2,915 research outputs found
A stochastic Monte Carlo approach to model real star cluster evolution, III. Direct integrations of three- and four-body interactions
Spherically symmetric equal mass star clusters containing a large amount of
primordial binaries are studied using a hybrid method, consisting of a gas
dynamical model for single stars and a Monte Carlo treatment for relaxation of
binaries and the setup of close resonant and fly-by encounters of single stars
with binaries and binaries with each other (three- and four-body encounters).
What differs from our previous work is that each encounter is being integrated
using a highly accurate direct few-body integrator which uses regularized
variables. Hence we can study the systematic evolution of individual binary
orbital parameters (eccentricity, semi-major axis) and differential and total
cross sections for hardening, dissolution or merging of binaries (minimum
distance) from a sampling of several ten thousands of scattering events as they
occur in real cluster evolution including mass segregation of binaries,
gravothermal collapse and reexpansion, binary burning phase and ultimately
gravothermal oscillations. For the first time we are able to present empirical
cross sections for eccentricity variation of binaries in close three- and
four-body encounters. It is found that a large fraction of three-body and
four-body encounters results in merging. Previous cross sections obtained by
Spitzer and Gao for strong encounters can be reproduced, while for weak
encounters non-standard processes like formation of hierarchical triples occur.Comment: 16 pages, 19 figures, Latex in the MN style, submitted to MNRA
Monte-Carlo Simulations of Globular Cluster Evolution - I. Method and Test Calculations
We present a new parallel supercomputer implementation of the Monte-Carlo
method for simulating the dynamical evolution of globular star clusters. Our
method is based on a modified version of Henon's Monte-Carlo algorithm for
solving the Fokker-Planck equation. Our code allows us to follow the evolution
of a cluster containing up to 5x10^5 stars to core collapse in < 40 hours of
computing time. In this paper we present the results of test calculations for
clusters with equal-mass stars, starting from both Plummer and King model
initial conditions. We consider isolated as well as tidally truncated clusters.
Our results are compared to those obtained from approximate, self-similar
analytic solutions, from direct numerical integrations of the Fokker-Planck
equation, and from direct N-body integrations performed on a GRAPE-4
special-purpose computer with N=16384. In all cases we find excellent agreement
with other methods, establishing our new code as a robust tool for the
numerical study of globular cluster dynamics using a realistic number of stars.Comment: 35 pages, including 8 figures, submitted to ApJ. Revised versio
Glassy dynamics of kinetically constrained models
We review the use of kinetically constrained models (KCMs) for the study of
dynamics in glassy systems. The characteristic feature of KCMs is that they
have trivial, often non-interacting, equilibrium behaviour but interesting slow
dynamics due to restrictions on the allowed transitions between configurations.
The basic question which KCMs ask is therefore how much glassy physics can be
understood without an underlying ``equilibrium glass transition''. After a
brief review of glassy phenomenology, we describe the main model classes, which
include spin-facilitated (Ising) models, constrained lattice gases, models
inspired by cellular structures such as soap froths, models obtained via
mappings from interacting systems without constraints, and finally related
models such as urn, oscillator, tiling and needle models. We then describe the
broad range of techniques that have been applied to KCMs, including exact
solutions, adiabatic approximations, projection and mode-coupling techniques,
diagrammatic approaches and mappings to quantum systems or effective models.
Finally, we give a survey of the known results for the dynamics of KCMs both in
and out of equilibrium, including topics such as relaxation time divergences
and dynamical transitions, nonlinear relaxation, aging and effective
temperatures, cooperativity and dynamical heterogeneities, and finally
non-equilibrium stationary states generated by external driving. We conclude
with a discussion of open questions and possibilities for future work.Comment: 137 pages. Additions to section on dynamical heterogeneities (5.5,
new pages 110 and 112), otherwise minor corrections, additions and reference
updates. Version to be published in Advances in Physic
Current issues in finite- density-functional theory and Warm-Correlated Matter
Finite-temperature DFT has become of topical interest, partly due to the
increasing ability to create novel states of warm-correlated matter (WCM).
Subclasses of WCM are Warm-dense matter (WDM), ultra-fast matter (UFM), and
high-energy density matter (HEDM), containing electyrons (e) and ions (i).
Strong e-e, i-i and e-i correlation effects and partial degeneracies are found
in these systems where the electron temperature is comparable to the
electron Fermi energy. The ion subsystem may be solid, liquid or plasma, with
many states of ionization with ionic charge . Quasi-equilibria with the
ion temperature are common. The ion subsystem in WCM can no longer
be treated as a passive "external potential", as is customary in density
functional theory (DFT) dominated by solid-state theory or quantum chemistry.
Hohenberg-Kohn-Mermin theory can be used for WCMs if finite-
exchange-correlation (XC) functionals are available. They are functionals of
both the one-body electron density and the one-body ion densities
. A method of approximately but accurately mapping the quantum
electrons to a classical Coulomb gas enables one to treat electron-ion systems
entirely classically at any temperature and arbitrary spin polarization, using
exchange-correlation effects calculated {\it in situ}, directly from the
pair-distribution functions. This eliminates the need for any XC-functionals,
or the use of a Born-Oppenheimer approximation. This classical map has been
used to calculate the equation of state of WDM systems, and construct a
finite- XC functional that is found to be in close agreement with recent
quantum path-integral simulation data. In this review current developments and
concerns in finite- DFT, especially in the context of non-relativistic
warm-dense matter and ultra-fast matter will be presented.Comment: Presented at the DFT16 meeting in Debrecen, Hungary, September 2015,
held on the 50th anniversary of Kohn-Sham Theory, 10 pages, 3 figure
Magnetars: the physics behind observations
Magnetars are the strongest magnets in the present universe and the
combination of extreme magnetic field, gravity and density makes them unique
laboratories to probe current physical theories (from quantum electrodynamics
to general relativity) in the strong field limit. Magnetars are observed as
peculiar, burst--active X-ray pulsars, the Anomalous X-ray Pulsars (AXPs) and
the Soft Gamma Repeaters (SGRs); the latter emitted also three "giant flares,"
extremely powerful events during which luminosities can reach up to 10^47 erg/s
for about one second. The last five years have witnessed an explosion in
magnetar research which has led, among other things, to the discovery of
transient, or "outbursting," and "low-field" magnetars. Substantial progress
has been made also on the theoretical side. Quite detailed models for
explaining the magnetars' persistent X-ray emission, the properties of the
bursts, the flux evolution in transient sources have been developed and
confronted with observations. New insight on neutron star asteroseismology has
been gained through improved models of magnetar oscillations. The long-debated
issue of magnetic field decay in neutron stars has been addressed, and its
importance recognized in relation to the evolution of magnetars and to the
links among magnetars and other families of isolated neutron stars. The aim of
this paper is to present a comprehensive overview in which the observational
results are discussed in the light of the most up-to-date theoretical models
and their implications. This addresses not only the particular case of magnetar
sources, but the more fundamental issue of how physics in strong magnetic
fields can be constrained by the observations of these unique sources.Comment: 81 pages, 24 figures, This is an author-created, un-copyedited
version of an article submitted to Reports on Progress in Physic
Close Approach during Hard Binary--Binary Scattering
We report on an extensive series of numerical experiments of binary--binary
scattering, analysing the cross--section for close approach during interactions
for a range of hard binary parameters of interest in globular cluster cores. We
consider the implied rate for tidal interactions for different globular
clusters and compare our results with previous, complementary estimates of
stellar collision rates in globular clusters. We find that the collision rate
for binary--binary encounters dominates in low density clusters if the binary
fraction in the cluster is larger than for wide main--sequence binaries.
In dense clusters binary--single interactions dominate the collision rate and
the core binary fraction must be \ltorder 0.1 per decade in semi--major axis
or too many collisions take place compared to observations. The rates are
consistent if binaries with semi--major axes are overabundant in
low density clusters or if breakup and ejection substantially lowers the binary
fraction in denser clusters. Given reasonable assumptions about fractions of
binaries in the cores of low density clusters such as NGC~5053, we cannot
account for all the observed blue stragglers by stellar collisions during
binary encounters, suggesting a substantial fraction may be due to coalescence
of tight primordial binaries.Comment: 13 pages including 13 ps figures. MNRAS in pres
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