Stellar Escape from Globular Clusters. I. Escape Mechanisms and Properties at Ejection

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.Comment: 28 pages, 6 figures, 4 tables, accepted to Ap

Modeling Dense Star Clusters in the Milky Way and beyond with the Cluster Monte Carlo Code

We describe the public release of the Cluster Monte Carlo (CMC) code, a parallel, star-by-star N-body code for modeling dense star clusters. CMC treats collisional stellar dynamics using Hénon’s method, where the cumulative effect of many two-body encounters is statistically reproduced as a single effective encounter between nearest-neighbor particles on a relaxation timescale. The star-by-star approach allows for the inclusion of additional physics, including strong gravitational three- and four-body encounters, two-body tidal and gravitational-wave captures, mass loss in arbitrary galactic tidal fields, and stellar evolution for both single and binary stars. The public release of CMC is pinned directly to the COSMIC population synthesis code, allowing dynamical star cluster simulations and population synthesis studies to be performed using identical assumptions about the stellar physics and initial conditions. As a demonstration, we present two examples of star cluster modeling: first, we perform the largest (N = 108) star-by-star N-body simulation of a Plummer sphere evolving to core collapse, reproducing the expected self-similar density profile over more than 15 orders of magnitude; second, we generate realistic models for typical globular clusters, and we show that their dynamical evolution can produce significant numbers of black hole mergers with masses greater than those produced from isolated binary evolution (such as GW190521, a recently reported merger with component masses in the pulsational pair-instability mass gap)

4-cisim sistemlerinin uzun vadeli devinimin yarı analitik hesaplamaları

The observations of a large fraction of Sun-like stars in multiple-star systems and planet-forming circumstellar disks’ existence around them have triggered a renewed interest in the dynamical evolution and stability of planetary systems in binaries. In this thesis, we study the secular evolution of quadruple (N = 4) systems consisting of two planets around a member of a binary star system where the Kozai-Lidov mechanism plays a role. The standard Kozai-Lidov mechanism has been studied extensively for hierarchical triple systems in the literature and has a number of applications to the systems with cylindrical symmetry, i.e., circular binary orbits. In this mechanism, the conservation of the component of the angular momentum vector of a test particle along the symmetry axis restricts its orientation in space, i.e., prograde orbits cannot become retrograde. One way to break the cylindrical symmetry and thus to avoid this restriction is to make the perturber’s orbit eccentric and to go beyond the test particle approximation, which magnify the effects of high-order (octupole) terms in the disturbing function. These generalizations have been shown to cause large eccentricity excitations as well as orbit flips (i > 90◦ ) in 3-body systems. We investigate another way of removing the axial symmetry by adding one more body to triple systems. The presence of a fourth body allows visits to the parts of phase space unavailable to triples. Depending on the initial setup of the system, the fourth body may create effects similar to that of the high-order terms in the disturbing function in the 3-body problem. We observe that the addition of a second planet on a highly inclined orbit removes the cylindrical symmetry of the companion star on a circular orbit. This in turn induces dramatic changes in the orbital eccentricity of the inner planet and even flips its orientation. On the other hand, the fourth body may suppress the high-order effects present in triples by causing periapsis precession of the inner planet’s orbit at a faster rate. The strength of the coupling of the planets’ orbits determines the evolution and the stability of 4-body systems. In our work, we observe that especially weakly-coupled two-planet systems in binaries exhibit rich features. We calculate the secular interactions in these nearly Keplerian systems semi-analytically by combining two approximation methods: the Hamiltonian perturbation theory and the Gauss method.Thesis (M.S.) -- Graduate School of Natural and Applied Sciences. Physics

Modeling Dense Star Clusters in the Milky Way and beyond with the Cluster Monte Carlo Code

We describe the public release of the Cluster Monte Carlo (CMC) code, a parallel, star-by-star N-body code for modeling dense star clusters. CMC treats collisional stellar dynamics using Hénon’s method, where the cumulative effect of many two-body encounters is statistically reproduced as a single effective encounter between nearest-neighbor particles on a relaxation timescale. The star-by-star approach allows for the inclusion of additional physics, including strong gravitational three- and four-body encounters, two-body tidal and gravitational-wave captures, mass loss in arbitrary galactic tidal fields, and stellar evolution for both single and binary stars. The public release of CMC is pinned directly to the COSMIC population synthesis code, allowing dynamical star cluster simulations and population synthesis studies to be performed using identical assumptions about the stellar physics and initial conditions. As a demonstration, we present two examples of star cluster modeling: first, we perform the largest (N = 10$^{8}$) star-by-star N-body simulation of a Plummer sphere evolving to core collapse, reproducing the expected self-similar density profile over more than 15 orders of magnitude; second, we generate realistic models for typical globular clusters, and we show that their dynamical evolution can produce significant numbers of black hole mergers with masses greater than those produced from isolated binary evolution (such as GW190521, a recently reported merger with component masses in the pulsational pair-instability mass gap)