278 research outputs found
Binary--single-star scattering VI. Automatic Determination of Interaction Cross Sections
Scattering encounters between binaries and single stars play a central role
in determining the dynamical evolution of a star cluster. In addition,
three-body scattering can give rise to many interesting exceptional objects:
merging can produce blue stragglers; exchange can produce binaries containing
millisecond pulsars in environments quite different from those in which the
pulsars were spun up; various types of X-ray binaries can be formed, and their
activity can be either shut off or triggered as a result of triple
interactions.
To date, all published results on three-body scattering have relied on human
guidance for determining the correct parameter range for the envelope within
which to perform Monte--Carlo scattering experiments. In this paper, we
describe the first fully automatic determination of cross sections and reaction
rates for binary--single-star scattering. Rather than relying on human
inspection of pilot calculations, we have constructed a feedback system that
ensures near-optimal coverage of parameter space while guaranteeing
completeness. We illustrate our approach with a particular example, in which we
describe the results of a three-body encounter between three main-sequence
stars of different masses. We provide total cross sections, as well as
branching ratios for the various different types of two-body mergers,
three-body mergers, and exchanges, both non-resonant and resonant. The
companion paper in this series, Paper VII, provides a full survey of
unequal-mass three-body scattering for hard binaries in the point-mass limit.Comment: 20 pages, TeX + 5 ps-figures, to appear in Ap
Star cluster ecology IVa: Dissection of an open star cluster---photometry
The evolution of star clusters is studied using N-body simulations in which
the evolution of single stars and binaries are taken self-consistently into
account. Initial conditions are chosen to represent relatively young Galactic
open clusters, such as the Pleiades, Praesepe and the Hyades. The calculations
include a realistic mass function, primordial binaries and the external
potential of the parent Galaxy. Our model clusters are generally significantly
flattened in the Galactic tidal field, and dissolve before deep core collapse
occurs. The binary fraction decreases initially due to the destruction of soft
binaries, but increases later because lower mass single stars escape more
easily than the more massive binaries. At late times, the cluster core is quite
rich in giants and white dwarfs. There is no evidence for preferential
evaporation of old white dwarfs, on the contrary the formed white dwarfs are
likely to remain in the cluster. Stars tend to escape from the cluster through
the first and second Lagrange points, in the direction of and away from the
Galactic center. Mass segregation manifests itself in our models well within an
initial relaxation time. As expected, giants and white dwarfs are much more
strongly affected by mass segregation than main-sequence stars. Open clusters
are dynamically rather inactive. However, the combined effect of stellar mass
loss and evaporation of stars from the cluster potential drives its dissolution
on a much shorter timescale than if these effects are neglected. The often-used
argument that a star cluster is barely older than its relaxation time and
therefore cannot be dynamically evolved is clearly in error for the majority of
star clusters.Comment: reduced abstract, 33 pages (three separate color .jpg figures),
submitted to 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
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
Gravothermal Expansion in an -Body System
This paper describes the numerical evolution of an -body system with a
slight ``temperature inversion''; i.e. the maximum velocity dispersion occurs
not at the centre but further out. Fluid models predict that the core of such a
system expands on a time-scale of thousands of central relaxation times, and
here this behaviour is qualitatively confirmed for an -body system of over
3000 bodies. With certain qualifications, this demonstrates the existence in
N-body systems of one of the fundamental mechanisms which, in fluid models,
drive the gravothermal oscillations discovered by Bettwieser & Sugimoto.Comment: 25pp and 12 figures (available from [email protected]), te
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