Claire S. Ye

We present the first model of the Milky Way’s detectable compact object–luminous star binary population from disrupted dense star clusters. We bridge large-scale cosmological star cluster formation with high-resolution dynamical evolution of compact object binaries by mapping the predicted star clusters from the EMP- Pathfinder simulations to N -body Cluster Monte Carlo models. We predict that approximately 3 × 10 ⁵ white dwarfs (WDs), 1.5 × 10 ⁵ black holes (BHs), and 1 × 10 ³ neutron stars (NSs) in binaries with luminous companions are released to the Galaxy from now-disrupted dense star clusters throughout the history of the Milky Way. Synthetic observations modeled with the gaiamock pipeline reveal that the modeled Gaia DR3 yields are sparse (≈2 WDs, 0 NS, 0 BHs at 90% credibility), with the majority lying beyond the detection horizon. Gaia DR4 is expected to increase the observational yield of these systems only marginally, as the benefits of an expanded search volume are largely offset by the diminished astrometric and photometric precision of more distant sources (≈14 WDs, 0 NS, 0 BHs). While the underlying BH binary population is similar to that of WDs, they are detected far less frequently; they tend to pair with lower-mass, dimmer companions and have less temporal coverage of their long orbital periods. For NSs, we suggest that the observed overrepresentation of metal-poor, halo systems is inconsistent with an origin in disrupted dense star clusters. Instead, the observed Gaia NS population could reflect the accretion history of metal-poor, dwarf galaxies into the Milky Way, isolated binary star evolution, or supernova physics.

The dense cores of Milky Way globular clusters (GCs) play host to a variety of dynamical encounters between stellar objects, which can accelerate stars to velocities high enough to escape the GC. The most extreme examples of these encounters are interactions between single GC stars and binaries including at least one compact object. These interactions can result in ejection velocities of up to several hundred km s ⁻¹ , approaching or exceeding the escape velocity of the Galaxy itself. In order to study whether these interactions contribute to the Galactic population of hypervelocity stars (stars moving faster than the Galactic escape speed), we combine Monte Carlo N -body GC simulations, observations of Galactic GCs, and a particle spray code to generate realistic populations of stars that 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 ∼6300 stars from Galactic GCs, of which 83 9 − 67 + 70 have present-day velocities exceeding 500 km s ⁻¹ . Using mock photometric observations, we find that 29 0 − 23 + 28 ejected stars are detectable in Gaia Data Release 3; however, only 1 − 1 + 2 stars faster than 500 km s ⁻¹ are detectable. Even so, we show that observational prospects in the upcoming Legacy Survey of Space and Time are more optimistic, and future detected fast extratidal GC stars will serve as a useful probe of GC cores.

In the coming decade, the millihertz gravitational-wave observatory the Laser Interferometer Space Antenna (LISA) will provide the best constraints yet on the tens of thousands of close white dwarf binaries in the Milky Way, yielding unprecedented insights into the most abundant class of compact object binaries. Following inspiral via gravitational wave emission, interacting white dwarf binary pairs can lead to a multitude of outcomes, including AM Canum Venaticorum binaries, R Coronae Borealis stars, young, rapidly spinning single white dwarfs, (millisecond) magnetars, and a variety of explosive transients, most notably Type Ia supernovae. Current and future electromagnetic observations of these various outcomes coupled with the forthcoming flood of data from LISA place us on the precipice of a significant advance in our understanding of the long-term fate of white dwarf binaries. In this paper, we present a suite of mock catalogs of the Milky Way’s white dwarf merger history, created using the population synthesis code COSMIC combined with a metallicity-dependent star formation history from FIRE-2 galaxy simulations. We summarize the various merger outcomes expected (based upon varying white dwarf masses and chemical compositions) and explore ways the rates of these outcomes may vary with model uncertainties pertaining to binary evolution. We publicly release these merger catalogs as a tool for facilitating connections between gravitational-wave science and white dwarf binary astrophysics.

The Gaia mission has provided precise astrometry and spectrophotometry for billions of stars in the Milky Way, enabling the identification and kinematic characterization of stellar streams. These streams, remnants of disrupted globular clusters and dwarf galaxies, have revealed the structure of the Milky Way’s dark matter halo. We show that stellar streams also encode information about the initial mass function of globular clusters. We combine cold dark matter simulations that model the evolution and disruption of embedded globular clusters with observations of stellar streams and globular clusters to infer the initial cluster mass function. We find that initially more massive clusters produce more massive streams, but deposit a smaller fraction of their initial mass into those streams. Using stream mass and angular momentum measurements, we recover a declining, power-law-like initial mass function with a slope α = 1.4 ± 0.05 for streams ≳1000 M _⊙ (with a maximum value of ∼1.6 if we strictly use lower limits of stream mass measurements). This work establishes stellar streams as a novel probe of the early mass distribution of globular clusters.

The detection of fast radio bursts (FRBs) in both young and old stellar populations suggests multiple formation pathways, beyond just young magnetars from core-collapse supernovae (SNe). A promising delayed channel involves the formation of FRB-emitting neutron stars through the merger- or accretion-induced collapse of a massive white dwarf (WD). By simulating a realistic stellar population with both binaries and triples, we identify pathways to WD collapse that could produce FRB candidates. We find that: (i) triple dynamics open new merger channels inaccessible to isolated binaries, significantly enhancing the overall merger rate; (ii) triple-induced mergers broaden the delay-time distribution, producing long-delay (≳1–8 Gyr) events largely independent of metallicity, alongside a shorter-delay population (≲100 Myr) of rapid mergers; (iii) these long delays naturally yield FRBs in older environments, such as quiescent host galaxies and galactic halos; (iv) when convolved with the cosmic star formation history, binary channels track the star formation rate ( z _peak ∼ 2), while triple channels peak later ( z _peak ∼ 1), giving a combined local source rate of R ₀ ≈ 2 × 10 ⁴ Gpc ⁻³ yr ⁻¹ , consistent with observations; and (v) applying the same framework to SNe Ia, we find that triples extend the delay-time tail and roughly double the SNe Ia efficiency relative to binaries, yielding rates and redshift evolutions in good agreement with observations. If FRBs originate from the collapse of WDs, our results establish triples, alongside binaries, as a crucial and previously overlooked formation pathway whose predicted rates, host demographics, and redshift evolution offer clear tests for upcoming surveys.

The binary black hole merger GW231123 is both the most massive gravitational-wave event observed and has the highest component spins measured to date. The dimensionless spins of the more massive (primary) and less massive (secondary) black holes are measured to be χ 1 = 0.9 0 − 0.19 + 0.10 and χ 2 = 0.8 0 − 0.51 + 0.20 (90% credible intervals), respectively. Its large mass and extremal spins are challenging to explain through standard binary stellar physics, though a flurry of hypothetical scenarios have been proposed. Hierarchical assembly—i.e., mergers of black holes that are themselves formed from previous generations of mergers—is generally a promising way to explain massive and rapidly spinning black holes. Here, we investigate the possibility that GW231123 was assembled hierarchically in a dense star cluster as the merger of two second-generation black holes. Taking the inferred spin values at face value, we find that it is possible ( p ≈ 5%) that a compact binary with component spins like GW231123 could form in a cluster from hierarchical assembly.

The lensed Sunburst Arc ( z = 2.369) hosts a young (∼2–4 Myr), massive ( M _⋆ ∼ 10 ⁷ M _⊙ ), compact ( R _eff ∼ 8 pc) Lyman-continuum leaking super star cluster, which powers a compact (Z _⊙ and with an anomalously elevated nitrogen-to-oxygen ratio log ( N / O ) ∼ − 0.2 . We present semianalytic models and 3D magnetohydrodynamic simulations in an attempt to reproduce this system. The results indicate that the progenitor giant molecular cloud (GMC) may have M _cloud ≳ 3 × 10 ⁷ M _⊙ and R _cloud ∼ 70 pc, corresponding to a surface density ∼10 ³ –10 ⁴ M _⊙ pc ⁻² . Incorporating feedback from individual very massive stars (VMSs; ≥100 M _⊙ ) sampled from the Kroupa initial mass function, we find that their winds rapidly enrich ∼10 ⁴ M _⊙ of nearby gas with nitrogen (∼1 dex) and helium (∼0.1–0.2 dex). In the first 1–3 Myr, some cold gas falls to the system center where a central cluster builds up from subcluster mergers. There, the gas is photoionized, pressurized, and chemically enriched by the newly formed VMSs, before being radiatively expelled in the next ∼1 Myr. We find that both VMS feedback and a high-surface-density progenitor GMC are necessary to reproduce the observed nebular properties, such as high N/O, high pressure, and stellar proximity. Low metallicity ( Z ≤ 0.004) may be essential to avoid overproduction of carbon from WC stars. Such enrichment processes localized to compact starbursts may have caused strong nitrogen emission from dense ionized gas as observed in high-redshift galaxies such as GN-z11 and GS_3073.

Dense star clusters are thought to contribute significantly to the merger rates of stellar-mass binary black holes (BBHs) detected by the LIGO–Virgo–KAGRA collaboration. We combine N -body dynamic models of realistic dense star clusters with cluster formation histories to estimate the merger rate distribution as a function of primary mass for merging BBHs formed in these environments. It has been argued that dense star clusters—most notably old globular clusters—predominantly produce BBH mergers with primary masses M _p ≈ 30 M _⊙ . We show that dense star clusters forming at lower redshifts—and thus having higher metallicities—naturally produce lower-mass BBH mergers. We find that cluster BBH mergers span a wide range of primary mass, from about 6 M _⊙ to above 100 M _⊙ , with a peak near 8 M _⊙ , reproducing the overall merger rate distribution inferred from gravitational wave detections. Our results show that most low-mass BBH mergers (about 95% with M _p ≲ 20 M _⊙ ) originate in metal-rich ( Z ∼ Z _⊙ ) dense star clusters, while more massive BBH mergers form predominately in metal-poor globular clusters. We also discuss the role of hierarchical mergers in shaping the BBH mass distribution. Gravitational wave detection of dynamically formed low-mass BBH mergers—potentially identifiable by features such as isotropic spin distributions—may serve as probes of cluster formation histories in metal-rich environments at low redshifts.

The centers of massive elliptical galaxies exhibit a wide range in density profiles, from central cusps to resolved cores with order kiloparsec sizes. The cored ellipticals have been linked to the presence of supermassive black hole binaries that excavate their hosts’ central stellar populations through three-body encounters. This connection between cores and black holes similarly operates in globular clusters, which also exhibit a bimodality in cored and core-collapsed architectures, rich and depleted, respectively, in stellar black holes. We report new estimates of the total black hole mass in 25 Galactic globular clusters based on a suite of roughly 150 Monte Carlo N -body simulations that fit observed surface brightness and velocity dispersion profiles. We show that both globular clusters and massive elliptical galaxies individually exhibit strong correlations between total black hole mass ( M _• ) and core radius ( r _c ) and that these individual relations share a common power-law exponent to within 1 σ statistical precision: M • ∼ r c 1.3 . The individual relations appear to be offset, suggesting swarms of stellar black holes scour globular cluster cores more efficiently than lone supermassive black holes scour the cores of massive ellipticals. Yet the shared basis of core scouring via black hole binaries hints at a unified M _• – r _c connection across over 10 orders of magnitude in M _• . Our findings imply core radius measurements may offer a powerful observational constraint on black hole merger rates, from kilohertz sources detectable by LIGO/Virgo/KAGRA formed in globular clusters to millihertz and nanohertz sources formed in massive elliptical galaxies.