Mass segregation

About: Mass segregation is a research topic. Over the lifetime, 1024 publications have been published within this topic receiving 57729 citations.

The Clustering Dynamics of Primordial Black Boles in N-Body Simulations

Abstract: We explore the possibility that Dark Matter (DM) may be explained by a nonuniform background of approximately stellar mass clusters of Primordial Black Holes (PBHs) by simulating the evolution from recombination to the present with over 5000 realisations using a Newtonian N-body code. We compute the cluster rate of evaporation and extract the binary and merged sub-populations along with their parent and merger tree histories, lifetimes and formation rates, the dynamical and orbital parameter profiles, the degree of mass segregation and dynamical friction and power spectrum of close encounters. Overall, we find that PBHs can constitute a viable DM candidate, and that their clustering presents a rich phenomenology throughout the history of the Universe. We show that binary systems constitute about 9.5% of all PBHs at present, with mass ratios of q¯B=0.154, and total masses of m¯T,B=303M⊙. Merged PBHs are rare, about 0.0023% of all PBHs at present, with mass ratios of q¯B=0.965 with total and chirp masses of m¯T,B=1670M⊙ and m¯c,M=642M⊙, respectively. We find that cluster puffing up and evaporation leads to bubbles of these PBHs of order 1 kpc containing at present times about 36% of objects and mass, with one-hundred pc-sized cores. We also find that these PBH sub-haloes are distributed in wider PBH haloes of order hundreds of kpc, containing about 63% of objects and mass, coinciding with the sizes of galactic halos. We find at last high rates of close encounters of massive Black Holes (M∼1000M⊙), with ΓS=(1.2+5.9−0.9)×107yr−1Gpc−3 and mergers with ΓM=1337±41yr−1Gpc−3.

Deep HST/FOC Imaging of the Central Density Cusp of the Globular Cluster M15

Abstract: Using the Faint Object Camera on the repaired Hubble Space Telescope, we have observed two fields in the globular cluster M15: the central density cusp, and a field at r = 20''. These are the highest-resolution images ever taken of this cluster's dense core, and are the first to probe the distribution of stars well below the main-sequence turnoff. After correction for incompleteness, we measure a logarithmic cusp slope (d log \sigma / d log r) of -0.70 +- 0.05 (1-sigma) for turnoff (~ 0.8 \Msun) stars over the radial range from 0.3'' to 10''; this slope is consistent with previous measurements. We also set an approximate upper limit of ~1.5'' (90% confidence limit) on the size of any possible constant-surface-density core, but discuss uncertainties in this limit that arise from crowding corrections. We find that fainter stars in the cusp also have power-law density profiles: a mass group near 0.7 \Msun has a logarithmic slope of -0.56 +- 0.05 (1-sigma) over the radial range from 2'' to 10''. Taken together, the two slopes are not well matched by the simplest core-collapse or black-hole models. We also measure a mass function at r = 20'', outside of the central cusp. Both of the FOC fields show substantial mass segregation, when compared with a mass function measured with the WFPC2 at r = 5'. In comparing the overall mass functions of the two FOC fields and the r = 5' field, we find that the radial variation of the mass function is somewhat less than that predicted by a King-Michie model of the cluster, but greater than that predicted by a Fokker-Planck model taken from the literature.

A Dynamical Survey of Stellar-mass Black Holes in 50 Milky Way Globular Clusters

Abstract: Recent numerical simulations of globular clusters (GCs) have shown that stellar-mass black holes (BHs) play a fundamental role in driving cluster evolution and shaping their present-day structure. Rapidly mass-segregating to the center of GCs, BHs act as a dynamical energy source via repeated super-elastic scattering, delaying onset of core collapse and limiting mass segregation for visible stars. While recent discoveries of BH candidates in Galactic and extragalactic GCs have further piqued interest in BH-mediated cluster dynamics, numerical models show that even if significant BH populations remain in today's GCs, they are typically in configurations that are not directly detectable. We demonstrated in Weatherford et al. (2018) that an anti-correlation between a suitable measure of mass segregation ($\Delta$) in observable stellar populations and the number of retained BHs in GC models can be applied to indirectly probe BH populations in real GCs. Here, we estimate the number and total mass of BHs presently retained in 50 Milky Way GCs from the ACS Globular Cluster Survey by measuring $\Delta$ between populations of main sequence stars, using correlations found between $\Delta$ and BH retention in the CMC Cluster Catalog models. We demonstrate that the range in $\Delta$'s distribution from our models matches that for observed GCs to a remarkable degree. Our results further provide the narrowest constraints to-date on the retained BH populations in the GCs analyzed. Of these 50 GCs, we identify NGCs 2808, 5927, 5986, 6101, and 6205 to presently contain especially large BH populations, each with total BH mass exceeding $10^3\,\rm{M_{\odot}}$.

Do open star clusters evolve towards energy equipartition

Abstract: We investigate whether open clusters (OCs) tend to energy equipartition, by means of direct N-body simulations with a broken power-law mass function. We nd that the simulated OCs become strongly mass segregated, but the local velocity dispersion does not depend on the stellar mass for most of the mass range: the curve of the velocity dispersion as a function of mass is nearly at even after several half-mass relaxation times, regardless of the adopted stellar evolution recipes and Galactic tidal eld model. This result holds both if we start from virialized King models and if we use clumpy sub-virial initial conditions. The velocity dispersion of the most massive stars and stellar remnants tends to be higher than the velocity dispersion of the lighter stars. This trend is particularly evident in simulations without stellar evolution. We interpret this result as a consequence of the strong mass segregation, which leads to Spitzer’s instability. Stellar winds delay the onset of the instability. Our simulations strongly support the result that OCs do not attain equipartition, for a wide range of initial conditions.

The influence of stellar dynamical ejections and collisions on the relation between the maximum stellar and star cluster mass

Abstract: We perform the largest currently available set of direct N-body calculations of young star cluster models to study the dynamical influence, especially through the ejections of the most massive star in the cluster, on the current relation between the maximum stellar mass and the star cluster mass. We vary several initial parameters such as the initial half-mass radius of the cluster, the initial binary fraction and the degree of initial mass segregation. Two different pairing methods are used to construct massive binaries for more realistic initial conditions of massive binaries. We find that lower mass clusters () do not shoot out their heaviest star. In the case of massive clusters (), no most massive star escapes the cluster within 3 Myr regardless of the initial conditions if clusters have initial half-mass radii, r0.5, ≥0.8 pc. However, a few of the initially smaller sized clusters (r0.5= 0.3 pc), which have a higher density, eject their most massive star within 3 Myr. If clusters form with a compact size and their massive stars are born in a binary system with a mass ratio biased towards unity, the probability that the mass of the most massive star in the cluster changes due to the ejection of the initially most massive star can be as large as 20 per cent. Stellar collisions increase the maximum stellar mass in a large number of clusters when clusters are relatively dense ( and r0.5= 0.3 pc) and binary rich. Overall, we conclude that dynamical effects hardly influence the observational maximum stellar mass–cluster mass relation.