Spatial mixing of binary stars in multiple-population globular clusters

We present the results of a study aimed at investigating the effects of dynamical evolution on the spatial distribution and mixing of primordial binary stars in multiple-population globular clusters. Multiple stellar population formation models predict that second-generation (SG) stars form segregated in the inner regions of a more extended first-generation (FG) cluster. Our study, based on the results of a survey of N-body simulations, shows that the spatial mixing process for binary stars is more complex than that of single stars since additional processes such as binary ionization, recoil and ejection following binary-single and binary-binary interactions play a key role in determining the spatial distribution of the population of surviving binaries. The efficiency and relative importance of these additional effects depends on the binary binding energy and determines the timescale of the spatial mixing of FG and SG binaries. Our simulations illustrate the role of ionization, recoil and ejection combined with the effects of mass segregation driven by two-body relaxation and show that the complex interplay of all these processes results in a significant extension of the time needed for the complete spatial mixing of FG and SG binaries compared to that of single stars. Clusters in which FG and SG single stars have already reached complete spatial mixing might be characterized by a significant radial gradient in the ratio of the FG-to-SG binary fraction. The implications of the delayed mixing of FG and SG binaries for the differences between the kinematics of the two populations are discussed.Comment: 8 pages, 7 figures, 1 table; accepted for publication in MNRA

Binary Black Hole Mergers from Globular Clusters: the Impact of Globular Cluster Properties

The dense environment of globular clusters (GCs) can facilitate the formation of binary black holes (BBHs), some of which can merge with gravitational waves (GW) within the age of the Universe. We have performed a survey of Monte-Carlo simulations following the dynamical evolution of GCs with different masses, sizes and binary fractions and explored the impact of the host GC properties on the formation of BBH mergers. We find that the number of BBH mergers from GCs is determined by the GC's initial mass, size and primordial binary fraction. We identify two groups of BBH mergers: a primordial group whose formation does not depend on cluster's dynamics and a dynamical group whose formation is driven by the cluster's dynamical evolution. We show how the BBH origin affects the BBH mergers' main properties such as the chirp mass and merging time distributions. We provide analytic expressions for the dependence of the number of BBH mergers from individual GCs on the main cluster's structural properties and the time evolution of the merger rates of these BBHs. These expressions provide an essential ingredient for a general framework allowing to estimate the merger rate density. Using the relations found in our study, we find a local merger rate density of 0.18$-$1.8 ${\rm Gpc}^{-3}{\rm yr}^{-1}$ for primordial BBH mergers and 0.6$-$18 ${\rm Gpc}^{-3}{\rm yr}^{-1}$ for dynamical BBH mergers, depending on the GC mass and size distributions, initial binary fraction and the number density of GCs in the Universe.Comment: 12 pages, 12 figures, 2 tables, accepted for publication in MNRA

Evolution of Binary Stars in Multiple-Population Globular Clusters

The discovery of multiple stellar populations in globular clusters has implications for all the aspects of the study of these stellar systems. In this paper, by means of N-body simulations, we study the evolution of binary stars in multiple-population clusters and explore the implications of the initial differences in the spatial distribution of different stellar populations for the evolution and survival of their binary stars. Our simulations show that initial differences between the spatial distribution of first-generation (FG) and second-generation (SG) stars can leave a fingerprint in the current properties of the binary population. SG binaries are disrupted more efficiently than those of the FG population resulting in a global SG binary fraction smaller than that of the FG. As for surviving binaries, dynamical evolution produces a difference between the SG and the FG binary binding energy distribution with the SG population characterized by a larger fraction of high binding energy (more bound) binaries. We have also studied the dependence of the binary properties on the distance from the cluster centre. Although the global binary fraction decreases more rapidly for the SG population, the local binary fraction measured in the cluster inner regions may still be dominated by SG binaries. The extent of the differences between the surviving FG and SG binary binding energy distribution also varies radially within the cluster and is larger in the cluster inner regions.Comment: 10 pages, 12 figures. Accepted for publication in MNRA

Evolution of Binary Stars in Multiple-Population Globular Clusters - II. Compact Binaries

We present the results of a survey of N-body simulations aimed at exploring the evolution of compact binaries in multiple-population globular clusters.We show that as a consequence of the initial differences in the structural properties of the first-generation (FG) and the second-generation (SG) populations and the effects of dynamical processes on binary stars, the SG binary fraction decreases more rapidly than that of the FG population. The difference between the FG and SG binary fraction is qualitatively similar to but quantitatively smaller than that found for wider binaries in our previous investigations.The evolution of the radial variation of the binary fraction is driven by the interplay between binary segregation, ionization and ejection. Ionization and ejection counteract in part the effects of mass segregation but for compact binaries the effects of segregation dominate and the inner binary fraction increases during the cluster evolution. We explore the variation of the difference between the FG and the SG binary fraction with the distance from the cluster centre and its dependence on the binary binding energy and cluster structural parameters. The difference between the binary fraction in the FG and the SG populations found in our simulations is consistent with the results of observational studies finding a smaller binary fraction in the SG population.Comment: 9 pages, 12 figures. Accepted for publication in MNRA

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. X. The radial distribution of stellar populations in NGC 2808

Due to their extreme helium abundance, the multiple stellar populations of the globular cluster NGC 2808 have been widely investigated from a photometric, spectroscopic, and kinematic perspective. The most striking feature of the color-magnitude diagram of NGC 2808 is the triple main sequence (MS), with the red MS corresponding to a stellar population with primordial helium, and the middle and the blue MS being enhanced in helium up to Y$\sim$0.32 and $\sim$0.38, respectively. A recent study has revealed that this massive cluster hosts at least five distinct stellar populations (A, B, C, D, and E). Among them populations A, B, and C correspond to the red MS, while populations C and D are connected to the middle and the blue MS. In this paper we exploit Hubble-Space-Telescope photometry to investigate the radial distribution of the red, the middle and the blue MS from the cluster center out to about 8.5 arcmin. Our analysis shows that the radial distribution of each of the three MSs is different. In particular, as predicted from multiple-population formation models, both the blue MS and the middle MS appears to be more concentrated than the red MS with a significance level for this result wich is above 3{\sigma}.Comment: Accepted for publication in MNRA

MOCCA SURVEY Database I: Binary Black Hole Mergers from Globular Clusters with Intermediate Mass Black Holes

The dynamical formation of black hole binaries in globular clusters that merge due to gravitational waves occurs more frequently in higher stellar density. Meanwhile, the probability to form intermediate mass black holes (IMBHs) also increases with the density. To explore the impact of the formation and growth of IMBHs on the population of stellar mass black hole binaries from globular clusters, we analyze the existing large survey of Monte-Carlo globular cluster simulation data (MOCCA SURVEY Database I). We show that the number of binary black hole mergers agrees with the prediction based on clusters' initial properties when the IMBH mass is not massive enough or the IMBH seed forms at a later time. However, binary black hole formation and subsequent merger events are significantly reduced compared to the prediction when the present-day IMBH mass is more massive than $\sim10^4 \rm M_{\odot}$ or the present-day IMBH mass exceeds about 1 per cent of cluster's initial total mass. By examining the maximum black hole mass in the system at the moment of black hole binary escaping, we find that $\sim$ 90 per cent of the merging binary black holes escape before the formation and growth of the IMBH. Furthermore, large fraction of stellar mass black holes are merged into the IMBH or escape as single black holes from globular clusters in cases of massive IMBHs, which can lead to the significant under-population of binary black holes merging with gravitational waves by a factor of 2 depending on the clusters' initial distributions.Comment: 9 pages, 8 figures, Accepted for publication in MNRA

Dynamics of primordial binary stars in multiple-population globular clusters

AbstractWe have performed a survey of N-body simulations to explore the dynamics of primordial binaries in multiple-population globular clusters. We show that, as a consequence of the initial differences between the spatial distribution of first-generation (FG) and second-generation (SG) stars, SG binaries are disrupted more efficiently than FG binaries. The effects of dynamical evolution on the surviving binaries produces a difference between the SG and the FG binary binding energy distribution with the SG population characterized by a larger fraction of high binding energy (more bound) binaries. We also explore the evolution of the radial variation of the SG-to-FG binary number ratio and find that although the global binary fraction decreases more rapidly for the SG population, the local binary fraction measured in the cluster inner regions may still be dominated by SG binaries