Stellar Encounters with Multiple Star Systems and the Blue Straggler Problem (original) (raw)
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Monthly Notices of the Royal Astronomical Society, 2011
We present a technique to identify the most probable dynamical formation scenario for an observed binary or triple system containing one or more merger products or, alternatively, to rule out the possibility of a dynamical origin. Our method relies on an analytic prescription for energy conservation during stellar encounters. With this, observations of the multiple star system containing the merger product(s) can be used to work backwards in order to constrain the initial orbital energies of any single, binary or triple systems that went into the encounter. The initial semi-major axes of the orbits provide an estimate for the collisional cross section and therefore the time-scale for the encounter to occur in its host cluster.
Demographics of triple systems in dense star clusters. (arXiv:2007.11605v1 [astro-ph.GA])
arXiv (Cornell University), 2020
Depending on the stellar type, more than ∼ 50% and ∼ 15% of stars in the field have at least one and two stellar companions, respectively. Hierarchical systems can be assembled dynamically in dense star clusters, as a result of few-body encounters among stars and/or compact remnants in the cluster core. In this paper, we present the demographics of stellar and compact-object triples formed via binary-binary encounters in the CMC Cluster Catalog, a suite of cluster simulations with presentday properties representative of the globular clusters (GCs) observed in the Milky Way. We show how the initial properties of the host cluster set the typical orbital parameters and formation times of the formed triples. We find that a cluster typically assembles hundreds of triples with at least one black hole (BH) in the inner binary, while only clusters with sufficiently small virial radii are efficient in producing triples with no BHs, as a result of the BH-burning process. We show that a typical GC is expected to host tens of triples with at least one luminous component at present day. We discuss how the Lidov-Kozai mechanism can drive the inner binary of the formed triples to high eccentricities, whenever it takes place before the triple is dynamically reprocessed by encountering another cluster member. Some of these systems can reach sufficiently large eccentricities to form a variety of transients and sources, such as blue stragglers, X-ray binaries, Type Ia Supernovae, Thorne-Zytkow objects, and LIGO/Virgo sources.
Demographics of Triple Systems in Dense Star Clusters
The Astrophysical Journal, 2020
Depending on the stellar type, more than ∼ 50% and ∼ 15% of stars in the field have at least one and two stellar companions, respectively. Hierarchical systems can be assembled dynamically in dense star clusters, as a result of few-body encounters among stars and/or compact remnants in the cluster core. In this paper, we present the demographics of stellar and compact-object triples formed via binary-binary encounters in the CMC Cluster Catalog, a suite of cluster simulations with presentday properties representative of the globular clusters (GCs) observed in the Milky Way. We show how the initial properties of the host cluster set the typical orbital parameters and formation times of the formed triples. We find that a cluster typically assembles hundreds of triples with at least one black hole (BH) in the inner binary, while only clusters with sufficiently small virial radii are efficient in producing triples with no BHs, as a result of the BH-burning process. We show that a typical GC is expected to host tens of triples with at least one luminous component at present day. We discuss how the Lidov-Kozai mechanism can drive the inner binary of the formed triples to high eccentricities, whenever it takes place before the triple is dynamically reprocessed by encountering another cluster member. Some of these systems can reach sufficiently large eccentricities to form a variety of transients and sources, such as blue stragglers, X-ray binaries, Type Ia Supernovae, Thorne-Zytkow objects, and LIGO/Virgo sources.
2011
In this thesis, we present theoretical and statistical techniques broadly related to systems of dynamically-interacting particles. We apply these techniques to observations of dense star clusters in order to study gravitational interactions between stars. These include both long-and short-range interactions, as well as encounters leading to direct collisions and mergers. The latter have long been suspected to be an important formation channel for several curious types of stars whose origins are unknown. The former drive the structural evolution of star clusters and, by leading to their eventual dissolution and the subsequent dispersal of their stars throughout the Milky Way Galaxy, have played an important role in shaping its history. Within the last few decades, theoretical work has painted a comprehensive picture for the evolution of star clusters.
Evolution of Stellar Collision Products in Globular Clusters. II. Off‐Axis Collisions
The Astrophysical Journal, 2001
We continue our exploration of collisionally merged stars in the blue straggler region of the color-magnitude diagram. We report the results of new SPH calculations of parabolic collisions between two main-sequence stars, with the initial structure and composition profiles of the parent stars having been determined from stellar evolution calculations. Parallelization of the SPH code has permitted much higher numerical resolution of the hydrodynamics. We also present evolutionary tracks for the resulting collision products, which emerge as rapidly rotating blue stragglers. The rotating collision products are brighter, bluer and remain on the main sequence longer than their non-rotating counterparts. In addition, they retain their rapid rotation rates throughout their main sequence lifetime. Rotationally-induced mixing strongly affects the evolution of the collision products, although it is not sufficient to mix the entire star. We discuss the implications of these results for studies of blue straggler populations in clusters. This work shows that off-axis collision products cannot become blue stragglers unless they lose a large fraction of their initial angular momentum. The mechanism for this loss is not apparent, although some possibilities are discussed.
Double Blue Straggler Sequences in Globular Clusters: The Case of NGC 362
The Astrophysical Journal, 2013
We used high-quality images acquired with the WFC3 on board the HST to probe the blue straggler star (BSS) population of the Galactic globular cluster NGC 362. We have found two distinct sequences of BSS: this is the second case, after M 30, where such a feature has been observed. Indeed the BSS location, their extension in magnitude and color and their radial distribution within the cluster nicely resemble those observed in M 30, thus suggesting that the same interpretative scenario can be applied: the red BSS sub-population is generated by mass transfer binaries, the blue one by collisions. The discovery of four new W UMa stars, three of which lying along the red-BSS sequence, further supports this scenario. We also found that the inner portion of the density profile deviates from a King model and is well reproduced by either a mild power-law (α ∼ −0.2) or a double King profile. This feature supports the hypothesis that the cluster is currently undergoing the core collapse phase. Moreover, the BSS radial distribution shows a central peak and monotonically decreases outward without any evidence of an external rising branch. This evidence is a further indication of the advanced dynamical age of NGC 362: in fact, together with M 30, NGC 362 belongs to the family of dynamically old clusters (Family III) in the "dynamical clock" classification proposed by . The observational evidence presented here strengthens the possible connection between the existence of a double BSS sequence and a quite advanced dynamical status of the parent cluster.
Spectroscopic properties and dynamical evolution of the merging system AM 1003-435
Astronomy and Astrophysics, 2006
Aims. We study the system AM 1003-435, which is composed of two strong interacting galaxies. Methods. We obtained long-slit optical spectra of twelve zones of the system, and performed numerical simulations of the encounter between the components following the evolution of their stellar and gaseous contents.
Monthly Notices of the Royal Astronomical Society, 2019
Many possible scenarios for the formation of multiple stellar populations (MSPs) in globular clusters (GCs) have been discussed so far, including the involvement of asymptotic giant branch stars, fast-rotating main-sequence stars, very massive main-sequence stars and mass-transferring massive binaries based on stellar evolution modelling. But self-consistent, dynamical simulations of very young GCs are usually not considered. In this work, we perform direct N-body modelling of such systems with total masses up to 3.2 × 105 M⊙, taking into account the observationally constrained primordial binary properties, and discuss the stellar mergers driven both by binary stellar evolution and dynamical evolution of GCs. The occurrence of stellar mergers is enhanced significantly in binary-rich clusters such that stars forming from the gas polluted by merger-driven ejection/winds would appear as MSPs. We thus emphasize that stellar mergers can be an important process that connects MSP formation...
Collisions of Main-Sequence Stars and the Formation of Blue Stragglers in Globular Clusters
The Astrophysical Journal, 1996
We report the results of new SPH calculations of parabolic collisions between two main-sequence stars in a globular cluster. Such collisions are directly relevant to the formation of blue stragglers. In particular, we consider parent stars of mass M/M T O = 0.2, 0.5, 0.75, and 1, where M T O is the cluster turnoff mass (typically about 0.8 M ⊙). Low-mass stars (with M = 0.2M T O or 0.5M T O) are assumed to be fully convective and are therefore modeled as n = 1.5 polytropes. Stars at the turnoff (with M = M T O) are assumed to be mostly radiative and are modeled as n = 3 polytropes. Intermediate-mass stars (with M = 0.75M T O) are modeled as composite polytropes consisting of a radiative core with polytropic index n = 3 and a convective envelope with n = 1.5. We focus our study on the question of hydrodynamic mixing of helium and hydrogen, which plays a crucial role in determining the observable characteristics of blue stragglers. In all cases we find that there is negligible hydrodynamic mixing of helium into the outer envelope of the merger remnant. The amount of hydrogen mixed into the core of the merger depends strongly on the entropy profiles of the two colliding stars. For two stars with nearly equal masses (and hence entropy profiles) very little hydrodynamic mixing occurs at all, especially if they are close to the turnoff point. This is because the hydrogen-rich material from both stars maintains, on average, a higher specific entropy than the helium-rich material. If the two parent stars are close to turnoff, very little hydrogen is present at the center of the merger remnant and the main-sequence lifetime of the blue straggler could be very short. In contrast, during a collision between two stars of sufficiently different masses (mass ratio q ∼ < 0.5), the hydrogen-rich material
The Evolution of Compact Binary Star Systems
Living Reviews in Relativity, 2006
We review the formation and evolution of compact binary stars consisting of white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and BHs are thought to be the primary astrophysical sources of gravitational waves (GWs) within the frequency band of ground-based detectors, while compact binaries of WDs are important sources of GWs at lower frequencies to be covered by space interferometers (LISA). Major uncertainties in the current understanding of properties of NSs and BHs most relevant to the GW studies are discussed, including the treatment of the natal kicks which compact stellar remnants acquire during the core collapse of massive stars and the common envelope phase of binary evolution. We discuss the coalescence rates of binary NSs and BHs and prospects for their detections, the formation and evolution of binary WDs and their observational manifestations. Special attention is given to AM CVn-stars -compact binaries in which the Roche lobe is filled by another WD or a low-mass partially degenerate helium-star, as these stars are thought to be the best LISA verification binary GW sources.