arXiv subjects

Frederic A. Rasio

Publications and source records attributed to Frederic A. Rasio.

At least 19 recordsLinked to original sources

arXiv2026

Red vs. Blue: How metallicity shapes black hole dynamics and mergers in dense star clusters

Dense star clusters are a well-established environment for the formation of gravitational wave sources through dynamical interactions. Recent LIGO-Virgo-KAGRA (LVK) events such as GW241011 and GW241110 provide some of the best evidence yet for a dynamical origin. However, their relatively low component masses are in tension with predictions from low-metallicity globular cluster models (which typically produce more massive black holes), hinting that these events may have originated in higher-metallicity environments. Here we present a new set of Monte Carlo star cluster simulations with refined coverage in metallicity, focusing specifically on clusters with [Fe/H] $\geq-1$, similar to the ''red'' globular cluster subpopulation observed in most galaxies. We show that metallicity has a significant effect on the mass function of black holes and black hole mergers, the total number of black hole mergers per cluster, black hole retention from natal kicks, the mass segregation time for black-hole-driven cluster dynamics, and the merger delay time distribution. We also show that high-metallicity cluster models produce low-mass hierarchical mergers consistent with the mass ratios and component masses of GW241011 and GW241110, motivating the importance of high-metallicity clusters in the astrophysical interpretation of future LVK catalogs.

astro-ph.GA↗

arXiv2026

Getting Tilted: Random Walk of Binary Black Hole Spin-Orbit Alignment in Dense Star Clusters

It is commonly assumed that the spin-orbit angles of binary black holes (BBHs) originating from dense stellar environments rapidly converge to an isotropic distribution following a number of strong gravitational encounters. We challenge this assumption by modeling the evolution of the BBH orbital angular momentum orientation through successive binary--single encounters as a random walk on the unit sphere, yielding an exact solution for the orientation distribution after $n$ encounters and a closed-form expression for the number of encounters required to reach isotropy. To characterize the step distribution, we conduct a large suite of Newtonian point-particle scattering experiments with an equal-mass binary, varying the mass of the single, and obtain semi-analytic expressions for both the mean step size and the full distribution of steps. Applying these results to BBHs with initially aligned spins, as may arise from the evolution of primordial massive binaries, we find that spin-orbit alignment can survive several strong encounters before being erased. This has direct implications for the slight trend toward spin-orbit alignment reported in GWTC-5.0 as well as for the retention of hierarchical merger products, such as the components of GW231123.

astro-ph.HE↗

arXiv2026

Black hole mergers from dense star clusters with realistic binary populations

We present a suite of 24 full-lifetime simulations of dense star clusters with the Cluster Monte Carlo (CMC) code, featuring updated input physics and a realistic distribution of initial binary systems. The latter encompasses a mass-dependent binary fraction, period distribution, and eccentricity distribution based on observations of well-studied stellar populations in the Solar neighborhood and nearby star-forming regions. We predict the cosmic rate, masses, and spins of binary black hole (BBH) mergers formed through dynamical assembly, primordial binary evolution, and hierarchical mergers within dense clusters. As with previous model grids with fewer binaries, dynamically assembled first-generation (1G) mergers dominate the rate of cluster-derived mergers, and the total merger rate is consistent with that inferred from LIGO-Virgo-KAGRA observations as of GWTC-5.0. Our models naturally reproduce key features of the inferred BBH population, including the broken-power-law behavior of the primary BH mass spectrum for $m_1 \gtrsim 20 M_\odot$, the shallower (steeper) slope of the secondary mass spectrum relative to the primary for $m_2 \lesssim 10 M_\odot$ ($m_2 \gtrsim 30 M_\odot$), and the shape of the mass-ratio distribution in the low- and high-mass domains. We predict broad distributions of the spin parameters $\chi_{\mathrm{eff}}$ and $\chi_{\mathrm{p}}$, consistent with previous studies of dynamical assembly in clusters. The merger rate from primordial binary systems within clusters is a small fraction of the total; however, their merger products are frequently involved in subsequent hierarchical mergers, with the result that the hierarchical merger rate evolves more steeply than the 1G dynamical merger rate with redshift.

astro-ph.GA↗

arXiv2026

An analytical approach to binary populations in globular clusters

Globular clusters (GCs) display much lower binary fractions than found among main-sequence stars in the solar neighborhood. The physical cause of this difference is debatable: does it reflect different star formation outcomes at low metallicity and/or high density, the dynamical processing of primordial binaries over cluster lifetimes, or a combination of the two? Starting from the assumption that the initial binary distribution in GCs is the same as the binary distribution observed in the solar neighborhood, we show with straightforward analytical calculations that the dynamical dissolution of "soft" primordial binaries can fully explain the main-sequence binary fractions in present-day GCs. We validate our estimates against a detailed N-body simulation with the Cluster Monte Carlo code. Adopting the view that the observed binary fraction in a given cluster constrains the location of the hard/soft boundary at birth, we infer that surviving Milky Way GCs had a similar distribution of birth radii to young massive clusters in the local universe. Our findings underscore the crucial role of stellar black holes (through "black hole burning") in sculpting GC binary populations and reinforce the need for realistic initial conditions in theoretical modeling of GC dynamics.

astro-ph.GA↗

arXiv2026

Survival Analysis of Intermediate-Mass Black Holes in Dense Star Clusters

Recently, an intermediate-mass black hole (IMBH) candidate was announced in the Galactic globular cluster Omega Centauri. IMBHs at the lower end of the traditional mass range have also been detected through gravitational-wave transients, though their formation and subsequent growth linking the two mass scales remains a mystery. One way IMBHs may be produced is through the collapse of very massive stars produced by stellar collisions in dense stellar environments. However, IMBHs may be ejected from such environments by either dynamical recoil from binary-single scattering or gravitational-wave recoil following the merger of two black holes. We conduct Newtonian and post-Newtonian binary-single scattering experiments to study dynamical ejection in greater detail. We obtain fits to the probabilities for dynamical ejection, gravitational wave capture, and per-encounter hardening as a function of the binary mass ratio and hardness with respect to its environment. We borrow techniques from survival analysis (commonly used in studies of medicine, epidemiology, engineering, etc.) to develop a model to calculate the probability of IMBH binary ejection vs in-cluster merger. We confirm that the dynamical ejection probability strongly depends on both the mass ratio of the IMBH compared to other BHs in its environment and on the environment's velocity dispersion. We estimate that for a typical Milky Way globular cluster, IMBHs with mass $\lesssim10^3\,\mathrm{M}_\odot$ are unlikely to be retained until the present. Our results also suggest that IMBH mergers with $q\lesssim0.2$ may be detectable at higher redshifts with future gravitational wave instruments such as the Einstein Telescope and Cosmic Explorer.

astro-ph.HE↗

arXiv2026

Machine Learning Methods for Stellar Collisions. I. Predicting Outcomes of SPH Simulations

Stellar collisions can occur frequently in dense cluster environments, and play a crucial role in producing exotic phenomena from blue stragglers in globular clusters to high-energy transients in galactic nuclei. Successive collisions and mergers of massive stars could also lead to the formation of massive black holes, serving as seeds for supermassive black hole in the early universe. While analytic fitting formulae exist for predicting collision outcomes, they do not generalize across different energy scales or stellar evolutionary phases. Smoothed particle hydrodynamics (SPH) simulations are often used to compute the outcomes of stellar collisions, but, even at low resolution, their computational cost makes running on-the-fly calculations during an $N$-body simulation quite challenging. Here we present a new grid of $27,720$ SPH calculations of main-sequence star collisions, spanning a wide range of masses, ages, relative velocities, and impact parameters. Using this grid, we train machine learning models to predict both collision outcomes (merger vs disruption, or flyby) and final remnant masses. We compare the performance of nearest neighbors, support vector machines, and neural networks, achieving classification balanced accuracy of $98.4\%$, and regression relative errors as low as $0.11\%$ and $0.15\%$ for the final stars $1$ and $2$, respectively. We make our trained models publicly available as part of the package collAIder, enabling rapid predictions of stellar collision outcomes in $N$-body models of dense star cluster dynamics.

astro-ph.HE↗

arXiv2026

Intermediate-Mass Black Hole Formation from Hierarchical Mergers in Galactic Nuclei

Dense stellar environments like nuclear star clusters (NSCs) can dynamically assemble gravitational wave (GW) sources. We consider a population of single stellar mass black holes (BHs) in the inner $0.1$~pc of a NSC surrounding a $4 \times 10^6$~M$_\odot$ supermassive black hole (SMBH). Using a semianalytic model, we account for direct collisions between BHs and stars and GW capture between BHs. We explore the effect of the initial BH mass and spin distributions on their final properties and the production of GW sources. Specifically, we consider upper and lower limits for the BH initial mass distribution, and we account for the possibility that a subset of our initial population are the merger products of primordial BH binaries. We find that $\sim 500$ M$_{\odot}$ intermediate mass black holes (IMBHs) can form for our upper limit mass distribution, while our lower limit mass distribution forms none. Most IMBHs $\gtrsim 200$~M$_\odot$ eventually sink towards the center of the cluster and merge with the SMBH. We also find successive BH-star collisions can produce low-spinning BHs with $\chi \lesssim 0.2$. Our results have implications for LIGO-Virgo-KAGRA sources. We find that the overall merger rate depends primarily on the initial BH mass distribution and is $\gtrsim 10^{-9}$~yr$^{-1}$ per Milky Way-like galaxy for our range of initial conditions. However, primordial binaries can change the number of GW mergers with second and higher generation progenitor BHs by an order of magnitude.

astro-ph.GA↗

arXiv2025

The Interplay of Stellar Evolution and Collisions in Galactic Nuclei

Nuclear star clusters (NSCs) surrounding supermassive black holes (SMBHs) are among the densest stellar environments in the universe. In these environments, collisions can shape the stellar mass function and produce exotic stellar populations. In this work, we investigate how stellar collisions couple with stellar evolution in the inner parsec of an NSC. We simulate the evolution of a sample of $1000$ $1$ $M_\odot$ stars embedded in a uniform cluster of dynamically relaxed 0.5 $M_\odot$ stars. Using COSMIC to evolve stellar properties in time , we track the mass, radius, and evolutionary state of the stars as they collide in the cluster. Our results show that most stars within $0.1$~pc of the SMBH experience a collision while on the main-sequence. However, outside of this distance, stars collide during the red giant phase, when the stellar radius increases dramatically. We find that the most common type of collision -- main-sequence or red giant -- over the lifetime of the cluster depends on the steepness of the stellar cusp, which determines the spatial distribution of the stars in the cluster. These results show that stellar evolution plays a fundamental role in shaping the collisional history of stars in nuclear star clusters. Lastly, we consider whether the closest known stars to the Milky Way's SMBH have experienced a collision. We estimate that several of the S-stars have a high probability of experiencing a collision over their main-sequence lifetime, perhaps with implications for their observed youth and properties.

astro-ph.GA↗

arXiv2025

Beyond Hierarchical Mergers: Accretion-Driven Origins of Massive, Highly Spinning Black Holes in Dense Star Clusters

GW231123, the most massive binary black hole (BBH) merger detected by LIGO/Virgo/KAGRA, highlights the need to understand the origins of massive, high-spin stellar black holes (BHs). Dense star clusters provide natural environments for forming such systems, beyond the limits of standard massive star evolution to core collapse. While repeated BBH mergers can grow BHs through dynamical interactions (the so-called "hierarchical merger" channel), most star clusters with masses $\lesssim 10^6\,M_\odot$ have escape speeds too low to retain higher-generation BHs, limiting growth into or beyond the mass gap. In contrast, BH--star collisions with subsequent accretion of the collision debris can grow and retain BHs irrespective of the cluster escape speed. Using $N$-body (Cluster Monte Carlo) simulations, we study BH growth and spin evolution through this process and we find that accretion can drive BH masses up to at least $\sim200\,M_\odot$, with spins set by the details of the growth history. BHs up to about $150\,M_\odot$ can reach dimensionless spins $χ\gtrsim 0.7$ via single coherent episodes, while more massive BHs form through multiple stochastic accretion events and eventually spin down to $χ\lesssim 0.4$. These BHs later form binaries through dynamical encounters, producing BBH mergers that contribute up to $\sim10\%$ of all detectable events, comparable to predictions for the hierarchical channel. However, the two pathways predict distinct signatures: hierarchical mergers yield more unequal mass ratios, whereas accretion-grown BHs preferentially form near-equal-mass binaries. The accretion-driven channel allows dense clusters with low escape speeds, such as globular clusters, to produce highly spinning BBHs with both components in or above the mass gap, providing a natural formation pathway to GW231123-like systems.

astro-ph.HE↗

arXiv2025

Modeling Stellar Collisions in Galactic Nuclei Using Hydrodynamic Simulations and Machine Learning

Nuclear star clusters represent some of the most extreme collisional environments in the Universe. A nuclear star cluster like that of the Milky Way harbors a supermassive black hole at its center, which accelerates stars to high speeds ($\gtrsim 100$-$1000$ km/s) in a region where millions of other stars reside. Direct collisions occur in such high-density environments, where they can shape the stellar populations and influence the evolution of the cluster. We present a suite of a couple hundred high-resolution smoothed-particle hydrodynamics (SPH) simulations of collisions between $1$ M$_\odot$ stars, at impact speeds representative of galactic nuclei. We use our SPH dataset to develop physically-motivated fitting formulae for predicting collision outcomes. While collision-driven mass loss has been examined in detail in the literature, we present a new framework for understanding the effects of ``hit-and-run'' collisions on a star's trajectory. We demonstrate that the change in stellar velocity follows the tidal-dissipation limit for grazing encounters, while the deflection angle is well-approximated by point-particle dynamics for periapses $\gtrsim 0.3$ times the stellar radii. We use our SPH dataset to test two machine learning (ML) algorithms, k-Nearest Neighbors and neural networks, for predicting collision outcomes. We find that the neural network out-performs k-Nearest Neighbors and delivers results on par with and in some cases exceeding the accuracy of our fitting formulae. We conclude that both fitting formulae and ML have merits for modeling collisions in dense stellar environments, however ML may prove more effective as the parameter space of initial conditions expands.

astro-ph.GA↗

arXiv2025

Intruder Alert: Breaking Resonant Chains with Planetesimal Flybys

The orbital architectures of compact exoplanet systems record their complicated dynamical histories. Recent research supports the ``breaking-the-chains'' hypothesis, which proposes that compact systems typically form in chains of mean-motion resonances (MMRs) but subsequently break out on a $\sim 100$Myr timescale. We investigate a scenario for breaking the chains through intermittent flybys of planetesimals originating from a distant reservoir. Using $N$-body simulations and semi-analytical calculations, we characterize the disruption of MMRs through these flybys. We find a planetesimal reservoir of total mass $\gtrsim 0.04 M_{\oplus}$ is required to disrupt MMR chains, depending on the mass distribution and the typical number of flybys executed by each planetesimal. We verify that systems disrupted in this way are frequently unstable to close encounters within $\sim 100$Myr of the final flyby. This mechanism operates in systems with both a sufficiently massive reservoir and an efficient mechanism for planetesimal injection. Consequently, we predict an anti-correlation between resonant inner systems and dynamically active outer configurations.

astro-ph.EP↗

arXiv2025

Can slow pulsars in Milky Way globular clusters form via partial recycling?

Alongside the population of several hundred radio millisecond pulsars currently known in Milky Way globular clusters, a subset of six slowly spinning pulsars (spin periods $0.3-4\,$s) are also observed. With inferred magnetic fields $\gtrsim 10^{11}\,$G and characteristic ages $\lesssim10^8\,$yr, explaining the formation of these apparently young pulsars in old stellar populations poses a major challenge. One popular explanation is that these objects are not actually young but instead have been partially spun up via accretion from a binary companion. In this scenario, accretion in a typical low-mass X-ray binary is interrupted by a dynamical encounter with a neighboring object in the cluster. Instead of complete spin up to millisecond spin periods, the accretion is halted prematurely, leaving behind a ''partially recycled'' neutron star. In this Letter, we use a combination of analytic arguments motivated by low-mass X-ray binary evolution and $N$-body simulations to show that this partial-recycling mechanism is not viable. Realistic globular clusters are not sufficiently dense to interrupt mass transfer on the short timescales required to achieve such slow spin periods. We argue that collapse of massive white dwarfs and/or neutron star collisions are more promising ways to form slow pulsars in old globular clusters.

astro-ph.SR↗

arXiv2025

Spin-Orbit Alignment in Merging Binary Black Holes Following Collisions with Massive Stars

Merging binary black holes (BBHs) formed dynamically in dense star clusters are expected to have uncorrelated spin--orbit orientations since they are assembled through many random interactions. However, measured effective spins in BBHs detected by LIGO/Virgo/KAGRA hint at additional physical processes that may introduce anisotropy. Here we address this question by exploring the impact of stellar collisions, and accretion of collision debris, on the spin--orbit alignment in merging BBHs formed in dense star clusters. Through hydrodynamic simulations, we study the regime where the disruption of a massive star by a BBH causes the stellar debris to form individual accretion disks bound to each black hole. We show that these disks, which are randomly oriented relative to the binary orbital plane after the initial disruption of the star, can be reoriented by strong tidal torques in the binary near pericenter passages. Following accretion by the BHs on longer timescales, BBHs with small but preferentially positive effective spin parameters ($χ_{\rm eff} \lesssim 0.2$) are formed. Our results indicate that BBH collisions in young massive star clusters could contribute to the observed trend toward small positive $χ_{\rm eff}$, and we suggest that the standard assumption often made that dynamically assembled BBHs should have isotropically distributed BH spins is not always justified.

astro-ph.HE↗

arXiv2025

Black Hole Accretion and Spin-up Through Stellar Collisions in Dense Star Clusters

Dynamical interactions in dense star clusters could significantly influence the properties of black holes, leaving imprints on their gravitational-wave signatures. While previous studies have mostly focused on repeated black hole mergers for spin and mass growth, this work examines the impact of physical collisions and close encounters between black holes and (non-compact) stars. Using Monte Carlo N-body models of dense star clusters, we find that a large fraction of black holes retained upon formation undergo collisions with stars. Within our explored cluster models, the proportion of binary black hole mergers affected by stellar collisions ranges from $10\%$ to $60\%$. If all stellar-mass black holes are initially non-spinning, we find that up to $40\%$ of merging binary black holes may have components with dimensionless spin parameter $χ\gtrsim 0.2$ because of prior stellar collisions, while typically about $10\%$ have spins near $χ= 0.7$ from prior black hole mergers. We demonstrate that young star clusters are especially important environments as they can produce collisions of black holes with very massive stars, allowing significant spin up of the black holes through accretion. Our predictions for black hole spin distributions from these stellar collisions highlight their sensitivity to accretion efficiency, underscoring the need for detailed hydrodynamic calculations to better understand the accretion physics following these interactions.

astro-ph.HE↗

arXiv2024

Lower-mass-gap Black Holes in Dense Star Clusters

The existence of compact stellar remnants in the mass range $2-5\,M_{\odot}$ has long been debated. This so-called lower mass gap was initially suggested by the lack of low-mass X-ray binary observations with accretors about $2-5\,M_{\odot}$, but it has recently been called into question following newer observations, including a lower-mass-gap candidate with a millisecond pulsar companion in the dense globular cluster NGC 1851. Here we model NGC 1851 with a grid of similar dense star clusters utilizing the state-of-the-art Monte Carlo $N$-body code \texttt{CMC}, and we specifically study the formation of lower-mass-gap black holes. We demonstrate that both massive star evolution and dynamical interactions can contribute to forming lower-mass-gap black holes. In general, the collapse of massive remnants formed through mergers of neutron stars or massive white dwarfs produces the largest number of lower-mass-gap black holes among all formation channels. However, in more massive clusters, supernova core collapse can contribute comparable numbers. Our NGC 1851-like models can reproduce millisecond pulsar -- lower-mass-gap black hole binaries similar to the observed system. Additionally, the lower-mass-gap black holes can also become components of dynamically assembled binaries, and some will be in merging black hole - neutron star systems similar to the recently detected gravitational wave source GW230529. However, the corresponding merger rate is probably $\lesssim 1~{\rm Gpc^{-3}\,yr^{-1}}$.

astro-ph.HE↗

arXiv2024

IMBH Progenitors from Stellar Collisions in Dense Star Clusters

Very massive stars (VMSs) formed via a sequence of stellar collisions in dense star clusters have been proposed as the progenitors of massive black hole seeds. VMSs could indeed collapse to form intermediate-mass black holes (IMBHs), which would then grow by accretion to become the supermassive black holes observed at the centers of galaxies and powering high-redshift quasars. Previous studies have investigated how different cluster initial conditions affect the formation of a VMS, including mass segregation, stellar collisions, and binaries, among others. In this study, we investigate the growth of VMSs with a new grid of Cluster Monte Carlo (CMC) star cluster simulations -- the most expansive to date. The simulations span a wide range of initial conditions, varying the number of stars, cluster density, stellar initial mass function (IMF), and primordial binary fraction. We find a gradual shift in the mass of the most massive collision product across the parameter space; in particular, denser clusters born with top-heavy IMFs provide strong collisional regimes that form VMSs with masses easily exceeding 1000 solar masses. Our results are used to derive a fitting formula that can predict the typical mass of a VMS formed as a function of the star cluster properties. Additionally, we study the stochasticity of this process and derive a statistical distribution for the mass of the VMS formed in one of our models, recomputing the model 50 times with different initial random seeds.

astro-ph.GA↗

arXiv2024

Stellar Escape from Globular Clusters. II. Clusters May Eat Their Own Tails

We apply for the first time the Monte Carlo star cluster modeling method to study tidal tail and stellar stream formation from globular clusters, assuming a circular orbit in a smooth Galactic potential. Approximating energetically unbound bodies (potential escapers; PEs) as collisionless enables this fast but spherically symmetric method to capture asymmetric tidal phenomena with unprecedented detail. Beyond reproducing known stream features, including epicyclic overdensities, we show how 'returning tidal tails' may form after the stream fully circumnavigates the Galaxy back to the cluster, enhancing the stream's velocity dispersion. While a realistically clumpy, time-dependent Galactic potential may disrupt such tails, they warrant scrutiny as potentially excellent constraints on the Galactic potential's history and substructure. Re-examining the escape timescale $Δt$ of PEs, we find new behavior related to chaotic scattering in the three-body problem; the $Δt$ distribution features sharp plateaus corresponding to distinct locally smooth patches of the chaotic saddle separating the phase space basins of escape. We study for the first time $Δt$ in an evolving cluster, finding that $Δt\sim(E_{\rm J}^{-0.1},E_{\rm J}^{-0.4})$ for PEs with (low, high) Jacobi energy $E_{\rm J}$, flatter than for a static cluster ($E_{\rm J}^{-2}$). Accounting for cluster mass loss and internal evolution -- and (roughly) for ongoing relaxation among PEs -- lowers the median $Δt$ from ${\sim}10\,$Gyr to ${\lesssim}100\,$Myr. We finally outline future improvements to escape physics in the Monte Carlo method intended to enable both the first large-parameter-space studies of tidal tail/stellar stream formation from full globular cluster simulations and detailed comparisons to stream observations.

astro-ph.GA↗

arXiv2024

Binary black hole mergers and intermediate-mass black holes in dense star clusters with collisional runaways

Intermediate-mass black holes (IMBHs) are believed to be the missing link between the supermassive black holes (BHs) found at the centers of massive galaxies and BHs formed through stellar core collapse. One of the proposed mechanisms for their formation is a collisional runaway process in high-density young star clusters, where an unusually massive object forms through repeated stellar collisions and mergers, eventually collapsing to form an IMBH. This seed IMBH could then grow further through binary mergers with other stellar-mass BHs. Here we investigate the gravitational-wave (GW) signals produced during these later IMBH--BH mergers. We use a state-of-the-art semi-analytic approach to study the stellar dynamics and to characterize the rates and properties of IMBH--BH mergers. We also study the prospects for detection of these mergers by current and future GW observatories, both space-based (LISA) and ground-based (LIGO Voyager, Einstein Telescope, and Cosmic Explorer). We find that most of the merger signals could be detected, with some of them being multi-band sources. Therefore, GWs represent a unique tool to test the collisional runaway scenario and to constrain the population of dynamically assembled IMBHs.

astro-ph.GA↗