The repeating fast radio burst (FRB) localized to a globular cluster (GC) in M81 challenges our understanding of FRB models. In this Letter, we explore dynamical formation scenarios for objects in old GCs that may plausibly power FRBs. Using N-body simulations, we demonstrate that young neutron stars (NSs) may form in GCs at a rate of up to similar to 50 Gpc(-3) yr(-1) through a combination of binary white dwarf (WD) mergers, WD-NS mergers, binary NS mergers, and accretion-induced collapse of massive WDs in binary systems. We consider two FRB emission mechanisms: First, we show that a magnetically powered source (e.g., a magnetar with field strength greater than or similar to 10(14) G) is viable for radio emission efficiencies greater than or similar to 10(-4). This would require magnetic activity lifetimes longer than the associated spin-down timescales and longer than empirically constrained lifetimes of Galactic magnetars. Alternatively, if these dynamical formation channels produce young rotation-powered NSs with spin periods of similar to 10 ms and magnetic fields of similar to 10(11) G (corresponding to spin-down lifetimes of greater than or similar to 10(5) yr), the inferred event rate and energetics can be reasonably reproduced for order unity duty cycles. Additionally, we show that recycled millisecond pulsars or low-mass X-ray binaries similar to those well-observed in Galactic GCs may also be plausible channels, but only if their duty cycle for producing bursts similar to the M81 FRB is small.

As ancient, gravitationally bound stellar populations, globular clusters represent 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 CMC Cluster Catalog. Without performing 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 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. In addition, 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 python methods for examining the publicly accessible CMC Cluster Catalog, as well as any other models generated using CMC.

Orbital eccentricity is one of the most robust discriminators for distinguishing between dynamical and isolated formation scenarios of binary black hole mergers using gravitational-wave observatories such as LIGO and Virgo. Using state-of-the-art cluster models, we show how selection effects impact the detectable distribution of eccentric mergers from clusters. We show that the observation (or lack thereof) of eccentric binary black hole mergers can significantly constrain the fraction of detectable systems that originate from dynamical environments, such as dense star clusters. After roughly 150 observations, observing no eccentric binary signals would indicate that clusters cannot make up the majority of the merging binary black hole population in the local universe (95% credibility). However, if dense star clusters dominate the rate of eccentric mergers and a single system is confirmed to be measurably eccentric in the first and second gravitational-wave transient catalogs, clusters must account for at least 14% of detectable binary black hole mergers. The constraints on the fraction of detectable systems from dense star clusters become significantly tighter as the number of eccentric observations grows and will be constrained to within 0.5 dex once 10 eccentric binary black holes are observed.

Supersonically induced gas objects (SIGOs), are structures with little to no dark-matter component predicted to exist in regions of the universe with large relative velocities between baryons and dark matter at the time of recombination. They have been suggested to be the progenitors of present-day globular clusters. Using simulations, SIGOs have been studied on small scales (around 2 Mpc) where these relative velocities are coherent. However, it is challenging to study SIGOs using simulations on large scales due to the varying relative velocities at scales larger than a few Mpc. Here, we study SIGO abundances semi-analytically: using perturbation theory, we predict the number density of SIGOs analytically, and compare these results to small-box numerical simulations. We use the agreement between the numerical and analytic calculations to extrapolate the large-scale variation of SIGO abundances over different stream velocities. As a result, we predict similar large-scale variations of objects with high gas densities before reionization that could possibly be observed by JWST. If indeed SIGOs are progenitors of globular clusters, then we expect a similar variation of globular cluster abundances over large scales. Significantly, we find that the expected number density of SIGOs is consistent with observed globular cluster number densities. As a proof-of-concept, and because globular clusters were proposed to be natural formation sites for gravitational wave sources from binary black-hole mergers, we show that SIGOs should imprint an anisotropy on the gravitational wave signal on the sky, consistent with their distribution.

Numerical and observational evidence suggests that massive white dwarfs dominate the innermost regions of core-collapsed globular clusters by both number and total mass. Using NGC 6397 as a test case, we constrain the features of white dwarf populations in core-collapsed clusters, both at present day and throughout their lifetimes. The dynamics of these white dwarf subsystems have a number of astrophysical implications. We demonstrate that the collapse of globular cluster cores is ultimately halted by the dynamical burning of white dwarf binaries. We predict that core-collapsed clusters in the local universe yield a white dwarf merger rate of O(10) Gpc(-3) yr(-1), roughly 0.1%-1% of the observed Type Ia supernova rate. We show that prior to merger, inspiraling white dwarf binaries will be observable as gravitational-wave sources at millihertz and decihertz frequencies. Over 90% of these mergers have a total mass greater than the Chandrasekhar limit. We argue that the merger/collision remnants, if not destroyed completely in an explosive transient, may be observed in core-collapsed clusters either as young neutron stars/pulsars/magnetars (in the event of accretion-induced collapse) or as young massive white dwarfs offset from the standard white dwarf cooling sequence. Finally, we show that collisions between white dwarfs and main-sequence stars, which may be detectable as bright transients, occur at a rate of O(100) Gpc(-3) yr(-1) in the local universe. We find that these collisions lead to depletion of blue straggler stars and main-sequence star binaries in the centers of core-collapsed clusters.

We analyse Gaia EDR3 and re-calibrated HST proper motion data from the core-collapsed and non-core-collapsed globular clusters NGC 6397 and NGC 3201, respectively, with the Bayesian mass-orbit modelling code MAMPOSSt-PM. We use Bayesian evidence and realistic mock data sets constructed with Agama to select between different mass models. In both clusters, the velocities are consistent with isotropy within the extent of our data. We robustly detect a dark central mass (DCM) of roughly 1000M(circle dot) in both clusters. Our MAMPOSSt-PM fits strongly prefer an extended DCM in NGC 6397, while only presenting a mild preference for it in NGC 3201, with respective sizes of a roughly one and a few per cent of the cluster effective radius. We explore the astrophysics behind our results with the CMC Monte Carlo N-body code, whose snapshots best matching the phase space observations lead to similar values for the mass and size of the DCM. The internal kinematics are thus consistent with a population of hundreds of massive white dwarfs in NGC 6397, and roughly 100 segregated stellar-mass black holes in NGC 3201, as previously found with CMC. Such analyses confirm the accuracy of both mass-orbit modelling and Monte Carlo N-body techniques, which together provide more robust predictions on the DCM of globular clusters (core-collapsed or not). This opens possibilities to understand a vast range of interesting astrophysical phenomena in clusters, such as fast radio bursts, compact object mergers, and gravitational waves.

Globular clusters are considered to be likely breeding grounds for compact binary mergers. In this paper, we demonstrate how the gravitational-wave signals produced by compact object mergers can act as tracers of globular cluster formation and evolution. Globular cluster formation is a long-standing mystery in astrophysics, with multiple competing theories describing when and how globular clusters formed. The limited sensitivity of electromagnetic telescopes inhibits our ability to directly observe globular cluster formation. However, with future audio-band detectors sensitive out to redshifts of z approximate to 50 for GW150914-like signals, gravitational-wave astronomy will enable us to probe the Universe when the first globular clusters formed. We simulate a population of binary black hole mergers from theoretically motivated globular cluster formation models, and construct redshift measurements consistent with the predicted accuracy of third-generation detectors. We show that we can locate the peak time of a cluster formation epoch during reionization to within 0.05Gyr after 1yr of observations. The peak of a formation epoch that coincides with the Universal star formation rate can be measured to within 0.4-10.5Gyr after 1yr of observations, depending on the relative weighting of the model components.

Recent analyses have shown that close encounters between stars and stellar black holes occur frequently in dense star clusters. Depending upon the distance at closest approach, these interactions can lead to dissipating encounters such as tidal captures and disruptions, or direct physical collisions, all of which may be accompanied by bright electromagnetic transients. In this study, we perform a wide range of hydrodynamic simulations of close encounters between black holes and main-sequence stars that collectively cover the parameter space of interest, and we identify and classify the various possible outcomes. In the case of nearly head-on collisions, the star is completely disrupted with roughly half of the stellar material becoming bound to the black hole. For more distant encounters near the classical tidal-disruption radius, the star is only partially disrupted on the first pericenter passage. Depending upon the interaction details, the partially disrupted stellar remnant may be tidally captured by the black hole or become unbound (in some cases, receiving a sufficiently large impulsive kick from asymmetric mass loss to be ejected from its host cluster). In the former case, the star will undergo additional pericenter passages before ultimately being disrupted fully. Based on the properties of the material bound to the black hole at the end of our simulations (in particular, the total bound mass and angular momentum), we comment upon the expected accretion process and associated electromagnetic signatures that are likely to result.

Close encounters between neutron stars and main-sequence stars occur in globular clusters and may lead to various outcomes. Here we study encounters resulting in the tidal disruption of the star. Using N-body models, we predict the typical stellar masses in these disruptions and the dependence of the event rate on the host cluster properties. We find that tidal disruption events occur most frequently in core-collapsed globular clusters and that roughly 25% of the disrupted stars are merger products (i.e., blue straggler stars). Using hydrodynamic simulations, we model the tidal disruptions themselves (over timescales of days) to determine the mass bound to the neutron star and the properties of the accretion disks formed. In general, we find roughly 80%-90% of the initial stellar mass becomes bound to the neutron star following disruption. Additionally, we find that neutron stars receive impulsive kicks of up to about 20 km s(-1) as a result of the asymmetry of unbound ejecta; these kicks place these neutron stars on elongated orbits within their host cluster, with apocenter distances well outside the cluster core. Finally, we model the evolution of the (hypercritical) accretion disks on longer timescales (days to years after disruption) to estimate the accretion rate onto the neutron stars and accompanying spin-up. As long as greater than or similar to 1% of the bound mass accretes onto the neutron star, millisecond spin periods can be attained. We argue the growing numbers of isolated millisecond pulsars observed in globular clusters may have formed, at least in part, through this mechanism. In the case of significant mass growth, some of these neutron stars may collapse to form low-mass (less than or similar to 3 M (circle dot)) black holes.

We report the discovery of a new 5.78 ms period millisecond pulsar (MSP), PSR J1740-5340B (NGC 6397B), in an eclipsing binary system discovered with the Parkes radio telescope (now also known as Murriyang) in Australia and confirmed with the MeerKAT radio telescope in South Africa. The measured orbital period, 1.97 days, is the longest among all eclipsing binaries in globular clusters (GCs) and consistent with that of the coincident X-ray source U18, previously suggested to be a "hidden MSP." Our XMM-Newton observations during NGC 6397B's radio-quiescent epochs detected no X-ray flares. NGC 6397B is either a transitional MSP or an eclipsing binary in its initial stage of mass transfer after the companion star left the main sequence. The discovery of NGC 6397B potentially reveals a subgroup of extremely faint and heavily obscured binary pulsars, thus providing a plausible explanation for the apparent dearth of binary neutron stars in core-collapsed GCs as well as a critical constraint on the evolution of GCs.