385 research outputs found
Collisional dynamics around binary black holes in galactic centers
We follow the sinking of two massive black holes in a spherical stellar
system where the black holes become bound under the influence of dynamical
friction. Once bound, the binary hardens by three-body encounters with
surrounding stars. We find that the binary wanders inside the core, providing
an enhanced supply of reaction partners for the hardening. The binary evolves
into a highly eccentric orbit leading to coalescence well beyond a Hubble time.
These are the first results from a hybrid ``self consistent field'' (SCF) and
direct Aarseth N-body integrator (NBODY6), which combines the advantages of the
direct force calculation with the efficiency of the field method. The code is
designed for use on parallel architectures and is therefore applicable to
collisional N-body integrations with extraordinarily large particle numbers (>
10^5). This creates the possibility of simulating the dynamics of both globular
clusters with realistic collisional relaxation and stellar systems surrounding
supermassive black holes in galactic nuclei.Comment: 38 pages, 13 figures, submitted to ApJ, accepted, revised text and
added figure
A comprehensive set of simulations studying the influence of gas expulsion on star cluster evolution
We have carried out a large set of N-body simulations studying the effect of
residual-gas expulsion on the survival rate and final properties of star
clusters.
We have varied the star formation efficiency, gas expulsion timescale and
strength of the external tidal field, obtaining a three-dimensional grid of
models which can be used to predict the evolution of individual star clusters
or whole star cluster systems by interpolating between our runs. The complete
data of these simulations is made available on the Internet.
Our simulations show that cluster sizes, bound mass fraction and velocity
profile are strongly influenced by the details of the gas expulsion. Although
star clusters can survive star formation efficiencies as low as 10% if the
tidal field is weak and the gas is removed only slowly, our simulations
indicate that most star clusters are destroyed or suffer dramatic loss of stars
during the gas removal phase. Surviving clusters have typically expanded by a
factor 3 or 4 due to gas removal, implying that star clusters formed more
concentrated than as we see them today. Maximum expansion factors seen in our
runs are around 10. If gas is removed on timescales smaller than the initial
crossing time, star clusters acquire strongly radially anisotropic velocity
dispersions outside their half-mass radii. Observed velocity profiles of star
clusters can therefore be used as a constraint on the physics of cluster
formation.Comment: 12 pages, 9 figures, MNRAS accepte
The possible origin of the faint fuzzy star clusters in NGC 1023
In the lenticular galaxy NGC 1023 a new population of star clusters (``faint
fuzzies'') was recently discovered by Larsen & Brodie. These clusters are found
inside the disc and are faint (23 < V < 24 mag) and extended with effective
radii of r_eff approx. 7 to 15 pc. We present here N-body calculations of a
likely formation-scenario through merging star clusters in clusters of star
clusters (super-clusters). Such super-clusters are observed to form in
interacting galaxies. The resulting merger objects have masses comparable to
the ``faint fuzzies'' and show large effective radii (r_eff > 7 pc). Even
though these objects are suffering from strong tidal forces they are able to
survive and reach the estimated ages of the extended star clusters in NGC 1023.Comment: 19 pages, 12 figures, accepted by A
Star Clusters
This review concentrates almost entirely on globular star clusters. It
emphasises the increasing realisation that few of the traditional problems of
star cluster astronomy can be studied in isolation: the influence of the Galaxy
affects dynamical evolution deep in the core, and the spectrum of stellar
masses; in turn the evolution of the core determines the highest stellar
densities, and the rate of encounters. In this way external tidal effects
indirectly influence the formation and evolution of blue stragglers, binary
pulsars, X-ray sources, etc. More controversially, the stellar density appears
to influence the relative distribution of normal stars. In the opposite sense,
the evolution of individual stars governs much of the early dynamics of a
globular cluster, and the existence of large numbers of primordial binary stars
has changed important details of our picture of the dynamical evolution. New
computational tools which will become available in the next few years will help
dynamical theorists to address these questions.Comment: 10 pages, 3 figures, Te
On the structure of tidal tails
We examine the longitudinal distribution of the stars escaping from a cluster
along tidal tails. Using both theory and simulations, we show that, even in the
case of a star cluster in a circular galactic orbit, when the tide is steady,
the distribution exhibits maxima at a distance of many tidal radii from the
cluster.Comment: 6 pages, 4 figures, accepted for publication in MNRA
Casimir Force on a Micrometer Sphere in a Dip: Proposal of an Experiment
The attractive Casimir force acting on a micrometer-sphere suspended in a
spherical dip, close to the wall, is discussed. This setup is in principle
directly accessible to experiment. The sphere and the substrate are assumed to
be made of the same perfectly conducting material.Comment: 11 pages, 1 figure; to appear in J. Phys. A: Math. Ge
Mapping the three-body system - decay time and reversibility
In this paper we carry out a quantitative analysis of the three-body systems
and map them as a function of decaying time and intial conguration, look at
this problem as an example of a simple deterministic system, and ask to what
extent the orbits are really predictable. We have investigated the behavior of
about 200 000 general Newtonian three body systems using the simplest initial
conditions. Within our resolution these cover all the possible states where the
objects are initially at rest and have no angular momentum. We have determined
the decay time-scales of the triple systems and show that the distribution of
this parameter is fractal in appearance. Some areas that appear stable on large
scales exhibit very narrow strips of instability and the overall pattern,
dominated by resonances, reminds us of a traditional Maasai warrior shield.
Also an attempt is made to recover the original starting conguration of the
three bodies by backward integration. We find there are instances where the
evolution to the future and to the past lead to different orbits, in spite of
time symmetric initial conditions. This implies that even in simple
deterministic systems there exists an Arrow of Time.Comment: 8 pages, 9 figures. Accepted for publication in MNRAS. Includes
low-resolution figures. High-resolution figures are available as PNG
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