arXiv subjects

Frederic A. Rasio

Publications and source records attributed to Frederic A. Rasio.

At least 55 records · Page 3Linked to original sources

arXiv2021

No Black Holes in NGC 6397

Recently, \citet{vitral2021does} detected a central concentration of dark objects in the core-collapsed globular cluster NGC 6397, which could be interpreted as a subcluster of stellar-mass black holes. However, it is well established theoretically that any significant number of black holes in the cluster would provide strong dynamical heating and is fundamentally inconsistent with this cluster's core-collapsed profile. Claims of intermediate-mass black holes in core-collapsed clusters should similarly be treated with suspicion, for reasons that have been understood theoretically for many decades. Instead, the central dark population in NGC 6397 is exactly accounted for by a compact subsystem of white dwarfs, as we demonstrate here by inspection of a previously published model that provides a good fit to this cluster. These central subclusters of heavy white dwarfs are in fact a generic feature of core-collapsed clusters, while central black hole subclusters are present in all {\em non\/}-collapsed clusters.

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arXiv2021

Matching Globular Cluster Models to Observations

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}.

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arXiv2021

The Observed Rate of Binary Black Hole Mergers can be Entirely Explained by Globular Clusters

Since the first signal in 2015, the gravitational-wave detections of merging binary black holes (BBHs) by the LIGO and Virgo collaborations (LVC) have completely transformed our understanding of the lives and deaths of compact object binaries, and have motivated an enormous amount of theoretical work on the astrophysical origin of these objects. We show that the phenomenological fit to the redshift-dependent merger rate of BBHs from Abbott et al. (2020) is consistent with a purely dynamical origin for these objects, and that the current merger rate of BBHs from the LVC could be explained entirely with globular clusters alone. While this does not prove that globular clusters are the dominant formation channel, we emphasize that many formation scenarios could contribute a significant fraction of the current LVC rate, and that any analysis that assumes a single (or dominant) mechanism for producing BBH mergers is implicitly using a specious astrophysical prior.

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arXiv2021

Intermediate-mass Black Holes from High Massive-star Binary Fractions in Young Star Clusters

Black holes formed in dense star clusters, where dynamical interactions are frequent, may have fundamentally different properties than those formed through isolated stellar evolution. Theoretical models for single star evolution predict a gap in the black hole mass spectrum from roughly $40-120\,M_{\odot}$ caused by (pulsational) pair-instability supernovae. Motivated by the recent LIGO/Virgo event GW190521, we investigate whether black holes with masses within or in excess of this "upper-mass gap" can be formed dynamically in young star clusters through strong interactions of massive stars in binaries. We perform a set of $N$-body simulations using the CMC cluster-dynamics code to study the effects of the high-mass binary fraction on the formation and collision histories of the most massive stars and their remnants. We find that typical young star clusters with low metallicities and high binary fractions in massive stars can form several black holes in the upper-mass gap and often form at least one intermediate-mass black hole. These results provide strong evidence that dynamical interactions in young star clusters naturally lead to the formation of more massive black hole remnants.

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arXiv2021

Black Hole Mergers from Star Clusters with Top-Heavy Initial Mass Functions

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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arXiv2020

On the Origin of GW190521-like events from repeated black hole mergers in star clusters

LIGO and Virgo have reported the detection of GW190521, from the merger of a binary black hole (BBH) with a total mass around $150$ M$_\odot$. While current stellar models limit the mass of any black hole (BH) remnant to about $40 - 50$ M$_\odot$, more massive BHs can be produced dynamically through repeated mergers in the core of a dense star cluster. The process is limited by the recoil kick (due to anisotropic emission of gravitational radiation) imparted to merger remnants, which can escape the parent cluster, thereby terminating growth. We study the role of the host cluster metallicity and escape speed in the buildup of massive BHs through repeated mergers. Almost independent of host metallicity, we find that a BBH of about $150$ M$_\odot$ could be formed dynamically in any star cluster with escape speed $\gtrsim 200$ km s$^{-1}$, as found in galactic nuclear star clusters as well as the most massive globular clusters and super star clusters. Using an inspiral-only waveform, we compute the detection probability for different primary masses ($\ge 60$ M$_\odot$) as a function of secondary mass and find that the detection probability increases with secondary mass and decreases for larger primary mass and redshift. Future additional detections of massive BBH mergers will be of fundamental importance for understanding the growth of massive BHs through dynamics and the formation of intermediate-mass BHs.

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arXiv2020

Black Hole Mergers from Hierarchical Triples in Dense Star Clusters

Hierarchical triples are expected to be produced by the frequent binary-mediated interactions in the cores of globular clusters. In some of these triples, the tertiary companion can drive the inner binary to merger following large eccentricity oscillations, as a result of the eccentric Kozai-Lidov mechanism. In this paper, we study the dynamics and merger rates of black hole (BH) hierarchical triples, formed via binary--binary encounters in the CMC Cluster Catalog, a suite of cluster simulations with present-day properties representative of the Milky Way's globular clusters. We compare the properties of the mergers from triples to the other merger channels in dense star clusters, and show that triple systems do not produce significant differences in terms of mass and effective spin distribution. However, they represent an important pathway for forming eccentric mergers, which could be detected by LIGO--Virgo/KAGRA (LVK), and future missions such as LISA and DECIGO. We derive a conservative lower limit for the merger rate from this channel of $0.35$ Gpc$^{-3}$yr$^{-1}$ in the local Universe and up to $\sim9\%$ of these events may have a detectable eccentricity at LVK design sensitivity. Additionally, we find that triple systems could play an important role in retaining second-generation BHs, which can later merge again in the core of the host cluster.

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arXiv2020

Populating the upper black hole mass gap through stellar collisions in young star clusters

Theoretical modeling of massive stars predicts a gap in the black hole (BH) mass function above $\sim 40-50\,M_{\odot}$ for BHs formed through single star evolution, arising from (pulsational) pair-instability supernovae. However, in dense star clusters, dynamical channels may exist that allow construction of BHs with masses in excess of those allowed from single star evolution. The detection of BHs in this so-called "upper-mass gap" would provide strong evidence for the dynamical processing of BHs prior to their eventual merger. Here, we explore in detail the formation of BHs with masses within or above the pair-instability gap through collisions of young massive stars in dense star clusters. We run a suite of 68 independent cluster simulations, exploring a variety of physical assumptions pertaining to growth through stellar collisions, including primordial cluster mass segregation and the efficiency of envelope stripping during collisions. We find that as many as $\sim20\%$ of all BH progenitors undergo one or more collisions prior to stellar collapse and up to $\sim1\%$ of all BHs reside within or above the pair-instability gap through the effects of these collisions. We show that these BHs readily go on to merge with other BHs in the cluster, creating a population of massive BH mergers at a rate that may compete with the "multiple-generation" merger channel described in other analyses. This has clear relevance for the formation of very massive BH binaries as recently detected by LIGO/Virgo in GW190521. Finally, we describe how stellar collisions in clusters may provide a unique pathway to pair-instability supernovae and briefly discuss the expected rate of these events and other electromagnetic transients.

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arXiv2020

COSMIC Variance in Binary Population Synthesis

The formation and evolution of binary stars is a critical component of several fields in astronomy. The most numerous sources for gravitational wave observatories are inspiraling and/or merging compact binaries, while binary stars are present in nearly every electromagnetic survey regardless of the target population. Simulations of large binary populations serve to both predict and inform observations of electromagnetic and gravitational wave sources. Binary population synthesis is a tool that balances physical modeling with simulation speed to produce large binary populations on timescales of days. We present a community-developed binary population synthesis suite: COSMIC which is designed to simulate compact-object binary populations and their progenitors. As a proof of concept, we simulate the Galactic population of compact binaries and their gravitational wave signal observable by the Laser Interferometer Space Antenna (LISA). We find that $\sim10^8$ compact binaries reside in the Milky Way today, while $\sim10^4$ of them may be resolvable by LISA.

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arXiv2020

Demographics of triple systems in dense star clusters

Depending on the stellar type, more than $\sim 50$\% and $\sim 15\%$ of stars in the field have at least one and two stellar companions, respectively. Hierarchical systems can be assembled dynamically in dense star clusters, as a result of few-body encounters among stars and/or compact remnants in the cluster core. In this paper, we present the demographics of stellar and compact-object triples formed via binary--binary encounters in the \texttt{CMC Cluster Catalog}, a suite of cluster simulations with present-day properties representative of the globular clusters (GCs) observed in the Milky Way. We show how the initial properties of the host cluster set the typical orbital parameters and formation times of the formed triples. We find that a cluster typically assembles hundreds of triples with at least one black hole (BH) in the inner binary, while only clusters with sufficiently small virial radii are efficient in producing triples with no BHs, as a result of the BH-burning process. We show that a typical GC is expected to host tens of triples with at least one luminous component at present day. We discuss how the Lidov-Kozai mechanism can drive the inner binary of the formed triples to high eccentricities, whenever it takes place before the triple is dynamically reprocessed by encountering another cluster member. Some of these systems can reach sufficiently large eccentricities to form a variety of transients and sources, such as blue stragglers, X-ray binaries, Type Ia Supernovae, Thorne-Zytkow objects, and LIGO/Virgo sources.

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arXiv2020

A Dynamical Survey of Stellar-Mass Black Holes in 50 Milky Way Globular Clusters

Recent numerical simulations of globular clusters (GCs) have shown that stellar-mass black holes (BHs) play a fundamental role in driving cluster evolution and shaping their present-day structure. Rapidly mass-segregating to the center of GCs, BHs act as a dynamical energy source via repeated super-elastic scattering, delaying onset of core collapse and limiting mass segregation for visible stars. While recent discoveries of BH candidates in Galactic and extragalactic GCs have further piqued interest in BH-mediated cluster dynamics, numerical models show that even if significant BH populations remain in today's GCs, they are typically in configurations that are not directly detectable. We demonstrated in Weatherford et al. (2018) that an anti-correlation between a suitable measure of mass segregation ($Δ$) in observable stellar populations and the number of retained BHs in GC models can be applied to indirectly probe BH populations in real GCs. Here, we estimate the number and total mass of BHs presently retained in 50 Milky Way GCs from the ACS Globular Cluster Survey by measuring $Δ$ between populations of main sequence stars, using correlations found between $Δ$ and BH retention in the CMC Cluster Catalog models. We demonstrate that the range in $Δ$'s distribution from our models matches that for observed GCs to a remarkable degree. Our results further provide the narrowest constraints to-date on the retained BH populations in the GCs analyzed. Of these 50 GCs, we identify NGCs 2808, 5927, 5986, 6101, and 6205 to presently contain especially large BH populations, each with total BH mass exceeding $10^3\,\rm{M_{\odot}}$.

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arXiv2020

GW190412 as a Third-Generation Black Hole Merger from a Super Star Cluster

We explore the possibility that GW190412, a binary black hole merger with a non-equal-mass ratio and significantly spinning primary, was formed through repeated black hole mergers in a dense super star cluster. Using a combination of semi-analytic prescriptions for the remnant spin and recoil kick of black hole mergers, we show that the mass ratio and spin of GW190412 are consistent with a binary black hole whose primary component has undergone two successive mergers from a population of $\sim 10M_{\odot}$ black holes in a high-metallicity environment. We then explore the production of GW190412-like analogs in the CMC Cluster Catalog, a grid of 148 $N$-body star cluster models, as well as a new model, behemoth, with nearly $10^7$ particles and initial conditions taken from a cosmological MHD simulation of galaxy formation. We show that the production of binaries with GW190412-like masses and spins is dominated by massive super star clusters with high metallicities and large central escape speeds. While many are observed in the local universe, our results suggest that a careful treatment of these massive clusters, many of which may have been disrupted before the present day, is necessary to characterize the production of unique gravitational-wave events produced through dynamics.

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arXiv2020

Gravitational-wave captures by intermediate-mass black holes in galactic nuclei

Intermediate-mass black holes (IMBHs) have not been detected beyond any reasonable doubt, despite their potential role as massive seeds for quasars and sources of tidal disruption events, ultra-luminous X-ray sources, dwarf galaxy feedback, and hypervelocity stars. Gravitational wave (GW) observations can help to find and confirm the existence of IMBHs. Current and upcoming detectors, such as LIGO, Virgo, KAGRA, LISA, ET, and DECIGO promise to identify the full range from stellar-mass to supermassive black holes (SMBHs). In this paper, we address the question of whether IMBHs can produce GWs in galactic nuclei. We consider the possibility that stellar black holes (SBHs) form bound systems and later coalesce with an IMBH through gravitational captures in the dense nucleus. We show that this mechanism is efficient for IMBH masses in the range $\sim 3\times 10^3$ M$_\odot$--$2\times 10^4$ M$_\odot$. We find that the typical distributions of peak frequencies and merger timescales depend mainly on the IMBH mass. In particular, the typical peak frequency is about $0.2\,$Hz, $0.1\,$Hz, $0.09\,$Hz, and $0.05\,$Hz for $M_{\rm IMBH}=5\times 10^3$ M$_\odot$, $8\times 10^3$ M$_\odot$, $1\times 10^4$ M$_\odot$, and $2\times 10^4$ M$_\odot$, respectively. Our results show that, at design sensitivity, both DECIGO and ET should be able to detect these IMBH--SBH mergers. Furthermore, most of the mergers will appear eccentric ($e \gtrsim 0.1$), providing an indication of their dynamical origin.

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arXiv2020

Merging Black Holes in the Low-mass and High-mass Gaps from 2+2 Quadruple Systems

The origin of the black hole (BH) binary mergers observed by LIGO-Virgo is still uncertain, as are the boundaries of the stellar BH mass function. Stellar evolution models predict a dearth of BHs both at masses $\gtrsim 50$ M$_\odot$ and $\lesssim 5$ M$_\odot$, thus leaving low- and high-mass gaps in the BH mass function. A natural way to form BHs of these masses is through mergers of neutron stars (NSs; for the low-mass gap) or lower-mass BHs (for the high-mass gap); the low- or high-mass-gap BH produced as a merger product can then be detected by LIGO-Virgo if it merges again with a new companion. We show that the evolution of a 2+2 quadruple system can naturally lead to BH mergers with component masses in the low- or high-mass gaps. In our scenario, the BH in the mass gap originates from the merger of two NSs, or two BHs, in one of the two binaries and the merger product is imparted a recoil kick (from anisotropic gravitational wave emission), which triggers its interaction with the other binary component of the quadruple system. The outcome of this three-body interaction is usually a new eccentric compact binary containing the BH in the mass gap, which can then merge again. The merger rate is $\sim 10^{-7} - 10^{-2}$ Gpc$^{-3}$ yr$^{-1}$ and $\sim 10^{-3} - 10^{-2}$ Gpc$^{-3}$ yr$^{-1}$ for BHs in the low-mass and high-mass gap, respectively. As the sensitivity of gravitational wave detectors improves, tighter constraints will soon be placed on the stellar BH mass function.

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arXiv2020

Modeling Dense Star Clusters in the Milky Way and Beyond with the $\texttt{CMC}$ Cluster Catalog

We present a set of 148 independent $N$-body simulations of globular clusters (GCs) computed using the code $\texttt{CMC}$ ($\texttt{Cluster Monte Carlo}$). At an age of $\sim10-13\,$Gyr, the resulting models cover nearly the full range of cluster properties exhibited by the Milky Way GCs, including total mass, core and half-light radii, metallicity, and galactocentric distance. We use our models to investigate the role that stellar-mass black holes play in the process of core collapse. Furthermore, we study how dynamical interactions affect the formation and evolution of several important types of sources in GCs, including low-mass X-ray binaries, millisecond pulsars, blue stragglers, cataclysmic variables, Type Ia supernovae, calcium-rich transients, and merging compact binaries. While our focus here is on old, low-metallicity GCs, our $\texttt{CMC}$ simulations follow the evolution of clusters over a Hubble time, and they include a wide range of metallicities (up to solar), so that our results can also be used to study younger and higher-metallicity star clusters.

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arXiv2019

On the Rate of Neutron Star Binary Mergers from Globular Clusters

The first detection of gravitational waves from a neutron star - neutron star (NS-NS) merger, GW170817, and the increasing number of observations of short gamma-ray bursts (SGRBs) have greatly motivated studies of the origins of NS-NS and neutron star - black hole (NS-BH) binaries. We calculate the merger rates of NS-NS and NS-BH binaries from globular clusters (GCs) using realistic GC simulations with the \texttt{CMC} cluster catalog. We use a large sample of models with a range of initial numbers of stars, metallicities, virial radii and galactocentric distances, representative of the present-day Milky Way GCs, to quantify the inspiral times and volumetric merger rates as a function of redshift, both inside and ejected from clusters. We find that over the complete lifetime of most GCs, stellar BHs dominate the cluster cores and prevent the mass segregation of NSs, thereby reducing the dynamical interaction rates of NSs so that at most a few NS binary mergers are ever produced. We estimate the merger rate in the local universe to be $\sim\rm{0.02\,Gpc^{-3}\,yr^{-1}}$ for both NS-NS and NS-BH binaries, or a total of $\sim 0.04$~Gpc$^{-3}$~yr$^{-1}$ for both populations. These rates are about 5 orders of magnitude below the current empirical merger rate from LIGO/Virgo. We conclude that dynamical interactions in GCs do not play a significant role in enhancing the NS-NS and NS-BH merger rates.

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arXiv2019

Probing the Survival of Planetary Systems in Globular Clusters with Tidal Disruption Events

Among the growing list of confirmed exoplanets, the number of planets identified in dense star clusters remains sparse. Previous analyses have suggested this may be due in part to dynamical interactions that unbind large fractions of planets from their host stars, limiting the survival of planetary systems in clusters. Thus, alternative detection strategies may be necessary to study planets in clusters that may no longer be bound to a host star. Here, we use the cluster Monte Carlo code CMC to explore the evolution of planetary systems in dense star clusters. Depending on a number of initial conditions, we show that $10-50\%$ of primordial planetary systems are broken through dynamical encounters over a cluster's full lifetime, populating clusters with "free-floating" planets. Furthermore, a large number ($30-80\%$) of planets are ejected from their host cluster through strong dynamical encounters and/or tidal loss. Additionally, we show that planets naturally mix with stellar-mass black holes (BHs) in the central regions of their host cluster. As a consequence, up to a few hundreds of planets will be tidally disrupted through close passages of BHs. We show these BH-planet tidal disruption events (TDEs) occur in clusters at a rate of up to $10^{-5}\,\rm{yr}^{-1}$ in a Milky Way-type galaxy. We predict BH-planet TDEs should be detected by upcoming transient surveys such as LSST at a rate of roughly a few events per year. The observed rate of BH-planet TDEs would place new constraints upon the formation and survival of both planetary systems and BHs in dense star clusters. Additionally, depending on various assumptions including the initial number of planets and their orbital properties, we predict that typical globular clusters may contain a few dynamically-formed NS-planet systems at present as well as up to roughly 100 dynamically-formed WD-planet systems.

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arXiv2019

Black Holes: The Next Generation -- Repeated Mergers in Dense Star Clusters and their Gravitational-Wave Properties

When two black holes merge in a dense star cluster, they form a new black hole with a well-defined mass and spin. If that "second-generation" black hole remains in the cluster, it will continue to participate in dynamical encounters, form binaries, and potentially merge again. Using a grid of 96 dynamical models of dense star clusters and a cosmological model of cluster formation, we explore the production of binary black hole mergers where at least one component of the binary was forged in a previous merger. We create four hypothetical universes where every black hole born in the collapse of a massive star has a dimensionless Kerr spin parameter, $χ_{\rm birth}$, of 0.0, 0.1, 0.2, or 0.5. We show that if all stellar-born black holes are non-spinning ($χ_{\rm birth} = 0.0$), then more than 10% of merging binary black holes from clusters have components formed from previous mergers, accounting for more than 20% of the mergers from globular clusters detectable by LIGO/Virgo. Furthermore, nearly 7% of detectable mergers would have a component with a mass $\gtrsim 55M_{\odot}$, placing them clearly in the mass "gap" region where black holes cannot form from isolated collapsing stars due to the pulsational-pair instability mechanism. On the other hand, if black holes are born spinning, then the contribution from these second-generation mergers decreases, making up as little as 1% of all detections from globular clusters when $χ_{\rm birth} = 0.5$. We make quantitative predictions for the detected masses, mass ratios, and spin properties of first- and second-generation mergers from dense star clusters, and show how these distributions are highly sensitive to the birth spins of black holes.

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