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On Earth, biology, hydrology, and geology are interlinked such that certain types of life are often associated with specific conditions, including rock type, pressure, temperature, and chemistry. Life on Earth has established itself in diverse and extreme niches, presenting the possibility that Mars, too, may hold records of fossilized and/or extant life in diverse environments. Geologic, paleohydrologic, and climatic conditions through the evolution of Mars are similar in many respects to conditions occurring during the evolution of Earth and, as such, may point to environments on Mars with potential to have supported living systems. Here, we discuss examples of those Martian settings. Such extraterrestrial environments should be targeted by international robotic and/or manned missions to explore potential fossilized or extant life on Mars.

Hierarchical triples are expected to be produced by the frequent binary-mediated interactions in the cores of globular clusters. In some of these triples, the tertiary companion can drive the inner binary to merger following large eccentricity oscillations, as a result of the eccentric Kozai-Lidov mechanism. In this paper, we study the dynamics and merger rates of black hole (BH) hierarchical triples, formed via binary-binary encounters in the CMC Cluster Catalog, a suite of cluster simulations with present-day properties representative of the Milky Way's globular clusters. We compare the properties of the mergers from triples to the other merger channels in dense star clusters, and show that triple systems do not produce significant differences in terms of mass and effective spin distribution. However, they represent an important pathway for forming eccentric mergers, which could be detected by LIGO-Virgo/Kamioka Gravitational-Wave Detector (LVK), and future missions such as LISA and the DECi-hertz Interferometer Gravitational wave Observatory. We derive a conservative lower limit for the merger rate from this channel of 0.35 Gpc(-3) yr(-1) in the local universe and up to similar to 9% of these events may have a detectable eccentricity at LVK design sensitivity. Additionally, we find that triple systems could play an important role in retaining second-generation BHs, which can later merge again in the core of the host cluster.

Theoretical modeling of massive stars predicts a gap in the black hole (BH) mass function above similar to 40-50 M for BHs formed through single star evolution, arising from (pulsational) pair-instability supernovae (PISNe). However, in dense star clusters, dynamical channels may exist that allow construction of BHs with masses in excess of those allowed from single star evolution. The detection of BHs in this so-called "upper-mass gap" would provide strong evidence for the dynamical processing of BHs prior to their eventual merger. Here, we explore in detail the formation of BHs with masses within or above the pair-instability gap through collisions of young massive stars in dense star clusters. We run a suite of 68 independent cluster simulations, exploring a variety of physical assumptions pertaining to growth through stellar collisions, including primordial cluster mass segregation and the efficiency of envelope stripping during collisions. We find that as many as similar to 20% of all BH progenitors undergo one or more collisions prior to stellar collapse and up to similar to 1% of all BHs reside within or above the pair-instability gap through the effects of these collisions. We show that these BHs readily go on to merge with other BHs in the cluster, creating a population of massive BH mergers at a rate that may compete with the "multiple-generation" merger channel described in other analyses. This has clear relevance for the formation of very massive BH binaries as recently detected by the Laser Interferometer Gravitational-Wave Observatory/Virgo in GW190521. Finally, we describe how stellar collisions in clusters may provide a unique pathway to PISNe and briefly discuss the expected rate of these events and other electromagnetic transients.

LIGO's third observing run (O3) has reported several neutron star-black hole (NSBH) merger candidates. From a theoretical point of view, NSBH mergers have received less attention in the community than either binary black holes, or binary neutron stars. Here we examine single-single (sin-sin) gravitational wave (GW) captures in different types of star clusters-galactic nuclei, globular clusters, and young stellar clusters-and compare the merger rates from this channel to other proposed merger channels in the literature. There are currently large uncertainties associated with every merger channel, making a definitive conclusion about the origin of NSBH mergers impossible. However, keeping these uncertainties in mind, we find that sin-sin GW capture is unlikely to significantly contribute to the overall NSBH merger rate. In general, it appears that isolated binary evolution in the field or in clusters, and dynamically interacting binaries in triple configurations, may result in a higher merger rate.

Recent observations of globular clusters (GCs) provide evidence that the stellar initial mass function (IMF) may not be universal, suggesting specifically that the IMF grows increasingly top-heavy with decreasing metallicity and increasing gas density. Noncanonical IMFs can greatly affect the evolution of GCs, mainly because the high end determines how many black holes (BHs) form. Here we compute a new set of GC models, varying the IMF within observational uncertainties. We find that GCs with top-heavy IMFs lose most of their mass within a few gigayears through stellar winds and tidal stripping. Heating of the cluster through BH mass segregation greatly enhances this process. We show that, as they approach complete dissolution, GCs with top-heavy IMFs can evolve into "dark clusters" consisting of mostly BHs by mass. In addition to producing more BHs, GCs with top-heavy IMFs also produce many more binary BH (BBH) mergers. Even though these clusters are short-lived, mergers of ejected BBHs continue at a rate comparable to, or greater than, what is found for long-lived GCs with canonical IMFs. Therefore, these clusters, though they are no longer visible today, could still contribute significantly to the local BBH merger rate detectable by LIGO/Virgo, especially for sources with higher component masses well into the BH mass gap. We also report that one of our GC models with a top-heavy IMF produces dozens of intermediate-mass black holes (IMBHs) with masses M > 100 M-circle dot M > 500 M-circle dot. Ultimately, additional gravitational wave observations will provide strong constraints on the stellar IMF in old GCs and the formation of IMBHs at high redshift.

Black holes formed in dense star clusters, where dynamical interactions are frequent, may have fundamentally different properties than those formed through isolated stellar evolution. Theoretical models for single-star evolution predict a gap in the black hole mass spectrum from roughly 40-120 M caused by (pulsational) pair-instability supernovae. Motivated by the recent LIGO/Virgo event GW190521, we investigate whether black holes with masses within or in excess of this "upper-mass gap" can be formed dynamically in young star clusters through strong interactions of massive stars in binaries. We perform a set of N-body simulations using the CMC cluster-dynamics code to study the effects of the high-mass binary fraction on the formation and collision histories of the most massive stars and their remnants. We find that typical young star clusters with low metallicities and high binary fractions in massive stars can form several black holes in the upper-mass gap and often form at least one intermediate-mass black hole. These results provide strong evidence that dynamical interactions in young star clusters naturally lead to the formation of more massive black hole remnants.

Observational evidence suggests that the majority of stars may have been born in stellar clusters or associations. Within these dense environments, dynamical interactions lead to high rates of close stellar encounters. A variety of recent observational and theoretical indications suggest stellar-mass black holes may be present and play an active dynamical role in stellar clusters of all masses. In this study, we explore the tidal disruption of main-sequence stars by stellar-mass black holes in young star clusters. We compute a suite of over 3000 independent N-body simulations that cover a range of cluster mass, metallicity, and half-mass radii. We find stellar-mass black hole tidal disruption events (TDEs) occur at an overall rate of up to roughly 200 Gpc(-3) yr(-1) in young stellar clusters in the local universe. These TDEs are expected to have several characteristic features, namely, fast rise times of order a day, peak X-ray luminosities of at least 10(44) erg s(-1), and bright optical luminosities (roughly 10(41)-10(44) erg s(-1)) associated with reprocessing by a disk wind. In particular, we show these events share many features in common with the emerging class of Fast Blue Optical Transients.

The recent release of the second Gravitational-Wave Transient Catalog (GWTC-2) has increased significantly the number of known GW events, enabling unprecedented constraints on formation models of compact binaries. One pressing question is to understand the fraction of binaries originating from different formation channels, such as isolated field formation versus dynamical formation in dense stellar clusters. In this paper, we combine the COSMIC binary population synthesis suite and the CMC code for globular cluster evolution to create a mixture model for black hole binary formation under both formation scenarios. For the first time, these code bodies are combined self-consistently, with CMC itself employing COSMIC to track stellar evolution. We then use a deep-learning enhanced hierarchical Bayesian analysis to continuously sample over and constrain the common envelope efficiency alpha assumed in COSMIC, the initial cluster virial radius r(nu) adopted in CMC, and the intrinsic mixture fraction f between each channel. Under specific assumptions about other uncertain aspects of isolated binary and globular cluster evolution, we report the median and 90% confidence interval of three physical parameters for the intrinsic population (f, alpha, r(nu))= (0.20(-0.18)(+0.32), 2.26(-1.84)(+2.65), 2.71(-1.17)(+0.83)). This simultaneous constraint agrees with observed properties of globular clusters in the Milky Way and is an important first step in the pathway toward learning the astrophysics of compact binary formation through GW observations.

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.