Tuesday, June 17
Dynamic Relationship Between Star Cluster Evolution and Tidal Stream Formation
Tidal streams originating from star clusters preserve the dynamical evolution history of their host clusters as well as the influences from the galactic environment. By implementing the Galpy and AGAMA modules within the PeTar code, we can simultaneously simulate star cluster evolution under a time-dependent Galactic tidal field. We have applied our framework to study the tidal disruption of star clusters near the Galactic center and investigated the tidal streams of Hyades, Meingast I, C-19, and Palomar 5. Our results show that the properties of tidal streams can serve as important indicators of the formation conditions of their host clusters, offering constraints on the initial mass function (IMF), the presence of black holes, primordial binaries, and the occurrence of gas expulsion. We also examine how the Galactic bar, spiral arms, and the Large Magellanic Cloud (LMC) affect the characteristics of tidal streams.
The most metal poor stellar stream as a probe of globular cluster formation and evolution
The discovery of the most metal-poor stream, C-19, provides us a fossil record of a stellar structure born very soon after the Big Bang. Surprisingly, it concurrently has a large velocity dispersion and a small metallicity dispersion, which suggests its progenitor is an unusual globular cluster. The C-19 stream gives us a test stone of cluster formation and evolution, such as its binary fraction, variations of IMF, as well as heating mechanisms from various dynamical processes. Among them, the multi-epoch spectroscopic observations are crucial to study its binarity nature, which is more attainable in globular cluster streams than their progenitors. Moreover, a newly established library of such streams after Gaia mission, would open a window to study the compact objects and binaries in these stellar systems.
Black holes in Globular Clusters: a structural and kinematic perspective
The internal dynamical state of globular clusters (GCs) provides key insights into their formation conditions and long-term evolution. In this context, the increasing number of stellar-mass black holes (BHs) detected in Galactic GCs has triggered extensive studies on the retention of exotic objects (such as BHs and neutron stars) in massive stellar systems and their role in the structural and dynamical evolution of the host cluster. Accounting for the presence of BHs is thus critical to model the dynamical evolution of GCs and to interpret their present-day properties. Several recent works have attempted to constrain the overall BH population in GCs by adopting different techniques generally based on scaling relations from numerical simulations. However, this approach may yield degenerate results due to multiple possible interpretations and underlying physical processes at play, requiring further studies. In this context, I will present a novel approach to constrain the BH mass fraction in GCs based on two observable quantities that probe the GC internal structure and kinematics. The combination of these two quantities has the advantage of breaking the degeneracies previous analyses were prone to, finally allowing us to disentangle the roles of the long-term dynamical evolution and the presence of a large BH population within the system. We then compared the results from Monte Carlo numerical simulations of cluster long-term evolution with multi-epoch HST observations for several Galactic GCs, providing deeper insights into their current dynamical state and BH content. Finally, I will discuss how future astrometric and photometric missions might help us shed new light on the subject.
Black holes on the move: from globular clusters to galactic nuclei
Nuclear star clusters (NSCs) are building blocks for understanding the assembly of massive black holes (BHs) and their coevolution with the host galaxy. In such extremely dense stellar environments, binary BHs undergo strong dynamical interactions that efficiently produce hierarchical mergers, distinctive gravitational-wave signals and massive BH seeds. However, our understanding remains limited by significant uncertainties in how NSCs and their BH populations form. In this talk, I will explore how the formation pathways of NSCs shape their BH populations. I will model the inward migration of globular clusters (GCs) to quantify how dynamical interactions and hierarchical mergers within the parent GC affect the BHs that populate the galactic nuclei. I will consider GC populations from detailed galaxy models that track star cluster formation across cosmic time. I will then track their evolution using a new population-synthesis approach that captures the progressive GC dissolution, orbital decay and core dynamics that drive BH ejection and merger. I will characterize the BH populations that sink to the galactic center across a range of host galaxy masses, and assess whether this mechanism can seed NSCs with intermediate-mass BHs. Also, I will investigate the gravitational-wave signals produced in GCs, with emphasis on the most distinctive signatures that can disentangle their formation channel. Not only pair-instability masses, but also spins, eccentricity and redshift evolution can unveil the dynamical origin of these sources. These findings will help constrain the role of GCs and NSCs in the formation of gravitational waves and the BH assembly across cosmic time.
Formation and Evolution of Black Hole Binaries in Dense Star Clusters
The discovery of gravitational waves (GWs) by the LIGO-Virgo-Kagra detectors has kickstarted one of the most exciting decades of astrophysics. We now have a significantly updated understanding of how high-mass stars evolve and form compact objects including black holes (BHs). On the other hand, we have a dramatically improved understanding of the interplay between the evolution of dense star clusters and the dynamics of the BHs they harbour. We find that the evolution, the observable global properties, and even the survival of dense star clusters depend intricately on the number of BHs in the cluster. The BH binaries a cluster can produce depends strongly on the global dynamical properties of the host cluster and not on the details of initial binary properties. However, the time-dependent formation of mass-gap and intermediate-mass BHs depends critically on the high-mass binary fraction of the clusters.
Black Hole and Neutron Star Dynamics in Dense Star Clusters
Frequent dynamical encounters in globular clusters significantly enhance the production of multi-messenger phenomena. It is now well established that globular clusters host robust populations of compact objects, including low-mass X-ray binaries, millisecond pulsars, and gravitational wave sources. The evolution of these compact objects is intricately linked within dense star clusters. Black holes impact the dynamics of neutron stars and white dwarfs. In turn, the collapse of white dwarfs and neutron stars through accretion or mergers plays a crucial role in explaining various observations of more massive compact objects. In this talk, I will use binary black hole mergers, millisecond pulsars, and fast radio burst observations as examples to illustrate the dynamical evolution of black holes, neutron stars, and white dwarfs in dense star clusters. I will demonstrate how we can connect these dynamics with rich observational data to understand compact object formation and evolution.
Black Holes Quench Observable Velocity Dispersion in Globular Cluster Stellar Streams
The velocity dispersion (VD) in stellar streams from globular clusters (GCs) reflects the form of the GC’s parent halo and perturbative heating from Galactic substructure, including dark matter subhalos. Recent studies have thus leveraged the VD in observed GC streams to infer properties of dark matter. Yet such studies have not considered possible selection effects arising from internal GC dynamics—e.g., a VD that differs between low- and high-mass stars. We explore this possibility using N-body simulations to generate mock GC streams with realistic stellar mass and velocity distributions. The simulations predict the stream’s VD depends negligibly on stellar mass so long as the GC retains a dynamically significant black hole (BH) population. But once the GC loses its BHs, allowing the GC’s core to observably collapse and its stars to strongly mass-segregate, the stream’s VD becomes mass-dependent—increasing with star mass by up to a factor of ~3 in the range 0.5–0.8 Msun. This occurs since the most-massive stars at the GC’s dense center experience stronger kicks from binary interactions more frequently than the lower-mass stars inhabiting the GC’s sparse outskirts. Gaia observations of a stream in a nearby core-collapsed GC, NGC 6397, suggestively support our simulations. In summary, magnitude-limited observations of streams from core-collapsed GCs (~20% of Galactic GCs) may be biased to higher VD (colder inferred dark matter) than would be predicted by typical models neglecting the VD’s mass dependence. Fortunately, however, such a selection effect is unlikely in the other ~80% of GC streams.
The kinematics of Omega Centauri studied in 3D: velocity dispersion, kinematic distance, anisotropy, and energy equipartition
Omega Centauri (ω Cen) is the Milky Way’s most massive globular cluster and one of the most extreme cases of the multiple stellar population phenomenon. It is likely the stripped nucleus of an accreted dwarf galaxy and the host of a central intermediate-mass black hole. We recently created oMEGACat, the largest spectroscopic, photometric, and astrometric catalog for any star cluster. In this talk, I will present the ongoing kinematic analysis of this unique dataset. I present velocity dispersion profiles and kinematic maps with significantly improved precision and spatial resolution in all 3 velocity dimensions. The comparison between spectroscopic and astrometric measurements allows a precise determination of the distance to the cluster. The subset of data with precise metallicity measurements shows no correlation between metallicity and kinematics, supporting the picture of well-mixed stellar populations within the half-light radius of ω Cen. Finally, I present an analysis of the degree of energy equipartition using a significantly enlarged range of stellar masses. We find partial energy equipartition in the center that decreases towards large radii. The spatial dependence of the radial energy equipartition is stronger than the tangential energy equipartition. The new kinematic observations can serve as a new reference for future dynamical modeling efforts that will help to further disentangle the complex mass distribution within ω Cen.
Stability of Mass Transfer and Common Envelope Evolution of Binary Stars
Mass transfer and common envelope (CE) evolution are crucial processes in the evolution of binary stars, significantly influencing the formation of compact binaries, including binary black holes, binary neutron stars, and Type Ia supernova progenitors. The stability of mass transfer determines whether a binary evolves through stable Roche lobe overflow or enters a CE phase, leading to dramatic orbital shrinkage or binary merger. Despite decades of study, fundamental questions remain regarding the conditions for stable mass transfer, the mechanisms governing CE evolution, and the efficiency of envelope ejection. In this talk, I will discuss the latest theoretical developments and observational constraints on mass transfer stability and CE evolution.
Tracing the ancient Omega Centauri dwarf galaxy
Omega Centauri, the most massive and luminous globular cluster in the Milky Way, stands out due to its complex and extensive stellar populations, which challenge the idea of it being a typical globular cluster. Growing evidence now supports the notion that Omega Centauri is the remnant nuclear star cluster (NSC) of a dwarf galaxy that was accreted during a major merger event which also produced the Gaia-Sausage-Enceladus (GSE) structure. In this work, we reconstruct the evolutionary history of this disrupted system by combining chemical evolution modelling with spectroscopic data from APOGEE and GALAH, as well as astrometric and spectrophotometric measurements from Gaia XP. Our chemical evolution models help to explain Omega Centauri’s metallicity distribution function, showing that the GSE debris accounts for its missing portion and complements its chemical pattern. Additionally, we explore how the Sequoia population may trace a stellar population stripped during the initial interaction of the dwarf galaxy and the Milky Way, providing insights into the merger processes that led to GSE and Omega Centauri.
Ultraluminous X-ray Sources in Extragalactic Star Clusters
Intermediate mass black holes (with masses greater than 100 times the mass of the Sun, but less than a million times the mass of the Sun) are thought to be one of the primary explanations for how supermassive black holes grow and evolve. They are also prime targets for the Laser Interferometer Space Antenna (LISA), with planned launch date in 2037. Although there is very little observational evidence for intermediate mass black holes, many theoretical simulations suggest that they may be present in young star clusters. However, this can now be backed up by systematic, multiwavelength studies. We are currently undertaking a census of the X-ray binary population of star clusters in 30 galaxies within 10 Mpc to identify and characterise potential intermediate mass black holes in star clusters, and to follow them up in the future with multiwavelength studies to rule them in as intermediate mass black hole candidates, or definitively rule them out.
Understanding Multiple Stellar populations through cluster internal dynamics
Globular clusters (GCs) are ancient relics of the Milky Way’s past and laboratories that offer crucial insights into the formation of the Galactic Halo, Bulge, as well as stellar formation and evolution. Once believed to be simple, coeval populations, they are now known to host multiple stellar populations (MPs) with distinct chemical compositions and structural properties. This discovery has reshaped our understanding of cluster formation, yet the origin of MPs remains one of astrophysics’ most enduring mysteries. Despite decades of research, it is still unclear what drives their formation and whether globular clusters undergo multiple episodes of star formation. One promising way to unravel the origin of MPs is through the internal dynamics of cluster stars. Indeed, the kinematics of cluster stars encode crucial information about their formation history, evolution, and interactions with the Galactic environment. In this study, we present a detailed large-scale investigation of the internal dynamics of MPs in 28 Galactic GCs, mapping their kinematics from their dense cores to their distant outskirts. Using astro-photometric data from ground-based surveys, Gaia DR3, and HST, we identify first-generation (1P) and second-generation (2P) stars and analyze their motions. Our findings reveal a clear dynamical contrast: 1P stars remain nearly isotropic, while 2P stars become increasingly radially anisotropic beyond the half-light radius. These differences are closely connected with cluster age, relaxation state, and Galactic environment. Younger, less evolved clusters show the most pronounced kinematic differences, while in older clusters, dynamical evolution gradually mixes MPs. Moreover, clusters closer to the Galactic center experience stronger dynamical differences, likely due to tidal interactions with the Milky Way. Our findings suggest that 2P stars formed in a more centrally concentrated environment, and gradually expanded. More broadly, they highlight the significant influence of the Galactic environment on the internal kinematics of GCs, reinforcing the connection between cluster evolution and interaction with the Milky Way.
Comparing the collision and merger products of stars using magnetohydrodynamics
Blue stragglers are stars that appear hotter and brighter than expected for main-sequence stars in a cluster, suggesting they have gained mass since the cluster’s formation. Theoretical studies link their formation to stellar interactions such as mass transfer, collisions, and mergers. In dense star cluster cores, frequent gravitational encounters often lead to stellar collisions. Additionally, close binaries in these dense environments can harden over time through binary-single interactions and tidal dissipation, eventually undergoing mass transfer and merging via gravitational inspiral. Using the moving-mesh magnetohydrodynamics code AREPO, we conduct the first simulations comparing two formation channels — collisions and mergers — for 5–10 Msun main-sequence stars to examine the similarities and differences in the properties of their resulting products. This mass range is particularly interesting as the resulting products are massive stars that can lead to supernovae, gravitational waves, pulsars, and more. Our simulations show that both collisions and mergers lead to significant internal mixing, replenishing the stellar core with hydrogen from the envelope and extending the main-sequence lifetime. This mixing also amplifies the stellar magnetic field by up to 10 orders of magnitude. The resulting stars rotate rapidly and differentially, near break-up velocity. Key differences emerge between the two channels: merger products develop more distinct disk-like structures in the merger plane compared to collision products. This structural variation makes merger products more extended and luminous. These findings highlight the impact of star cluster dynamics and binary interactions in shaping the formation and evolution of blue stragglers.
Simulations of the evolution of globular clusters with multiple stellar populations
The formation of many populations of stars in globular clusters and their further evolution is still the subject of much debate and awaits resolution. Many scenarios have been proposed to explain their formation. One of the most commonly proposed is the AGB scenario, in which chemically processed gas from the envelopes of AGB stars mixes with re-accreted primary gas flowing into the center of the cluster. Based on this scenario, more than two hundred MOCCA code simulations of cluster evolution have been carried out, taking into account additional physical processes mainly related to the external environment in which globular clusters live. These processes are related to: the time shift of the formation of the second population of stars, the different initial concentrations between the populations, their mass functions, deviations from virial equilibrium, and the migration of globular clusters in the galactic field and the impact of the capture of dwarf galaxies by the Milky Way. Analysis of the simulation results shows that the most of observational parameters of multiple stellar populations and the global parameters of clusters associated with the Milky Way are well reproduced except the correlation between cluster mass and the ratio between the number of population two stars and the total number of stars. It seems that this ratio can be reproduced if one takes into account the variability of the environment in which globular clusters live. I will present a speculative picture of globular cluster evolution leading to a possible reconstruction of the observed properties of globular clusters in the Milky Way, including the correlation between the mass of the cluster and the fraction of stars of the second population. This scenario additionally predicts that under certain circumstances it is possible to reconstruct the observational fact that for some globular clusters the first population is more concentrated than the second.
Dynamical co-evolution of the Multiple Stellar Populations in Globular Clusters.
Globular clusters (GCs) were initially considered ideal laboratories for studying simple (single) stellar populations. However, this picture has undergone significant changes. Modern observations (including those using advanced instruments such as JWST and HST) confirm the ubiquitous presence of at least two or more stellar generations in virtually all well-studied GCs. Theoretical modeling of such systems, consisting of multiple stellar populations but united by a common evolutionary dynamic within the Galactic field, is a pressing and relevant task in modern theoretical astrophysics, particularly in N-body numerical simulations. In this work, using two different N-body codes (phi-GPU and nbody6++gpu), we examined the dynamic evolution of several test GC systems, specifically constructed from several different stellar populations. In the first part of the study, we focused on investigating the joint dynamic evolution within the Galaxy of nested spherical stellar systems (King models) with different concentration and mass ratios of the first and second stellar populations. In the second part of the study, we complicated the task by investigating rotating stellar disk systems as the second population embedded in initially non-rotating spherical systems of the first generation of stars. The work examined various relaxation processes and the interplay of dynamics between the first and second stellar generations in such complex GC systems.
Formation of hierarchical systems in dense globular star clusters
Globular clusters (GCs) are fascinating astronomical objects because in the cores of GCs one can find substantially larger densities than in the field of the Milky Way. Thus, in GCs the strong dynamical interactions and physical collision between the stars are common and it can lead to creation of many exotic objects, like X-ray binaries, cataclysmic variables, BH-BH mergers and many more. I’m one of the developers of the MOCCA code, which is able to perform detailed numerical simulations of globular clusters of any size within a few days (http://moccacode.net). Because of its speed and a close agreement with N-body codes MOCCA code is perfect to perform a large grid of simulations for various initial conditions, which is currently beyond the capabilities of any N-body codes. Recently, the MOCCA code was updated heavily by adding e.g. multiple populations, ability to move star clusters to outer orbits around host galaxies, and Tsunami code which allows us to include tidal forces and post-Newtonian terms to close dynamical interactions between stars. However, the newest addition to MOCCA, which is currently being tested, is the full treatment of the dynamical formation and evolution of hierarchical systems (triples, quadruples and potentially more complex hierarchies). During the talk, I would like to present the first results for a number of MOCCA simulations which will show how the hierarchical systems are formed in dense globular clusters. For now, the initial conditions for these models are varied with initial number of stars (up to 1M) and binary fractions (from 0%, to 95%). This is the first set of simulations designed to test how the hierarchical systems form, in which part of globular clusters, what is their typical lifetime and a cause of their dissolution. I would also like to present also how the population of hierarchical systems is related to the global parameters of GCs.
The Binary Fraction of Stars in the Dwarf Galaxy Ursa Minor via DESI
We utilize multi-epoch radial velocity measurements from the Milky Way Survey (MWS) of the Dark Energy Spectroscopic Instrument (DESI) to estimate the binary fraction for member stars in the dwarf spheroidal galaxy Ursa Minor (UMi). Our dataset comprises 766 distinct member stars, with a total of more than 4,000 observations collected over approximately one year. We constrain the binary fraction for UMi to be 0.667^{+0.107}_{-0.100} and 0.739^{+0.115}_{-0.113}, based on the models by Duquennoy & Mayor in 1991 and Moe & Stefano in 2017, respectively. Furthermore, by dividing our data into two subsamples based on metallicity, we identify a difference in the binary fraction between the metal-rich and metal-poor populations, which roughly represent the younger and older stellar populations in the dwarf galaxy. The binary fraction for the metal-rich population is higher than that of the metal-poor population. Based on Moe & Stefano’s model, the best constrained binary fractions for metal-rich and metal-poor populations in UMi are 0.915^{+0.061}_{-0.102} and 0.239^{+0.125}_{-0.100}, respectively. After a thorough examination, this offset cannot be attributed to sample selection effects, suggesting that the binaries in the early formed metal-poor populations may be disrupted more by dynamical interactions during the evolutionary process of dwarf galaxies.
High-velocity and hypervelocity stars ejected from compact object binaries in Milky Way globular clusters
The dense cores of Milky Way globular clusters (GCs) play host to a variety of dynamical encounters, some of which can accelerate stars to velocities high enough to escape the GC. The most extreme examples of these encounters are interactions between GC stars and binaries including a compact object. These interactions can result in ejection velocities of up to several hundred km/s, sometimes even in excess of the escape velocity of the Galaxy itself. We combine N-body GC simulations, observations of Galactic GCs, and a particle spray code to generate realistic populations of stars which have escaped from Milky Way GCs following star + compact object binary (S+COB) interactions. We find that over the last 500 Myr, S+COB interactions have likely ejected thousands of stars from Galactic GCs, many hundreds of which are unbound to the Galaxy. In this talk, I will discuss the results of this study further, outlining where these escaped stars will be found, which clusters and compact object binary types are most responsible for them, whether any should in principle be detectable in current or near-future surveys of the Galaxy, and what they can reveal about dense GC cores.