44 research outputs found
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 0.181.8 for primordial BBH
mergers and 0.618 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 Y0.32 and
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 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 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