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.

astro-ph.SR↗

arXiv2023

The Dragon-II simulations -- I. Evolution of single and binary compact objects in star clusters with up to 1 million stars

We present the first results of the \textsc{Dragon-II} simulations, a suite of 19 $N$-body simulations of star clusters with up to $10^6$ stars, with up to $33\%$ of them initially paired in binaries. In this work, we describe the main evolution of the clusters and their compact objects (COs). All \textsc{Dragon-II} clusters form in their centre a black hole (BH) subsystem with a density $10-100$ times larger than the stellar density, with the cluster core containing $50-80\%$ of the whole BH population. In all models, the BH average mass steeply decreases as a consequence of BH burning, reaching values $\langle m_{\rm BH}\rangle < 15$ M$_\odot$ within $10-30$ relaxation times. Generally, our clusters retain only BHs lighter than $30$ M$_\odot$ over $30$ relaxation times. Looser clusters retain a higher binary fraction, because in such environments binaries are less likely disrupted by dynamical encounters. We find that BH-main sequence star binaries have properties similar to recently observed systems. Double CO binaries (DCOBs) ejected from the cluster exhibit larger mass ratios and heavier primary masses than ejected binaries hosting a single CO (SCOBs). Ejected SCOBs have BH masses $m_{\rm BH} = 3-20$ M$_\odot$, definitely lower than those in DCOBs ($m_{\rm BH} = 10-100$ M$_\odot$).

astro-ph.GA↗

arXiv2022

Dynamics of binary black holes in young star clusters: the impact of cluster mass and long-term evolution

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 ($\sim{500-800}$ M$_\odot$) and relatively high-mass star clusters ($\ge{5000}$ M$_\odot$). 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 $\sim{100}$ M$_\odot$) and driven by dynamical exchanges. Tidal disruption dramatically quenches the formation and dynamical evolution of BBHs in low-mass clusters on a very short timescale ($\lesssim{100}$ 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\% of BBHs have primary mass in the pair-instability mass gap, 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.

astro-ph.GA↗

arXiv2022

Gravitational background from dynamical binaries and detectability with 2G detectors

We study the impact of young clusters on the gravitational wave background from compact binary coalescence. We simulate a catalog of sources from population I/II isolated binary stars and stars born in young clusters, corresponding to one year of observations with second-generation (2G) detectors. Taking into account uncertainties on the fraction of dynamical binaries and star formation parameters, we find that the background is dominated by the population of binary black holes, and we obtain a value of $Ω_{gw}(25 \rm{Hz}) = 1.2^{+1.38}_{-0.65} \times 10^{-9}$ for the energy density, in agreement with the actual upper limits derived from the latest observation run of LIGO--Virgo. We demonstrate that a large number of sources in a specific corrected mass range yields to a bump in the background. This background could be detected with 8 years of coincident data by a network of 2G detectors.

astro-ph.CO↗

arXiv2022

Compact Object Mergers in Hierarchical Triples from Low-Mass Young Star Clusters

A binary star orbited by an outer companion constitutes a hierarchical triple system. The outer body may excite the eccentricity of the inner binary through the von~Zeipel-Lidov-Kozai (ZLK) mechanism, triggering the gravitational wave (GW) coalescence of the inner binary when its members are compact objects. Here, we study a sample of hierarchical triples with an inner black hole (BH) -- BH binary, BH -- neutron star (NS) binary, and BH -- white dwarf (WD) binary, formed via dynamical interactions in low-mass young star clusters. Our sample of triples was obtained self-consistently from direct N-body simulations of star clusters which included up-to-date stellar evolution. We find that the inner binaries in our triples cannot merge via GW radiation alone, and the ZLK mechanism is essential to trigger their coalescence. Contrary to binaries assembled dynamically in young star clusters, binary BHs merging in triples have preferentially low mass ratios (q ~ 0.3) and higher primary masses (m_p > 40 MSun). We derive a local merger rate density of 0.60, 0.11 and 0.5 yr^-1 Gpc^-3 for BH-BH, BH-NS and BH-WD binaries, respectively. Additionally, we find that merging binaries have high eccentricities across the GW spectrum, including the LIGO-Virgo-KAGRA (LVK), LISA, and DECIGO frequencies. About 7% of BH-BH and 60% of BH-NS binaries will have detectable eccentricities in the LVK band. Our results indicate that the eccentricity and the mass spectrum of merging binaries are the strongest features for the identification of GW mergers from triples.

astro-ph.HE↗

arXiv2021

New insights on binary black hole formation channels after GWTC-2: young star clusters versus isolated binaries

With the recent release of the second gravitational-wave transient catalogue (GWTC-2), which introduced dozens of new detections, we are at a turning point of gravitational wave astronomy, as we are now able to directly infer constraints on the astrophysical population of compact objects. Here, we tackle the burning issue of understanding the origin of binary black hole (BBH) mergers. To this effect, we make use of state-of-the-art population synthesis and N-body simulations, to represent two distinct formation channels: BBHs formed in the field (isolated channel) and in young star clusters (dynamical channel). We then use a Bayesian hierarchical approach to infer the distribution of the mixing fraction $f$, with $f=0$ ($f=1$) in the pure dynamical (isolated) channel. %that controls the proportion of isolated and dynamical BBHs. We explore the effects of additional hyper-parameters of the model, such as the spread in metallicity $σ_{\text{Z}}$ and the parameter $σ_{\text{sp}}$, describing the distribution of spin magnitudes. We find that the dynamical model is slightly favoured with a median value of $f=0.26$, when $σ_{\text{sp}}=0.1$ and $σ_{\text{Z}}=0.4$. Models with higher spin magnitudes tend to strongly favour dynamically formed BBHs ($f\le{}0.1$ if $σ_{\text{sp}}=0.3$). Furthermore, we show that hyper-parameters controlling the rates of the model, such as $σ_{\rm Z}$, have a large impact on the inference of the mixing fraction, which rises from $0.18$ to $0.43$ when we increase $σ_{\text{Z}}$ from 0.2 to 0.6, for a fixed value of $σ_{\text{sp}}=0.1$. Finally, our current set of observations is better described by a combination of both formation channels, as a pure dynamical scenario is excluded at the $99\%$ credible interval, except when the spin magnitude is high.

astro-ph.HE↗

arXiv2021

GW190521 formation via three-body encounters in young massive star clusters

GW190521 is the most massive binary black hole (BBH) merger observed to date, and its primary component lies in the pair-instability (PI) mass gap. Here, we investigate the formation of GW190521-like systems via three-body encounters in young massive star clusters. We performed 2$\times10^5$ simulations of binary-single interactions between a BBH and a massive $\geq{60}\,$M$_{\odot}$ black hole (BH), including post-Newtonian terms up to the $2.5$ order and a prescription for relativistic kicks. In our initial conditions, we take into account the possibility of forming BHs in the PI mass gap via stellar collisions. If we assume that first-generation BHs have low spins, $\sim{0.17}\%$ of all the simulated BBH mergers have component masses, effective and precessing spin, and remnant mass and spin inside the $90\%$ credible intervals of GW190521. Seven of these systems are first-generation exchanged binaries, while five are second-generation BBHs. We estimate a merger rate density $\mathcal{R}_{\rm GW190521}\sim{0.03}\,$Gpc$^{-3}\,$yr$^{-1}$ for GW190521-like binaries formed via binary-single interactions in young star clusters. This rate is extremely sensitive to the spin distribution of first-generation BBHs. Stellar collisions, second-generation mergers and dynamical exchanges are the key ingredients to produce GW190521-like systems in young star clusters.

astro-ph.HE↗

arXiv2021

Dynamics of binary black holes in low-mass young star clusters

Young star clusters are dynamically active stellar systems and are a common birthplace for massive stars. Low-mass star clusters ($\sim{}300-10^3$ M$_\odot$) are more numerous than massive systems and are characterized by a two-body relaxation time scale of a few Myr: the most massive stars sink to the cluster core and dynamically interact with each other even before they give birth to compact objects. Here, we explore the properties of black holes (BHs) and binary black holes (BBHs) formed in low-mass young star clusters, by means of a suite of $10^5$ direct $N$-body simulations with a high original binary fraction (100 % for stars with mass $>5$ M$_\odot$). Most BHs are ejected in the first $\sim{}20$ Myr by dynamical interactions. Dynamical exchanges are the main formation channel of BBHs, accounting for $\sim{}40-80$ % of all the systems. Most BBH mergers in low-mass young star clusters involve primary BHs with mass $<40$ M$_\odot$ and low mass ratios are extremely more common than in the field. Comparing our data with those of more massive star clusters ($10^3-3\times{}10^4$ M$_\odot$), we find a strong dependence of the percentage of exchanged BBHs on the mass of the host star cluster. In contrast, our results show just a mild correlation between the mass of the host star cluster and the efficiency of BBH mergers.

astro-ph.GA↗

arXiv2021

Intermediate mass black holes from stellar mergers in young star clusters

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 analyze $10^4$ 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 9 IMBHs out of 218 form via binary BH mergers, with a mass $\sim{}100-140$ M$_\odot$. This channel is strongly suppressed by the low escape velocity of our star clusters. In contrast, IMBHs with masses up to $\sim{}438$ M$_{\odot}$ efficiently form via runaway stellar collisions, especially at low metallicity. Up to $\sim{}0.2$~% 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: $\sim{}70$% 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: $\sim{}8$% ($\sim{}1$%) of the simulated clusters with initial mass $10^4-3\times{}10^4$ M$_\odot$ ($10^3-5\times{}10^3$ M$_\odot$) host at least one IMBH.

astro-ph.GA↗