The existence of black holes (BHs) with masses in the range between stellar remnants and supermassive BHs has only recently become unambiguously established. GW190521, a gravitational wave signal detected by the LIGO/Virgo Collaboration, provides the first direct evidence for the existence of such intermediate-mass BHs (IMBHs). This event sparked and continues to fuel discussion on the possible formation channels for such massive BHs. As the detection revealed, IMBHs can form via binary mergers of BHs in the "upper mass gap" (approximate to 40-120 M-circle dot). Alternatively, IMBHs may form via the collapse of a very massive star formed through stellar collisions and mergers in dense star clusters. In this study, we explore the formation of IMBHs with masses between 120 and 500 M-circle dot in young, massive star clusters using state-of-the-art Cluster Monte Carlo models. We examine the evolution of IMBHs throughout their dynamical lifetimes, ending with their ejection from the parent cluster due to gravitational radiation recoil from BH mergers, or dynamical recoil kicks from few-body scattering encounters. We find that all of the IMBHs in our models are ejected from the host cluster within the first similar to 500 Myr, indicating a low retention probability of IMBHs in this mass range for globular clusters today. We estimate the peak IMBH merger rate to be R approximate to 2Gpc(-3)yr(-1) z approximate to 2.

Formation of supermassive black holes (BHs) remains a theoretical challenge. In many models, especially beginning from stellar relic 'seeds,' this requires sustained super-Eddington accretion. While studies have shown BHs can violate the Eddington limit on accretion disc scales given sufficient 'fuelling' from larger scales, what remains unclear is whether or not BHs can actually capture sufficient gas from their surrounding interstellar medium (ISM). We explore this in a suite of multiphysics high-resolution simulations of BH growth in magnetized, star-forming dense gas complexes including dynamical stellar feedback from radiation, stellar mass-loss, and supernovae, exploring populations of seeds with masses similar to 1-10(4) M-circle dot. In this initial study, we neglect feedback from the BHs: so this sets a strong upper limit to the accretion rates seeds can sustain. We show that stellar feedback plays a key role. Complexes with gravitational pressure/surface density below similar to 10(3) M-circle dot pc(-2) are disrupted with low star formation efficiencies so provide poor environments for BH growth. But in denser cloud complexes, early stellar feedback does not rapidly destroy the clouds but does generate strong shocks and dense clumps, allowing similar to 1 per cent of randomly initialized seeds to encounter a dense clump with low relative velocity and produce runaway, hyper-Eddington accretion (growing by orders of magnitude). Remarkably, mass growth under these conditions is almost independent of initial BH mass, allowing rapid intermediate-mass black hole (IMBH) formation even for stellar-mass seeds. This defines a necessary (but perhaps not sufficient) set of criteria for runaway BH growth: we provide analytic estimates for the probability of runaway growth under different ISM conditions.

The theory of stellar escape from globular clusters (GCs) dates back nearly a century, especially the gradual evaporation of GCs via two-body relaxation coupled with external tides. More violent ejection can also occur via strong gravitational scattering, supernovae, gravitational wave-driven mergers, tidal disruption events, and physical collisions, but comprehensive study of the many escape mechanisms has been limited. Recent exquisite kinematic data from the Gaia space telescope has revealed numerous stellar streams in the Milky Way (MW) and traced the origin of many to specific MWGCs, highlighting the need for further examination of stellar escape from these clusters. In this study, the first of a series, we lay the groundwork for detailed follow-up comparisons between Cluster Monte Carlo (CMC) GC models and the latest Gaia data on the outskirts of MWGCs, their tidal tails, and associated streams. We thoroughly review escape mechanisms from GCs and examine their relative contributions to the escape rate, ejection velocities, and escaper demographics. We show for the first time that three-body binary formation may dominate high-speed ejection from typical MWGCs, potentially explaining some of the hypervelocity stars in the MW. Due to their mass, black holes strongly catalyze this process, and their loss at the onset of observable core collapse, characterized by a steep central brightness profile, dramatically curtails three-body binary formation, despite the increased post-collapse density. We also demonstrate that even when born from a thermal eccentricity distribution, escaping binaries have significantly nonthermal eccentricities consistent with the roughly uniform distribution observed in the Galactic field.

The Laser Interferometer Space Antenna (LISA) will be a transformative experiment for gravitational wave astronomy, and, as such, it will offer unique opportunities to address many key astrophysical questions in a completely novel way. The synergy with ground-based and space-born instruments in the electromagnetic domain, by enabling multi-messenger observations, will add further to the discovery potential of LISA. The next decade is crucial to prepare the astrophysical community for LISA's first observations. This review outlines the extensive landscape of astrophysical theory, numerical simulations, and astronomical observations that are instrumental for modeling and interpreting the upcoming LISA datastream. To this aim, the current knowledge in three main source classes for LISA is reviewed; ultra-compact stellar-mass binaries, massive black hole binaries, and extreme or interme-diate mass ratio inspirals. The relevant astrophysical processes and the established modeling techniques are summarized. Likewise, open issues and gaps in our understanding of these sources are highlighted, along with an indication of how LISA could help making progress in the different areas. New research avenues that LISA itself, or its joint exploitation with upcoming studies in the electromagnetic domain, will enable, are also illustrated. Improvements in modeling and analysis approaches, such as the combination of numerical simulations and modern data science techniques, are discussed. This review is intended to be a starting point for using LISA as a new discovery tool for understanding our Universe.

We study close encounters of a 1 M (circle dot) middle-age main-sequence star (modeled using MESA) with massive black holes through hydrodynamic simulations, and explore in particular the dependence of the outcomes on the black hole mass. We consider here black holes in the intermediate-mass range, M (BH) = 100-10(4) M (circle dot). Possible outcomes vary from a small tidal perturbation for weak encounters all the way to partial or full disruption for stronger encounters. We find that stronger encounters lead to increased mass loss at the first pericenter passage, in many cases ejecting the partially disrupted star on an unbound orbit. For encounters that initially produce a bound system, with only partial stripping of the star, the fraction of mass stripped from the star increases with each subsequent pericenter passage and a stellar remnant of finite mass is ultimately ejected in all cases. The critical penetration depth that separates bound and unbound remnants has a dependence on the black hole mass when M (BH) less than or similar to 10(3) M (circle dot). We also find that the number of successive close passages before ejection decreases as we go from the stellar-mass black hole to the intermediate-mass black hole regime. For instance, after an initial encounter right at the classical tidal disruption limit, a 1 M (circle dot) star undergoes 16 (5) pericenter passages before ejection from a 10 M (circle dot) (100 M (circle dot)) black hole. Observations of periodic flares from these repeated close passages could in principle indicate signatures of a partial tidal disruption event.

Compact-object binaries including a white dwarf component are unique among gravitational-wave sources because their evolution is governed not just by general relativity and tides, but also by mass transfer. While the black hole and neutron star binaries observed with ground-based gravitational-wave detectors are driven to inspiral due to the emission of gravitational radiation-manifesting as a "chirp-like" gravitational-wave signal-the astrophysical processes at work in double white dwarf (DWD) systems can cause the inspiral to stall and even reverse into an outspiral. The dynamics of the DWD outspiral thus encode information about tides, which tell us about the behavior of electron-degenerate matter. We carry out a population study to determine the effect of the strength of tides on the distributions of the DWD binary parameters that the Laser Interferometer Space Antenna (LISA) will be able to constrain. We find that the strength of tidal coupling parameterized via the tidal synchronization timescale at the onset of mass transfer affects the distribution of gravitational-wave frequencies and frequency derivatives for detectably mass-transferring DWD systems. Using a hierarchical Bayesian framework informed by binary population synthesis simulations, we demonstrate how this parameter can be inferred using LISA observations. By measuring the population properties of DWDs, LISA will be able to probe the behavior of electron-degenerate matter.

Recent studies of nearby globular clusters have discovered excess dark mass in their cores, apparently in an extended distribution, and simulations indicate that this mass is composed mostly of white dwarfs (respectively stellar-mass black holes) in clusters that are core collapsed (respectively with a flatter core). We perform mass-anisotropy modelling of the closest globular cluster, M4, with intermediate slope for the inner stellar density. We use proper motion data from Gaia Early Data Release 3 (EDR3) and from observations by the Hubble Space Telescope. We extract the mass profile employing Bayesian Jeans modelling, and check our fits with realistic mock data. Our analyses return isotropic motions in the cluster core and tangential motions (ss approximate to -0.4 +/- 0.1) in the outskirts. We also robustly measure a dark central mass of roughly 800 +/- 300 M-circle dot, but it is not possible to distinguish between a point-like source, such as an intermediate-mass black hole (IMBH), and a dark population of stellar remnants of extent approximate to 0.016 pc similar or equal to 3300 au. However, when removing a high-velocity star from the cluster centre, the same mass excess is found, but more extended (similar to 0.034 pc approximate to 7000 au). We use Monte Carlo N-body models of M4 to interpret the second outcome, and find that our excess mass is not sufficiently extended to be confidently associated with a dark population of remnants. Finally, we discuss the feasibility of these two scenarios (i.e. IMBH versus remnants), and propose new observations that could help to better grasp the complex dynamics in M4's core.

We present a novel, few-body computational frame work designed to shed light on the likelihood of forming intermediate-mass (IM) and supermassive (SM) black holes (BHs) in nuclear star clusters (NSCs) through successive BH mergers, initiated with a single BH seed. Using observationally motivated NSC profiles, we find that the probability of an similar to 100 - M-circle dot BH to grow beyond similar to 1000 M-circle dot through successive mergers ranges from similar to 0.1 per cent in low-density, lo w-mass clusters to nearly 90 percent in high-mass, high-density clusters. Ho we ver, in the most massi ve NSCs, the gro wth time-scale can be very long (greater than or similar to 1 Gyr); vice versa, while growth is least likely in less massive NSCs, it is faster there, requiring as little as similar to 0 . 1 Gyr. The increased gravitational focusing in systems with lower velocity dispersions is the primary contributor to this behaviour. We find that there is a simple '7-strikes-and-you're-in' rule go v erning the growth of BHs: Our results suggest that if the seed survives 7-10 successive mergers without being ejected (primarily through gravitational wave recoil kicks), the growing BH will most likely remain in the cluster and will then undergo runaway, continuous growth all the way to the formation of an SMBH (under the simplifying assumption adopted here of a fixed background NSC). Furthermore, we find that rapid mergers enforce a dynamically mediated 'mass gap' between about 50 -300 M-circle dot in an NSC.

In order to gain insights on the conditions of aqueous alteration on asteroid Ryugu and the origin of water in the outer solar system, we developed the measurement of water content in magnetite at the micrometer scale by secondary ion mass spectrometry (NanoSIMS) and determined the H and Si content of coarse-grained euhedral magnetite grains (polyhedral magnetite) and coarse-grained fibrous (spherulitic) magnetite from the Ryugu polished section A0058-C1001. The hydrogen content in magnetite ranges between similar to 900 and similar to 3300 wt ppm equivalent water and is correlated with the Si content. Polyhedral magnetite has low and homogenous silicon and water content, whereas fibrous magnetite shows correlated Si and water excesses. These excesses can be explained by the presence of hydrous Si-rich amorphous nanoinclusions trapped during the precipitation of fibrous magnetite away from equilibrium and testify that fibrous magnetite formed from a hydrous gel with possibly more than 20 wt% water. An attempt to determine the water content in sub-mu m framboids indicates that additional calibration and contamination issues must be addressed before a safe conclusion can be drawn, but hints at elevated water content as well. The high water content in fibrous magnetite, expected to be among the first minerals to crystallize at low water-rock ratio, points to the control of water content by local conditions of magnetite precipitation rather than large-scale alteration conditions. Systematic lithological variations associated with water-rich and water-poor magnetite suggest that the global context of alteration may be better understood if local water concentrations are compared with millimeter-scale distribution of the various morphologies of magnetite. Finally, the high water content in the magnetite precursor gel indicates that the initial O isotopic composition in alteration water must not have been very different from that of the earliest magnetite crystals.

The northern Eastern Cordillera (EC) in the Colombian Andes is a wide (-200 km) high-elevation (> 2.5 km) subduction-related asymmetric plateau. This plateau is currently subjected to shortening and magmatic addition, and yet the nature of the lowermost crust and its transition to the underlying mantle remains poorly constrained. To improve our knowledge about the structure of the deep crust beneath the EC plateau, we conducted a joint inversion of travel times of local earthquakes and gravity data. Gravity contributes to up to 0.5% in dVp and dVs with no affectation of the shape or location of the discussed anomalies. Along the latitudinal profile with the highest resolution (similar to 5.7(degrees)N), two anomalies are identified at depths of 40-60 km beneath the plateau. A western low-velocity anomaly is interpreted as crustal material underthrust eastward beneath the northwestern EC. This process is triggering the abrupt change in topography between the adjacent low-elevation basin and the orogenic plateau. Additionally, a high-velocity anomaly beneath the eastern flank of the EC is likely related to mantle metasomatism and westward underthrusting of the foreland lithosphere. This metasomatism is either related to the interaction between mafic magmas and the uppermost mantle or silica enrichment that occurred during a past episode of flat subduction. Evidence for foreland underthrusting includes relocated earthquakes at 33-42 km depth within the thrust system between the EC and the foreland, along with increased shortening in the eastern part of the plateau, an increase in exhumation rates, the eastward migration of deformation towards the foreland, and dominated thick-skinned deformation in the upper crust of the foreland with an eastward vergence of thrusts and related folds. Furthermore, within the central part of the plateau, our results show low velocities between 30 and 40 km depth, consistent with previous constraints, suggesting magmatic underplating beneath the Paipa-Iza volcanic complex.