A novel generative method for star clusters from hydro-dynamical simulations

Most stars form in clumpy and sub-structured clusters. These properties also emerge in hydro-dynamical simulations of star-forming clouds, which provide a way to generate realistic initial conditions for $N-$body runs of young stellar clusters. However, producing large sets of initial conditions by hydro-dynamical simulations is prohibitively expensive in terms of computational time. We introduce a novel technique for generating new initial conditions from a given sample of hydro-dynamical simulations, at a tiny computational cost. In particular, we apply a hierarchical clustering algorithm to learn a tree representation of the spatial and kinematic relations between stars, where the leaves represent the single stars and the nodes describe the structure of the cluster at larger and larger scales. This procedure can be used as a basis for the random generation of new sets of stars, by simply modifying the global structure of the stellar cluster, while leaving the small-scale properties unaltered.Comment: 6 pages, 4 figures, 1 table. Proceedings IAU Symposium No. 362 "The Predictive Power of Computational Astrophysics as a Discovery Tool", 202

Stellar-mass black holes in the Hyades star cluster?

Astrophysical models of binary-black hole mergers in the Universe require a significant fraction of stellar-mass black holes (BHs) to receive negligible natal kicks to explain the gravitational wave detections. This implies that BHs should be retained even in open clusters with low escape velocities ($\lesssim1~\mathrm{km \, s^{-1}}$). We search for signatures of the presence of BHs in the nearest open cluster to the Sun - the Hyades - by comparing density profiles of direct $N$-body models to data from $Gaia$. The observations are best reproduced by models with $2-3$ BHs at present. Models that never possessed BHs have an half-mass radius $\sim30\%$ smaller than the observed value, while those where the last BHs were ejected recently ($\lesssim150~$Myr ago) can still reproduce the density profile. In 50% of the models hosting BHs, we find BHs with stellar companion(s). Their period distribution peaks at $\sim10^3$ yr, making them unlikely to be found through velocity variations. We look for potential BH companions through large $Gaia$ astrometric and spectroscopic errors, identifying 56 binary candidates - none of which consistent with a massive compact companion. Models with $2-3$ BHs have an elevated central velocity dispersion, but observations can not yet discriminate. We conclude that the present-day structure of the Hyades requires a significant fraction of BHs to receive natal kicks smaller than the escape velocity of $\sim 3\, \mathrm{km \, s^{-1}}$ at the time of BH formation and that the nearest BHs to the Sun are in, or near, Hyades.Comment: 20 pages, 15 figures, 5 tables. Accepted for publication in MNRAS. Comments welcom

Impact of gas hardening on the population properties of hierarchical black hole mergers in AGN disks

Hierarchical black hole (BH) mergers in active galactic nuclei (AGNs) are unique among formation channels of binary black holes (BBHs) because they are likely associated with electromagnetic counterparts and can efficiently lead to the mass growth of BHs. Here, we explore the impact of gas accretion and migration traps on the evolution of BBHs in AGNs. We have developed a new fast semi-analytic model, which allows us to explore the parameter space while capturing the main physical processes involved. We find that effective exchange of energy and angular momentum between the BBH and the surrounding gas (hereafter, gas hardening) during inspiral greatly enhances the efficiency of hierarchical mergers, leading to the formation of intermediate-mass BHs (up to 10.000 solar masses) and triggering spin alignment. Moreover, our models with efficient gas hardening show both an anti-correlation between BBH mass ratio and effective spin, and a correlation between primary BH mass and effective spin. In contrast, if gas hardening is inefficient, the hierarchical merger chain is already truncated after the first two or three generations. We compare the BBH population in AGNs with other dynamical channels as well as isolated binary evolution.Comment: 21 pages, 15 figures, submitted to A&A, comments welcom

Dinamica degli ammassi stellari giovani e della loro popolazione di buchi neri

Young star clusters are key objects to interpret a large number of astrophysical processes, from star and binary formation to the hierarchical assembly of the Milky Way. The spatial distribution and motions of young stars reflect the processes of cluster formation, while the kinematics of more evolved systems and the age dependence of their mass function inform us on how star clusters progressively dissolve into the field of the Galaxy. Finally, being the place where the most massive stars form, star clusters are building blocks for our comprehension of the formation of compact objects, which we can detect through gravitational-wave observations. Direct N-body simulations are usually adopted to integrate the evolution of star clusters since their earliest phases, but we often assume quite unrealistic initial conditions for such simulations. In Chapter 2, I provide an accurate modeling of the very first phases of the cluster's life. First, I make use of hydro-dynamical simulations of collapsing molecular clouds, which, coupled with appropriate recipes for star formation, yield realistic initial conditions. Then, I introduce a new algorithm to associate a primordial binary star population to such hydro-dynamical simulations. Finally, I quantify the impact of such primordial binaries on the global evolution of the cluster. I find that primordial binaries accelerate the star cluster dissolution, and enhance the formation of a hot core of massive objects. Hydro-dynamical simulations are an accurate method to obtain realistic initial conditions for star forming regions. However, producing large sets of hydro-dynamical simulations is prohibitively expensive in terms of computational time. In Chapter 3, I address this issue by introducing a novel algorithm to generate new star clusters from a given set of star masses, positions and velocities from a hydro-dynamical simulation. This method is based on a hierarchical clustering algorithm that learns a tree representation of the cluster phase-space. This is later turned into new realizations by modifying the initial branches of the tree, while preserving the characteristics of small scale structure responsible for most of the dynamical evolution. The new realizations qualitatively resemble the original simulation, and show a realistic evolution at all scales. In Chapter 4, I explore how dynamical interactions within young star clusters affect the properties of BBH mergers. I find that dynamically active environments produce more massive BBH mergers thanks to the high rate of dynamical exchanges, which favor the coupling of the most massive BHs. Also, the high initial cluster densities trigger a large number of stellar collisions. This, in turn, leads to a non-negligible number of BBH mergers with primary mass in the pair-instability mass gap, where BHs are not expected to form via isolated stellar evolution. In Chapter 5, I look for signatures of the presence of BHs the Hyades cluster, by comparing accurate $N-$body models to precise measurements from Gaia. I find that even a few BHs can affect the properties of visible stars in a quantifiable way, leading to less concentrated distributions. For the case of the Hyades, 3 BHs are favored to match the observed properties of the cluster. In massive star clusters, BHs born by the merger of other BHs can be retained, dynamically form new BBHs, and merge again. This hierarchical merger process can repeat several times and lead to a significant BH mass growth. In Chapter 6, I explore the process of hierarchical mergers in globular clusters. I investigate the importance of stellar evolution, two-body relaxation and tidal stripping by the host galaxy. My results indicate that globular clusters can only host hierarchical BH mergers up to the third generation, i.e. at least one generation less than what previously thought.Young star clusters are key objects to interpret a large number of astrophysical processes, from star and binary formation to the hierarchical assembly of the Milky Way. The spatial distribution and motions of young stars reflect the processes of cluster formation, while the kinematics of more evolved systems and the age dependence of their mass function inform us on how star clusters progressively dissolve into the field of the Galaxy. Finally, being the place where the most massive stars form, star clusters are building blocks for our comprehension of the formation of compact objects, which we can detect through gravitational-wave observations. Direct N-body simulations are usually adopted to integrate the evolution of star clusters since their earliest phases, but we often assume quite unrealistic initial conditions for such simulations. In Chapter 2, I provide an accurate modeling of the very first phases of the cluster's life. First, I make use of hydro-dynamical simulations of collapsing molecular clouds, which, coupled with appropriate recipes for star formation, yield realistic initial conditions. Then, I introduce a new algorithm to associate a primordial binary star population to such hydro-dynamical simulations. Finally, I quantify the impact of such primordial binaries on the global evolution of the cluster. I find that primordial binaries accelerate the star cluster dissolution, and enhance the formation of a hot core of massive objects. Hydro-dynamical simulations are an accurate method to obtain realistic initial conditions for star forming regions. However, producing large sets of hydro-dynamical simulations is prohibitively expensive in terms of computational time. In Chapter 3, I address this issue by introducing a novel algorithm to generate new star clusters from a given set of star masses, positions and velocities from a hydro-dynamical simulation. This method is based on a hierarchical clustering algorithm that learns a tree representation of the cluster phase-space. This is later turned into new realizations by modifying the initial branches of the tree, while preserving the characteristics of small scale structure responsible for most of the dynamical evolution. The new realizations qualitatively resemble the original simulation, and show a realistic evolution at all scales. In Chapter 4, I explore how dynamical interactions within young star clusters affect the properties of BBH mergers. I find that dynamically active environments produce more massive BBH mergers thanks to the high rate of dynamical exchanges, which favor the coupling of the most massive BHs. Also, the high initial cluster densities trigger a large number of stellar collisions. This, in turn, leads to a non-negligible number of BBH mergers with primary mass in the pair-instability mass gap, where BHs are not expected to form via isolated stellar evolution. In Chapter 5, I look for signatures of the presence of BHs the Hyades cluster, by comparing accurate $N-$body models to precise measurements from Gaia. I find that even a few BHs can affect the properties of visible stars in a quantifiable way, leading to less concentrated distributions. For the case of the Hyades, 3 BHs are favored to match the observed properties of the cluster. In massive star clusters, BHs born by the merger of other BHs can be retained, dynamically form new BBHs, and merge again. This hierarchical merger process can repeat several times and lead to a significant BH mass growth. In Chapter 6, I explore the process of hierarchical mergers in globular clusters. I investigate the importance of stellar evolution, two-body relaxation and tidal stripping by the host galaxy. My results indicate that globular clusters can only host hierarchical BH mergers up to the third generation, i.e. at least one generation less than what previously thought

Star Clusters and the nursery of Binary Black Holes

Star clusters are the most common birthplace of massive stars, which form as members of binary or multiple systems. In these dense stellar environments, dynamical interactions can shape the properties of binary stars, and govern the formation of binary black holes (BBHs), the best known sources of gravitational waves. In this contribution, I will investigate how the interplay between star clusters and their binary stars and BBHs affects the properties of gravitational-wave sources. Through direct N-body models, I find that the formation channels of BBHs in dynamically-active (massive and dense) and dynamically-quiet (low-mass and loose) star clusters are extremely different. Dynamically-quiet clusters host mainly low-mass BBHs born from binary evolution, while BBHs in dynamically-active clusters are relatively massive and driven by dynamical exchanges. I will show that star-star collisions and repeated black hole mergers are viable mechanisms for the formation of black holes in the pair-instability mass gap (60-120 MSun), which cannot form from isolated stellar and binary evolution. In particular, repeated black hole mergers can extend the black hole mass distribution to values larger than 60 MSun, consistently with the trend inferred from gravitational-wave detections

Star Clusters and the nursery of Binary Black Holes

Star clusters are the most common birthplace of massive stars, which form as members of binary or multiple systems. In these dense stellar environments, dynamical interactions can shape the properties of binary stars, and govern the formation of binary black holes (BBHs), the best known sources of gravitational waves. In this contribution, I will investigate how the interplay between star clusters and their binary stars and BBHs affects the properties of gravitational-wave sources. Through direct N-body models, I find that the formation channels of BBHs in dynamically-active (massive and dense) and dynamically-quiet (low-mass and loose) star clusters are extremely different. Dynamically-quiet clusters host mainly low-mass BBHs born from binary evolution, while BBHs in dynamically-active clusters are relatively massive and driven by dynamical exchanges. I will show that star-star collisions and repeated black hole mergers are viable mechanisms for the formation of black holes in the pair-instability mass gap (60-120 MSun), which cannot form from isolated stellar and binary evolution. In particular, repeated black hole mergers can extend the black hole mass distribution to values larger than 60 MSun, consistently with the trend inferred from gravitational-wave detections

Hierarchical generative models for star clusters from hydro-dynamical simulations

Star formation in molecular clouds is clumpy, hierarchically subclustered. Fractal structure also emerges in hydro-dynamical simulations of star-forming clouds. Simulating the formation of realistic star clusters with hydro-dynamical simulations is a computational challenge, considering that only the statistically averaged results of large batches of simulations are reliable, due to the chaotic nature of the gravitational N-body problem. While large sets of initial conditions for N-body runs can be produced by hydro-dynamical simulations of star formation, this is prohibitively expensive in terms of computational time. Here we address this issue by introducing a new technique for generating many sets of new initial conditions from a given set of star masses, positions and velocities from a hydro-dynamical simulation. We use hierarchical clustering in phase space to learn a tree representation of the spatial and kinematic relations between stars. This constitutes the basis for the random generation of new sets of stars which share the same clustering structure of the original ones but have individually different masses, positions, and velocities. We apply this method to the output of a number of hydro-dynamical star-formation simulations, comparing the generated initial conditions to the original ones through a series of quantitative tests, including comparing mass and velocity distributions and fractal dimension. Finally, we evolve both the original and the generated star clusters using a direct N-body code, obtaining a qualitatively similar evolution.Comment: Updated version of the manuscript: "Seeing the forest for the trees: hierarchical generative models for star clusters from hydro-dynamical simulations". 15 pages, 15 figures, 2 tables. Comments welcom

Binary black hole mergers from Population III star clusters

International audienceBinary black holes (BBHs) born from the evolution of Population III (Pop. III) stars are one of the main high-redshift targets for next-generation ground-based gravitational-wave (GW) detectors. Their predicted initial mass function and lack of metals make them the ideal progenitors of black holes above the upper edge of the pair-instability mass gap, i.e. with a mass higher than $\approx{}134$ (241) M$_\odot$ for stars that become (do not become) chemically homogeneous during their evolution. Here, we investigate the effects of cluster dynamics on the mass function of BBHs born from Pop. III stars, by considering the main uncertainties on Pop. III star mass function, orbital properties of binary systems, star cluster's mass and disruption time. In our dynamical models, at least $\sim$5% and up to 100% BBH mergers in Pop. III star clusters have primary mass $m_1$ above the upper edge of the pair-instability mass gap. In contrast, only $\lesssim {} 3$% isolated BBH mergers have primary mass above the gap, unless their progenitors evolved as chemically homogeneous stars. The lack of systems with primary and/or secondary mass inside the gap defines a zone of avoidance with sharp boundaries in the primary mass - mass ratio plane. Finally, we estimate the merger rate density of BBHs and, in the most optimistic case, we find a maximum of $\mathcal{R}\approx200\,{\rm Gpc^{-3}\,yr^{-1}}$ at $z\sim15$ for BBHs formed via dynamical capture. For comparison, the merger rate density of isolated Pop. III BBHs is $\mathcal{R}\leq{}10\,{\rm Gpc^{-3}\,yr^{-1}}$, for the same model of Pop. III star formation history

The impact of binaries on the evolution of star clusters from turbulent molecular clouds

Contains fulltext : 238311.pdf (Publisher’s version ) (Open Access