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Showing papers on "Mass segregation published in 2021"


Journal ArticleDOI
TL;DR: In this article, a new set of GC models, varying the initial mass function (IMF) within observational uncertainties, are presented, and it is shown that GCs with top-heavy IMF lose most of their mass within a few Gyr through stellar winds and tidal stripping.
Abstract: Recent observations of globular clusters (GCs) provide evidence that the stellar initial mass function (IMF) may not be universal, suggesting specifically that the IMF grows increasingly top-heavy with decreasing metallicity and increasing gas density. Non-canonical IMFs can greatly affect the evolution of GCs, mainly because the high end determines how many black holes (BHs) form. Here we compute a new set of GC models, varying the IMF within observational uncertainties. We find that GCs with top-heavy IMFs lose most of their mass within a few Gyr through stellar winds and tidal stripping. Heating of the cluster through BH mass segregation greatly enhances this process. We show that, as they approach complete dissolution, GCs with top-heavy IMFs can evolve into 'dark clusters' consisting of mostly BHs by mass. In addition to producing more BHs, GCs with top-heavy IMFs also produce many more binary BH (BBH) mergers. Even though these clusters are short-lived, mergers of ejected BBHs continue at a rate comparable to, or greater than, what is found for long-lived GCs with canonical IMFs. Therefore these clusters, although they are no longer visible today, could still contribute significantly to the local BBH merger rate detectable by LIGO/Virgo, especially for sources with higher component masses well into the BH mass gap. We also report that one of our GC models with a top-heavy IMF produces dozens of intermediate-mass black holes (IMBHs) with masses $M>100\,{\rm M_\odot}$, including one with $M>500\,{\rm M_\odot}$. Ultimately, additional gravitational wave observations will provide strong constraints on the stellar IMF in old GCs and the formation of IMBHs at high redshift.

35 citations


Journal ArticleDOI
15 Jan 2021-Universe
TL;DR: In this article, the authors explore the possibility that dark matter may be explained by a nonuniform background of approximately stellar mass clusters of Primordial Black Holes (PBHs) by simulating the evolution from recombination to the present with over 5000 realisations using a Newtonian N-body code.
Abstract: We explore the possibility that Dark Matter (DM) may be explained by a nonuniform background of approximately stellar mass clusters of Primordial Black Holes (PBHs) by simulating the evolution from recombination to the present with over 5000 realisations using a Newtonian N-body code. We compute the cluster rate of evaporation and extract the binary and merged sub-populations along with their parent and merger tree histories, lifetimes and formation rates, the dynamical and orbital parameter profiles, the degree of mass segregation and dynamical friction and power spectrum of close encounters. Overall, we find that PBHs can constitute a viable DM candidate, and that their clustering presents a rich phenomenology throughout the history of the Universe. We show that binary systems constitute about 9.5% of all PBHs at present, with mass ratios of q¯B=0.154, and total masses of m¯T,B=303M⊙. Merged PBHs are rare, about 0.0023% of all PBHs at present, with mass ratios of q¯B=0.965 with total and chirp masses of m¯T,B=1670M⊙ and m¯c,M=642M⊙, respectively. We find that cluster puffing up and evaporation leads to bubbles of these PBHs of order 1 kpc containing at present times about 36% of objects and mass, with one-hundred pc-sized cores. We also find that these PBH sub-haloes are distributed in wider PBH haloes of order hundreds of kpc, containing about 63% of objects and mass, coinciding with the sizes of galactic halos. We find at last high rates of close encounters of massive Black Holes (M∼1000M⊙), with ΓS=(1.2+5.9−0.9)×107yr−1Gpc−3 and mergers with ΓM=1337±41yr−1Gpc−3.

34 citations


Journal ArticleDOI
TL;DR: In this paper, the authors analyzed proper motions from the Hubble Space Telescope (HST) and the second Gaia data release along with line-of-sight velocities from the MUSE spectrograph to detect imprints of an intermediate-mass black hole (IMBH) in the center of the nearby, core-collapsed, globular cluster NGC 6397.
Abstract: We analyze proper motions from the Hubble Space Telescope (HST) and the second Gaia data release along with line-of-sight velocities from the MUSE spectrograph to detect imprints of an intermediate-mass black hole (IMBH) in the center of the nearby, core-collapsed, globular cluster NGC 6397. For this, we use the new MAMPOSST-PM Bayesian mass-modeling code, along with updated estimates of the surface density profile of NGC 6397. We consider different priors on velocity anisotropy and on the size of the central mass, and we also separate the stars into components of different mean mass to allow for mass segregation. The velocity ellipsoid is very isotropic throughout the cluster, as expected in post-core collapsed clusters subject to as strong a Galactic tidal field as NGC 6397. There is strong evidence for a central dark component of 0.8 to 2% of the total mass of the cluster. However, we find robust evidence disfavoring a central IMBH in NGC 6397, preferring instead a diffuse dark inner subcluster of unresolved objects with a total mass of 1000 to 2000 M ⊙ , half of which is concentrated within 6 arcsec (2% of the stellar effective radius). These results require the combination of HST and Gaia data: HST for the inner diagnostics and Gaia for the outer surface density and velocity anisotropy profiles. The small effective radius of the diffuse dark component suggests that it is composed of compact stars (white dwarfs and neutron stars) and stellar-mass black holes, whose inner locations are caused by dynamical friction given their high progenitor masses. We show that stellar-mass black holes should dominate the mass of this diffuse dark component, unless more than 25% escape from the cluster. Their mergers in the cores of core-collapsed globular clusters could be an important source of the gravitational wave events detected by LIGO.

25 citations


Journal ArticleDOI
TL;DR: In this paper, the authors analyzed the structure of the NGC 2264 molecular cloud and compared the populations of clumps and young stellar objects (YSOs) in this region whose rich YSO population has shown evidence of sequential star formation.
Abstract: Context: The mass segregation of stellar clusters could be primordial rather than dynamical. Despite the abundance of studies of mass segregation for stellar clusters, those for stellar progenitors are still scarce, so the question concerning the origin and evolution of mass segregation is still open. Aims: Our goal is to characterize the structure of the NGC 2264 molecular cloud and compare the populations of clumps and young stellar objects (YSOs) in this region whose rich YSO population has shown evidence of sequential star formation. Methods: We separated the Herschel column density map of NGC 2264 into three subregions and compared their cloud power spectra using a multiscale segmentation technique. We extracted compact cloud fragments from the column density image, measured their basic properties, and studied their spatial and mass distributions. Results: In the whole NGC 2264 cloud, we identified a population of 256 clumps with typical sizes of ~0.1 pc and masses ranging from 0.08 M⊙ to 53 M⊙. Although clumps have been detected all over the cloud, most of the massive, bound clumps are concentrated in the central subregion of NGC 2264. The local surface density and the mass segregation ratio indicate a strong degree of mass segregation for the 15 most massive clumps, with a median Σ6 three times that of the whole clumps population and ΛMSR ≃ 8. We show that this cluster of massive clumps is forming within a high-density cloud ridge, which is formed and probably still fed by the high concentration of gas observed on larger scales in the central subregion. The time sequence obtained from the combined study of the clump and YSO populations in NGC 2264 suggests that the star formation started in the northern subregion, that it is now actively developing at the center, and will soon start in the southern subregion. Conclusions: Taken together, the cloud structure and the clump and YSO populations in NGC 2264 argue for a dynamical scenario of star formation. The cloud could first undergo global collapse, driving most clumps to centrally concentrated ridges. After their main accretion phase, some YSOs, and probably the most massive, would stay clustered while others would be dispersed from their birth sites. We propose that the mass segregation observed in some star clusters is inherited from that of clumps, originating from the mass assembly phase of molecular clouds.

17 citations


Journal ArticleDOI
TL;DR: In this paper, the authors carried out a systematic statistical study of the binary stars that evolve to form a BH--NS binary, considering different metallicities and taking into account the uncertainties on the natal kick distributions for BHs and NSs and on the common envelope phase of binary evolution.
Abstract: The long wait for the detection of merging black hole -- neutron star (BH--NS) binaries is finally over with the announcement by the LIGO/Virgo/Kagra collaboration of GW200105 and GW200115. Remarkably, the primary of GW200115 has a negative spin projection onto the orbital angular momentum, with about $90\%$ probability. Merging BH--NS binaries are expected to form mainly through the evolution of massive binary stars in the field, since their dynamical formation in dense star clusters is strongly suppressed by mass segregation. In this paper, we carry out a systematic statistical study of the binary stars that evolve to form a BH--NS binary, considering different metallicities and taking into account the uncertainties on the natal kick distributions for BHs and NSs and on the common envelope phase of binary evolution. Under the assumption that the initial stellar spins are aligned with the binary angular momentum, we show that both large natal kicks ($\gtrsim 150\kms$) and high efficiencies for common envelope ejection are required to simultaneously explain the inferred high merger rates and the large spin-orbit misalignment of GW200115.

13 citations


Posted Content
TL;DR: In this article, the authors identify hierarchical structures in the Vela OB2 complex and the cluster pair Collinder 135 and UBC 7 with Gaia EDR3 using the neural network machine learning algorithm StarGO.
Abstract: We identify hierarchical structures in the Vela OB2 complex and the cluster pair Collinder 135 and UBC 7 with Gaia EDR3 using the neural network machine learning algorithm StarGO. Five second-level substructures are disentangled in Vela OB2, which are referred to as Huluwa 1 (Gamma Velorum), Huluwa 2, Huluwa 3, Huluwa 4 and Huluwa 5. Huluwa 1-5 may have originated through sequential star formation. The clusters Huluwa 1-3 are the older generation, with ages of 10-20 Myr, and generated stellar feedback that caused turbulence which fostered the formation of the younger-generation clusters, Huluwa 4-5 (10 Myr). The cluster pair Collinder 135 and UBC 7 was likely formed from the same molecular cloud, are coeval (40 Myr). The 3D morphology of Huluwa 1-5 resembles a shell-like structure, right along the rim of the Vela IRAS shell. A supernova explosion located inside the Vela IRAS shell quenched star formation in the younger generation Huluwa 4-5 and rapidly expelled the remaining gas from the clusters. This resulted in mass stratification across the shell, with more low-mass stars located along the inner rim of the shell, and more massive stars in the outer region of the shell. Mass segregation is observed only in the lowest-mass cluster Huluwa 5. Significant expansion is observed in Vela OB2 with a 1D expansion rate of $(6.9-7.9)\times10^{-2}$~km $\rm s^{-1} pc^{-1}$. Expansion in the cluster pair is moderate. The velocity dispersions suggest that both Vela OB2 and the cluster pair are supervirial and are undergoing disruption. $N$-body simulations predict that Huluwa 1-5 in Vela OB2 and the cluster pair will continue to expand in the future 100 Myr and eventually dissolve.are supervirial and are undergoing disruption. $N$-body simulations predict that Huluwa 1-5 in Vela OB2 and the cluster pair will continue to expand in the future 100 Myr and eventually dissolve.

11 citations


Journal ArticleDOI
TL;DR: Using surface brightness and velocity dispersion profiles from the literature, the authors fit $59$ Milky Way globular clusters to dynamical models from the \texttt{CMC Cluster Catalog}.
Abstract: As ancient, gravitationally bound stellar populations, globular clusters are abundant, vibrant laboratories characterized by high frequencies of dynamical interactions coupled to complex stellar evolution. Using surface brightness and velocity dispersion profiles from the literature, we fit $59$ Milky Way globular clusters to dynamical models from the \texttt{CMC Cluster Catalog}. Without doing any interpolation, and without any directed effort to fit any particular cluster, $26$ globular clusters are well-matched by at least one of our models. We discuss in particular the core-collapsed clusters NGC 6293, NGC 6397, NGC 6681, and NGC 6624, and the non-core-collapsed clusters NGC 288, NGC 4372, and NGC 5897. As NGC 6624 lacks well-fitting snapshots on the main \texttt{CMC Cluster Catalog}, we run six additional models in order to refine the fit. We calculate metrics for mass segregation, explore the production of compact object sources such as millisecond pulsars, cataclysmic variables, low-mass X-ray binaries, and stellar-mass black holes, finding reasonable agreement with observations. Additionally, closely mimicking observational cuts, we extract the binary fraction from our models, finding good agreement except in the dense core regions of core-collapsed clusters. Accompanying this paper are a number of \textsf{python} methods for examining the publicly accessible \texttt{CMC Cluster Catalog}, as well as any other models generated using \texttt{CMC}.

10 citations


Journal ArticleDOI
TL;DR: In this paper, a study of the population of blue straggler stars (BSS) in the open clusters Trumpler 5 and NGC 2477 is presented. And the radial velocities of a sample of straggglers, for the sake of comparison with the mean radial velocity and the velocity dispersion of the clusters, are measured.
Abstract: We present a study, based on Gaia DR2, of the population of blue straggler stars (BSS) in the open clusters Trumpler 5, Trumpler 20, and NGC 2477. All candidates were selected according to their position in the color-magnitude diagram, to their proper motion components, and to their parallax. We also looked for yellow stragglers, i.e., possible evolved blue stragglers. We found that Trumpler 5 hosts a large BSS population, which allowed us to analyze their radial distribution as a probe of the cluster's dynamical status. The BSS distribution was compared with that of red giant branch stars (RGB) to evaluate mass segregation. Our results indicate that blue straggler stars are not more centrally concentrated than RGB stars in any of the clusters. The radial distribution of BSS in Trumpler 5 is flat. Additionally, using a multi-epoch radial velocity survey conducted with the high-resolution spectrograph FLAMES/GIRAFFE at VLT, we measured the radial velocities of a sample of stragglers, for the sake of comparison with the mean radial velocity and the velocity dispersion of the clusters. Based on the radial velocity variations for different epochs, we roughly classified these stars as possible close-or long-period binaries.

9 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigate the evolutionary nexus between the morphology and internal kinematics of the central regions of collisional, rotating, multi-mass stellar systems, with special attention to the spatial characterisation of the process of mass segregation.
Abstract: We investigate the evolutionary nexus between the morphology and internal kinematics of the central regions of collisional, rotating, multi-mass stellar systems, with special attention to the spatial characterisation of the process of mass segregation. We report results from idealized, purely $N$-body simulations that show multi-mass, rotating, and spherical systems rapidly form an oblate, spheroidal massive core, unlike single-mass rotating or multi-mass non-rotating configurations with otherwise identical initial properties, indicating that this evolution is a result of the interplay between the presence of a mass spectrum and angular momentum. This feature appears to be long-lasting, preserving itself for several relaxation times. The degree of flattening experienced by the systems is directly proportional to the initial degree of internal rotation. In addition, this morphological effect has a clear characterisation in terms of orbital architecture, as it lowers the inclination of the orbits of massive stars. We offer an idealised dynamical interpretation that could explain the mechanism underpinning this effect and we highlight possible useful implications, from kinematic hysteresis to spatial distribution of dark remnants in dense stellar systems.

8 citations


Journal ArticleDOI
TL;DR: In this article, the 3D morphology and kinematics of 13 open clusters (OCs) located within 500 pc of the Sun were analyzed using Gaia EDR3 and Kinematic data from literature.
Abstract: We analyze the 3D morphology and kinematics of 13 open clusters (OCs) located within 500 pc of the Sun, using Gaia EDR3 and kinematic data from literature. Members of OCs are identified using the unsupervised machine learning method StarGO, using 5D parameters (X, Y, Z, $\mu_\alpha \cos\delta, \mu_\delta$). The OC sample covers an age range of 25Myr--2.65Gyr. We correct the asymmetric distance distribution due to the parallax error using Bayesian inversion. The uncertainty in the corrected distance for a cluster at 500~pc is 3.0--6.3~pc, depending on the intrinsic spatial distribution of its members. We determine the 3D morphology of the OCs in our sample and fit the spatial distribution of stars within the tidal radius in each cluster with an ellipsoid model. The shapes of the OCs are well-described with oblate spheroids (NGC2547, NGC2516, NGC2451A, NGC2451B, NGC2232), prolate spheroids (IC2602, IC4665, NGC2422, Blanco1, Coma Berenices), or triaxial ellipsoids (IC2391, NGC6633, NGC6774). The semi-major axis of the fitted ellipsoid is parallel to the Galactic plane for most clusters. Elongated filament-like substructures are detected in three young clusters (NGC2232, NGC2547, NGC2451B), while tidal-tail-like substructures (tidal tails) are found in older clusters (NGC2516, NGC6633, NGC6774, Blanco1, Coma Berenices). Most clusters may be super-virial and expanding. $N$-body models of rapid gas expulsion with an SFE of $\approx 1/3$ are consistent with clusters more massive than $250\rm M_\odot$, while clusters less massive than 250$\rm M_\odot$ tend to agree with adiabatic gas expulsion models. Only six OCs (NGC2422, NGC6633, and NGC6774, NGC2232, Blanco1, Coma Berenices) show clear signs of mass segregation.

8 citations


Posted Content
TL;DR: In this article, the authors presented detailed and self-consistent 47 Tuc models computed with the cluster Monte Carlo code (\texttt{CMC}), including all relevant dynamical interactions coupled to stellar and binary evolution, and reproduce various observations, including surface brightness and velocity dispersion profiles, pulsar accelerations, and numbers of compact objects.
Abstract: The globular cluster 47 Tucanae (47 Tuc) is one of the most massive star clusters in the Milky Way and is exceptionally rich in exotic stellar populations. For several decades it has been a favorite target of observers, and yet it is computationally very challenging to model because of its large number of stars ($N\gtrsim 10^6$) and high density. Here we present detailed and self-consistent 47 Tuc models computed with the \texttt{Cluster Monte Carlo} code (\texttt{CMC}). The models include all relevant dynamical interactions coupled to stellar and binary evolution, and reproduce various observations, including the surface brightness and velocity dispersion profiles, pulsar accelerations, and numbers of compact objects. We show that the present properties of 47 Tuc are best reproduced by adopting an initial stellar mass function that is both bottom-heavy and top-light relative to standard assumptions \citep[as in, e.g.,][]{Kroupa2001}, and an initial Elson profile \citep{Elson1987} that is overfilling the cluster's tidal radius. We include new prescriptions in \texttt{CMC} for the formation of binaries through giant star collisions and tidal captures, and we show that these mechanisms play a crucial role in the formation of neutron star binaries and millisecond pulsars in 47 Tuc; our best-fit model contains $\sim 50$ millisecond pulsars, $80\%$ of which are formed through giant collisions and tidal captures. Our models also suggest that 47 Tuc presently contains up to $\sim 200$ stellar-mass black holes, $\sim 5$ binary black holes, $\sim 15$ low-mass X-ray binaries, and $\sim 300$ cataclysmic variables.

Journal ArticleDOI
TL;DR: In this article, the authors explore the clustering of galaxy groups in the Galaxy and Mass Assembly (GAMA) survey to investigate the dependence of group bias and profile on separation scale and group mass.
Abstract: We explore the clustering of galaxy groups in the Galaxy and Mass Assembly (GAMA) survey to investigate the dependence of group bias and profile on separation scale and group mass. Due to the inherent uncertainty in estimating the group selection function, and hence the group autocorrelation function, we instead measure the projected galaxy–group cross-correlation function. We find that the group profile has a strong dependence on scale and group mass on scales r⊥≲1h−1⁠. We also find evidence that the most massive groups live in extended, overdense, structures. In the first application of marked clustering statistics to groups, we find that group-mass marked clustering peaks on scales comparable to the typical group radius of r⊥ ≈ 0.5 h−1. While massive galaxies are associated with massive groups, the marked statistics show no indication of galaxy mass segregation within groups. We show similar results from the IllustrisTNG simulations and the L-GALAXIES model, although L-GALAXIES shows an enhanced bias and galaxy mass dependence on small scales.

Journal ArticleDOI
TL;DR: In this paper, the authors estimate the total and spectral-type-wise high mass-ratio (HMR) BF in 23 open clusters using unresolved binaries in color-magnitude diagrams using Gaia DR2 data.
Abstract: Binary stars play a vital role in astrophysical research, as a good fraction of stars are in binaries. Binary fraction (BF) is known to change with stellar mass in the Galactic field, but such studies in clusters require binary identification and membership information. Here, we estimate the total and spectral-type-wise high mass-ratio (HMR) BF ($f^{0.6}$) in 23 open clusters using unresolved binaries in color-magnitude diagrams using \textit{Gaia} DR2 data. We introduce the segregation index (SI) parameter to trace mass segregation of HMR (total and mass-wise) binaries and the reference population. This study finds that in open clusters, (1) HMR BF for the mass range 0.4--3.6 Msun (early M to late B type) has a range of 0.12 to 0.38 with a peak at 0.12--0.20, (2) older clusters have a relatively higher HMR BF, (3) the mass-ratio distribution is unlikely to be a flat distribution and BF(total) $\sim$ (1.5 to 2.5) $\times f^{0.6}$, (4) a decreasing BF(total) from late B-type to K-type, in agreement with the Galactic field stars, (5) older clusters show radial segregation of HMR binaries, (6) B and A/F type HMR binaries show radial segregation in some young clusters suggesting a primordial origin. This study will constrain the initial conditions and identify the major mechanisms that regulate binary formation in clusters. Primordial segregation of HMR binaries could result from massive clumps spatially segregated in the collapse phase of the molecular cloud.

Posted Content
TL;DR: In this paper, the authors investigated properties of intermediate-mass black holes (IMBHs) that escape from star clusters due to dynamical interactions and found that most of these objects escape in a binary with another black hole (BH) as their companion and they have masses between $100$ and $140$.
Abstract: In this work we investigate properties of intermediate-mass black holes (IMBHs) that escape from star clusters due to dynamical interactions. The studied models were simulated as part of the preliminary second survey carried out using the MOCCA code (MOCCA-SURVEY Database II), which is based on the Monte Carlo N-body method. We have found that IMBHs are more likely to be formed and ejected in models where both initial central density and central escape velocities have high values. Most of our studied objects escape in a binary with another black hole (BH) as their companion and they have masses between $100$ and $140\: M_{\odot}$. Escaping IMBHs tend to build-up mass most effectively through repeated mergers in a binary with BHs due to gravitational wave emission. Binaries play a key role in their ejection from the system as they allow these massive objects to gather energy needed for escape. The binaries in which IMBHs escape tend to have very high binding energy at the time of escape and the last interaction is strong but does not involve a massive intruder. These IMBHs gain energy needed to escape the cluster gradually in successive dynamical interactions. We present specific examples of the history of IMBH formation and escape from our star cluster models. We also discuss the observational implications of our findings and estimate that the merger rate of ejected IMBH binaries from star clusters is $\mathcal{R}=0.7\:\textrm{Gpc}^{-3}\:\textrm{yr}^{-1}$.

Journal ArticleDOI
TL;DR: In this article, the authors explore the evolution of rotating multi-mass star clusters during the violent relaxation phase, in the presence of a weak external tidal field, and show that massive stars tend to rotate more rapidly than low-mass stars around the axis of cluster rotation.
Abstract: We present the results of a study aimed at exploring, by means of N-body simulations, the evolution of rotating multi-mass star clusters during the violent relaxation phase, in the presence of a weak external tidal field. We study the implications of the initial rotation and the presence of a mass spectrum for the violent relaxation dynamics and the final properties of the equilibria emerging at the end of this stage. Our simulations show a clear manifestation of the evolution towards spatial mass segregation and evolution towards energy equipartition during and at the end of the violent relaxation phase. We study the final rotational kinematics and show that massive stars tend to rotate more rapidly than low-mass stars around the axis of cluster rotation. Our analysis also reveals that during the violent relaxation phase, massive stars tend to preferentially segregate into orbits with angular momentum aligned with the cluster's angular momentum, an effect previously found in the context of the long-term evolution of star clusters driven by two-body relaxation.

Journal ArticleDOI
TL;DR: Using surface brightness and velocity dispersion profiles from the literature, the authors fit $59$ Milky Way globular clusters to dynamical models from the \texttt{CMC Cluster Catalog}.
Abstract: As ancient, gravitationally bound stellar populations, globular clusters are abundant, vibrant laboratories characterized by high frequencies of dynamical interactions coupled to complex stellar evolution. Using surface brightness and velocity dispersion profiles from the literature, we fit $59$ Milky Way globular clusters to dynamical models from the \texttt{CMC Cluster Catalog}. Without doing any interpolation, and without any directed effort to fit any particular cluster, $26$ globular clusters are well-matched by at least one of our models. We discuss in particular the core-collapsed clusters NGC 6293, NGC 6397, NGC 6681, and NGC 6624, and the non-core-collapsed clusters NGC 288, NGC 4372, and NGC 5897. As NGC 6624 lacks well-fitting snapshots on the main \texttt{CMC Cluster Catalog}, we run six additional models in order to refine the fit. We calculate metrics for mass segregation, explore the production of compact object sources such as millisecond pulsars, cataclysmic variables, low-mass X-ray binaries, and stellar-mass black holes, finding reasonable agreement with observations. Additionally, closely mimicking observational cuts, we extract the binary fraction from our models, finding good agreement except in the dense core regions of core-collapsed clusters. Accompanying this paper are a number of \textsf{python} methods for examining the publicly accessible \texttt{CMC Cluster Catalog}, as well as any other models generated using \texttt{CMC}.

Journal ArticleDOI
TL;DR: In this paper, the authors carried out a systematic statistical study of the binary stars that evolve to form a BH--NS binary, considering different metallicities and taking into account the uncertainties on the natal kick distributions for BHs and NSs and on the common envelope phase of binary evolution.
Abstract: The long wait for the detection of merging black hole -- neutron star (BH--NS) binaries is finally over with the announcement by the LIGO/Virgo/Kagra collaboration of GW200105 and GW200115. Remarkably, the primary of GW200115 has a negative spin projection onto the orbital angular momentum, with about $90\%$ probability. Merging BH--NS binaries are expected to form mainly through the evolution of massive binary stars in the field, since their dynamical formation in dense star clusters is strongly suppressed by mass segregation. In this paper, we carry out a systematic statistical study of the binary stars that evolve to form a BH--NS binary, considering different metallicities and taking into account the uncertainties on the natal kick distributions for BHs and NSs and on the common envelope phase of binary evolution. Under the assumption that the initial stellar spins are aligned with the binary angular momentum, we show that both large natal kicks ($\gtrsim 150\kms$) and high efficiencies for common envelope ejection are required to simultaneously explain the inferred high merger rates and the large spin-orbit misalignment of GW200115.

Journal ArticleDOI
TL;DR: In this article, the authors presented 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 and compared their evolution to that of King and fractal models with different length scales.
Abstract: Most of massive stars form in binary or higher-order systems in clumpy, sub-structured 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 sub-clump 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% to 10% 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 absence of original binaries, the dynamically formed binaries present a mass dependent binary fraction, that mimics the trend of the observed one.

Journal ArticleDOI
TL;DR: In this article, the dissolution process of young embedded star clusters with different primordial mass segregation levels using fractal distributions was investigated by means of N-body simulations, and it was shown that the fraction of bound stellar mass can be well predicted just right after the gas expulsion but tends to be lower at later stages, as these systems evolve due to the stronger two-body interactions resulting from the inclusion of a realistic initial mass function.
Abstract: We investigate the dissolution process of young embedded star clusters with different primordial mass segregation levels using fractal distributions by means of N-body simulations. We combine several star clusters in virial and subvirial global states with Plummer and uniform density profiles to mimic the gas. The star clusters have masses of M-stars = 500 M-circle dot that follow an initial mass function where the stars have maximum distance from the centre of r = 1.5 pc. The clusters are placed in clouds that at the same radius have masses of M-cloud = 2000 M-circle dot, resulting in star formation efficiency of 0.2. We remove the background potential instantaneously at a very early phase, mimicking the most destructive scenario of gas expulsion. The evolution of the fraction of bound stellar mass is followed for a total of 16 Myr for simulations with stellar evolution and without. We compare our results with previous works using equal-mass particles where an analytical physical model was used to estimate the bound mass fraction after gas expulsion. We find that independent of the initial condition, the fraction of bound stellar mass can be well predicted just right after the gas expulsion but tends to be lower at later stages, as these systems evolve due to the stronger two-body interactions resulting from the inclusion of a realistic initial mass function. This discrepancy is independent of the primordial mass segregation level.

Journal ArticleDOI
TL;DR: In this paper, a Monte-Carlo approach is used to model the diffusion of both captured objects and compact-objects brought through two-body relaxation processes, and the rates of GW-inspirals resulting from relaxation-driven objects, and characterize EMRIs properties.
Abstract: The gravitational-wave (GW) inspirals of stellar-mass compact objects onto a supermassive black hole (MBH), are some of the most promising GW sources detectable by next-generation space-born GW-detectors. The rates and characteristics of such extreme mass ratio inspirals (EMRIs) sources are highly uncertain. They are determined by the dynamics of stars near MBHs, and the rate at which compacts objects are driven to the close proximity of the MBH. Here we consider weakly and strongly mass-segregated nuclear clusters, and the evolution of stars captured into highly eccentric orbits following binary disruptions by the MBH. We make use of a Monte-Carlo approach to model the diffusion of both captured objects, and compact-objects brought through two-body relaxation processes. We calculate the rates of GW-inspirals resulting from relaxation-driven objects, and characterize EMRIs properties. We correct previous studies and show that relaxation-driven sources produce GW-sources with lower-eccentricity than previously found, and provide the detailed EMRI eccentricity distribution in the weak and strong mass-segregation regimes. We also show that binary-disruption captured-stars could introduce low-eccentricity GW-sources of stellar black-hole EMRIs in mass-segregated clusters. The eccentricities of the GW-sources from the capture channel, however, are strongly affected by relaxation processes, and are significantly higher than previously suggested. We find that both the rate and eccentricity distribution of EMRIs could probe the dynamics near MBHs, and the contribution of captured stars, characterize the mass-function of stellar compact objects, and verify whether weak or strong mass-segregation processes take place near MBHs.

Journal ArticleDOI
TL;DR: In this paper, the authors present a comprehensive investigation of main-sequence (MS) binaries in the DRAGON simulations, which are the first one-million particles direct $N$-body simulations of globular clusters.
Abstract: We present a comprehensive investigation of main-sequence (MS) binaries in the DRAGON simulations, which are the first one-million particles direct $N$-body simulations of globular clusters. We analyse the orbital parameters of the binary samples in two of the DRAGON simulations, D1-R7- IMF93 and D2-R7-IMF01, focusing on their secular evolution and correlations up to 12 Gyr. These two models have different initial stellar mass functions: Kroupa 1993 (D1-R7-IMF93) and Kroupa 2001 (D2-R7-IMF01); and different initial mass ratio distributions: random paring (D1-R7-IMF93) and a power-law (D1-R7-IMF93). In general, the mass ratio of a population of binaries increases over time due to stellar evolution, which is less significant in D2-R7-IMF01. In D1-R7-IMF93, primordial binaries with mass ratio $q \approx$ 0.2 are most common, and the frequency linearly declines with increasing $q$ at all times. Dynamical binaries of both models have higher eccentricities and larger semi-major axes than primordial binaries. They are preferentially located in the inner part of the star cluster. Secular evolution of binary orbital parameters does not depend on the initial mass-ratio distribution, but is sensitive to the initial binary distribution of the system. At t = 12 Gyr, the binary fraction decreases radially outwards, and mass segregation is present. A color difference of 0.1 mag in $F330W-F814W$ and 0.2 mag in $NUV-y$ between the core and the outskirts of both clusters is seen, which is a reflection of the binary radial distribution and the mass segregation in the cluster. The complete set of data for primordial and dynamical binary systems at all snapshot intervals is made publicly available.

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TL;DR: In this paper, a maximum entropy method was used to systematically determine the long-term distribution of orientations of stellar stars in galactic nuclei and use it for a wide range of stellar orbital parameters and initial conditions.
Abstract: Supermassive black holes dominate the gravitational potential in galactic nuclei. In these dense environments, stars follow nearly Keplerian orbits and see their orbital planes relax through the potential fluctuations generated by the stellar cluster itself. For typical astrophysical galactic nuclei, the most likely outcome of this vector resonant relaxation (VRR) is that the orbital planes of the most massive stars spontaneously self-align within a narrow disc. We present a maximum entropy method to systematically determine this long-term distribution of orientations and use it for a wide range of stellar orbital parameters and initial conditions. The heaviest stellar objects are found to live within a thin equatorial disk. The thickness of this disk depends on the stars' initial mass function, and on the geometry of the initial cluster. This work highlights a possible (indirect) novel method to constrain the distribution of intermediate mass black holes in galactic nuclei.

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TL;DR: In this article, a deep learning framework is proposed to predict the mass distribution and age of globular clusters, and the shape of the mass-to-light profile and its global value with a mean error of 12 percent.
Abstract: Dynamical mass estimates of simple systems such as globular clusters (GCs) still suffer from up to a factor of 2 uncertainty. This is primarily due to the oversimplifications of standard dynamical models that often neglect the effects of the long-term evolution of GCs. Here, we introduce a new approach to measure the dynamical properties of GCs, based on the combination of a deep-learning framework and the large amount of data from direct N-body simulations. Our algorithm, π-DOC (Predicting Images for the Dynamics Of stellar Clusters) is composed of two convolutional networks, trained to learn the non-trivial transformation between an observed GC luminosity map and its associated mass distribution, age, and distance. The training set is made of V-band luminosity and mass maps constructed as mock observations from N-body simulations. The tests on π-DOC demonstrate that we can predict the mass distribution with a mean error per pixel of 27 per cent, and the age and distance with an accuracy of 1.5 Gyr and 6 kpc, respectively. In turn, we recover the shape of the mass-to-light profile and its global value with a mean error of 12 per cent, which implies that we efficiently trace mass segregation. A preliminary comparison with observations indicates that our algorithm is able to predict the dynamical properties of GCs within the limits of the training set. These encouraging results demonstrate that our deep-learning framework and its forward modelling approach can offer a rapid and adaptable tool competitive with standard dynamical models.

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TL;DR: In this paper, the authors used PopStar+Cloudy models to simulate the emission-line spectrum and found that the central emission line spectrum can be explained by a single young ionizing cluster with an age ~ 3.5 +/- 0.0004 and a stellar mass of about 2000 solar masses.
Abstract: We describe the observations of the low-metallicity nearby galaxy AGC198691 (Leoncino dwarf) obtained with the Integral Field Unit of the instrument MEGARA at the Gran Telescopio Canarias. The observations cover the wavelength ranges 4304 - 5198 A and 6098 - 7306 A with a resolving power R ~ 6000. We present 2D maps of the ionized gas, deriving the extension of the HII region and gas kinematics from the observed emission lines. We have not found any evidence of recent gas infall or loss of metals by means of outflows. This result is supported by the closed-box model predictions, consistent with the oxygen abundance found by other authors in this galaxy and points towards Leoncino being a genuine XMD galaxy. We present for the first time spatially resolved spectroscopy allowing the detailed study of a star forming region. We use PopStar+Cloudy models to simulate the emission-line spectrum. We find that the central emission line spectrum can be explained by a single young ionizing cluster with an age ~ 3.5 +/- 0.5Myr and a stellar mass of about 2000 solar masses. However, the radial profiles of [OIII]5007 A and the Balmer lines in emission demand photoionization by clusters of different ages between 3.5 and 6.5Myr that might respond either to the evolution of a single cluster evolving along the cooling time of the nebula (about 3Myr at the metallicity of Leoncino, Z ~ 0.0004) or to mass segregation of the cluster, being both scenarios consistent with the observed equivalent widths of the Balmer lines

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TL;DR: In this paper, the authors investigated more than 1000 star cluster models and obtained the local rate density of white dwarf (WD) tidal disruption events (TDEs) in globular clusters (GCs) and young massive clusters (YMCs).
Abstract: We investigate more than 1000 star cluster models (about half of all the cluster models in MOCCA-Survey Database I), and obtain the local rate density of white dwarf (WD) tidal disruption events (TDEs) in globular clusters (GCs) and young massive clusters (YMCs). We find that WD TDEs in a star cluster happen 1000 times more efficiently than predicted previously. We take into account WD TDEs in GCs, YMCs, and dwarf galaxies, and obtain the total WD TDE rate density in the local universe as $\sim 5.0 \times 10^2~{\rm yr}^{-1}~{\rm Gpc}^{-3}$, 90 % of which happens in GCs. The total WD TDE rate density is 50 times larger than estimated before. Our results show that thermonuclear explosions induced by WD TDEs can be observed at a rate of $\lesssim 550~{\rm yr}^{-1}$ by the next generation optical surveys, such as the Large Synoptic Survey Telescope. We also find that massive WDs are preferentially disrupted due to mass segregation, and that 20 % of exploding WDs have $\gtrsim 1.0 M_\odot$ despite of small population of such WDs. Such explosions can be as luminous and long as type Ia supernovae (SNe Ia), in contrast to previous arguments that such explosions are observed as more rapid and faint transients than SNe Ia due to their small radioactive mass ($\lesssim 0.1 M_\odot$) and ejecta mass ($\lesssim 0.6 M_\odot$).

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TL;DR: In this paper, a new set of GC models, varying the initial mass function (IMF) within observational uncertainties, are presented, and it is shown that GCs with top-heavy IMF lose most of their mass within a few Gyr through stellar winds and tidal stripping.
Abstract: Recent observations of globular clusters (GCs) provide evidence that the stellar initial mass function (IMF) may not be universal, suggesting specifically that the IMF grows increasingly top-heavy with decreasing metallicity and increasing gas density. Non-canonical IMFs can greatly affect the evolution of GCs, mainly because the high end determines how many black holes (BHs) form. Here we compute a new set of GC models, varying the IMF within observational uncertainties. We find that GCs with top-heavy IMFs lose most of their mass within a few Gyr through stellar winds and tidal stripping. Heating of the cluster through BH mass segregation greatly enhances this process. We show that, as they approach complete dissolution, GCs with top-heavy IMFs can evolve into 'dark clusters' consisting of mostly BHs by mass. In addition to producing more BHs, GCs with top-heavy IMFs also produce many more binary BH (BBH) mergers. Even though these clusters are short-lived, mergers of ejected BBHs continue at a rate comparable to, or greater than, what is found for long-lived GCs with canonical IMFs. Therefore these clusters, although they are no longer visible today, could still contribute significantly to the local BBH merger rate detectable by LIGO/Virgo, especially for sources with higher component masses well into the BH mass gap. We also report that one of our GC models with a top-heavy IMF produces dozens of intermediate-mass black holes (IMBHs) with masses $M>100\,{\rm M_\odot}$, including one with $M>500\,{\rm M_\odot}$. Ultimately, additional gravitational wave observations will provide strong constraints on the stellar IMF in old GCs and the formation of IMBHs at high redshift.

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TL;DR: In this paper, the authors presented deep UBV I and Halpha photometry for the young open cluster IC 1590 in the center of the H II region NGC 281.
Abstract: Young open clusters are ideal laboratories to understand star formation process. We present deep UBV I and Halpha photometry for the young open cluster IC 1590 in the center of the H II region NGC 281. Early-type members are selected from UBV photometric diagrams, and low-mass pre-main sequence (PMS) members are identified by using Halpha photometry. In addition, the published X-ray source list and Gaia astrometric data are also used to isolate probable members. A total of 408 stars are selected as members. The mean reddening obtained from early-type members is 1 M_sun). The signature of mass segregation is detected from the difference in the slopes of the initial mass functions for the inner (r < 2.'5) and outer region of this cluster. We finally discuss the star formation history in NGC 281.

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TL;DR: In this paper, the authors presented deep UBV I and Halpha photometry for the young open cluster IC 1590 in the center of the H II region NGC 281.
Abstract: Young open clusters are ideal laboratories to understand star formation process. We present deep UBV I and Halpha photometry for the young open cluster IC 1590 in the center of the H II region NGC 281. Early-type members are selected from UBV photometric diagrams, and low-mass pre-main sequence (PMS) members are identified by using Halpha photometry. In addition, the published X-ray source list and Gaia astrometric data are also used to isolate probable members. A total of 408 stars are selected as members. The mean reddening obtained from early-type members is 1 M_sun). The signature of mass segregation is detected from the difference in the slopes of the initial mass functions for the inner (r < 2.'5) and outer region of this cluster. We finally discuss the star formation history in NGC 281.

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TL;DR: In this article, the authors used the clustering algorithm HDBSCAN on Gaia parallaxes and proper motions to systematically look for members up to 50 pc from the cluster centers.
Abstract: The distribution of member stars in the surroundings of an Open Cluster (OC) can shed light on the process of its formation, evolution and dissolution. The analysis of structural parameters of OCs as a function of their age and position in the Galaxy brings constraints on theoretical models of cluster evolution. The Gaia catalogue is very appropriate to find members of OCs at large distance from their centers. We aim at revisiting the membership lists of OCs from the solar vicinity, in particular by extending these membership lists to the peripheral areas thanks to Gaia EDR3. We used the clustering algorithm HDBSCAN on Gaia parallaxes and proper motions to systematically look for members up to 50 pc from the cluster centers. We fitted a King's function on the radial density profile of these clusters and a Gaussian Mixture Model on their two dimensional distribution of members. We also evaluated the degree of mass segregation of the clusters. Our methodology performs well on 389 clusters out of the 467 selected ones. We report the detection of vast coronae around almost all the clusters and the detection of 71 OCs with tidal tails, multiplying by more than four the number of such structures identified. We find the size of the cores to be on average smaller for old clusters than for young ones. Also, the overall size of the clusters seems to slightly increase with age while the fraction of stars in the halo seems to decrease. As expected the mass segregation is more pronounced in the oldest clusters but a clear trend with age is not seen. OCs are more extended than previously expected, regardless of their age. The decrease in the proportion of stars populating the clusters halos highlights the different cluster evaporation processes and the short timescales they need to affect the clusters. Reported parameters all depend on cluster ages but can not be described as single functions of time.

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TL;DR: In this paper, the authors proposed a model to constrain the density and mass of the Orion Nebula Cluster progenitor clump, and used it to analyze the star formation histories of starburst clusters.
Abstract: We present a novel approach to the riddle of star cluster multiple populations. Stars form from molecular cores. But not all cores form stars. Following their initial compression, such 'failed' cores re-expand, rather than collapsing. We propose that their formation and subsequent dispersal regulate the gas density of cluster-forming clumps and, therefore, their core and star formation rates. Clumps for which failed cores are the dominant core type experience star formation histories with peaks and troughs. In contrast, too few failed cores results in smoothly decreasing star formation rates. We identify three main parameters shaping the star formation history of a clump: the star and core formation efficiencies per free-fall time, and the time-scale on which failed cores return to the clump gas. The clump mass acts as a scaling factor. We use our model to constrain the density and mass of the Orion Nebula Cluster progenitor clump, and to caution that the star formation histories of starburst clusters may contain close-by peaks concealed by stellar age uncertainties. Our model generates a great variety of star formation histories. Intriguingly, the chromosome maps and O-Na anti-correlations of old globular clusters also present diverse morphologies. This prompts us to discuss our model in the context of globular cluster multiple stellar populations. More massive globular clusters exhibit stronger multiple stellar population patterns, which our model can explain if the formation of the polluting stars requires a given stellar mass threshold.