2,272 research outputs found
Statistical Properties of Fermionic Molecular Dynamics
Statistical properties of Fermionic Molecular Dynamics are studied. It is
shown that, although the centroids of the single--particle wave--packets follow
classical trajectories in the case of a harmonic oscillator potential, the
equilibrium properties of the system are the quantum mechanical ones. A system
of weakly interacting fermions as well as of distinguishable particles is found
to be ergodic and the time--averaged occupation probabilities approach the
quantum canonical ones of Fermi--Dirac and Boltzmann statistics, respectively.Comment: 16 pages, several postscript figures, uses 'epsfig.sty'. More
information is available at http://www.gsi.de/~schnack/fmd.htm
Runaway of Line-Driven Winds Towards Critical and Overloaded solutions
Line-driven winds from hot stars and accretion disks are thought to adopt a
unique, critical solution which corresponds to maximum mass loss rate and a
particular velocity law. We show that in the presence of negative velocity
gradients, radiative-acoustic (Abbott) waves can drive shallow wind solutions
towards larger velocities and mass loss rates. Perturbations introduced
downstream from the wind critical point lead to convergence towards the
critical solution. By contrast, low-lying perturbations cause evolution towards
a mass-overloaded solution, developing a broad deceleration region in the wind.
Such a wind differs fundamentally from the critical solution. For sufficiently
deep-seated perturbations, overloaded solutions become time-dependent and
develop shocks and shells.Comment: Latex, 2 postscript figures Astrophysical Journal Letters, in pres
The nuclear liquid-gas phase transition within Fermionic Molecular Dynamics
The time evolution of excited nuclei, which are in equilibrium with the
surrounding vapour, is investigated. It is shown that the finite nuclear
systems undergo a first oder phase transition. The caloric curve is presented
for excited Oxygen, Magnesium, Aluminum and Calcium and the critical
temperature is estimated for Oxygen.Comment: 8 pages, 3 postscript figures, uses 'epsfig.sty'. Submitted to Phys.
Lett. B. More information available at http://www.gsi.de/~schnack/fmd.htm
Nuclear Structure based on Correlated Realistic Nucleon-Nucleon Potentials
We present a novel scheme for nuclear structure calculations based on
realistic nucleon-nucleon potentials. The essential ingredient is the explicit
treatment of the dominant interaction-induced correlations by means of the
Unitary Correlation Operator Method (UCOM). Short-range central and tensor
correlations are imprinted into simple, uncorrelated many-body states through a
state-independent unitary transformation. Applying the unitary transformation
to the realistic Hamiltonian leads to a correlated, low-momentum interaction,
well suited for all kinds of many-body models, e.g., Hartree-Fock or
shell-model. We employ the correlated interaction, supplemented by a
phenomenological correction to account for genuine three-body forces, in the
framework of variational calculations with antisymmetrised Gaussian trial
states (Fermionic Molecular Dynamics). Ground state properties of nuclei up to
mass numbers A<~60 are discussed. Binding energies, charge radii, and charge
distributions are in good agreement with experimental data. We perform angular
momentum projections of the intrinsically deformed variational states to
extract rotational spectra.Comment: 32 pages, 15 figure
X-ray photoionized bubble in the wind of Vela X-1 pulsar supergiant companion
Vela X-1 is the archetype of high-mass X-ray binaries, composed of a neutron
star and a massive B supergiant. The supergiant is a source of a strong
radiatively-driven stellar wind. The neutron star sweeps up this wind, and
creates a huge amount of X-rays as a result of energy release during the
process of wind accretion. Here we provide detailed NLTE models of the Vela X-1
envelope. We study how the X-rays photoionize the wind and destroy the ions
responsible for the wind acceleration. The resulting decrease of the radiative
force explains the observed reduction of the wind terminal velocity in a
direction to the neutron star. The X-rays create a distinct photoionized region
around the neutron star filled with a stagnating flow. The existence of such
photoionized bubbles is a general property of high-mass X-ray binaries. We
unveiled a new principle governing these complex objects, according to which
there is an upper limit to the X-ray luminosity the compact star can have
without suspending the wind due to inefficient line drivingComment: accepted for publication in ApJ, 6 pages, 4 figure
Clusters, Halos, And S-Factors In Fermionic Molecular Dynamics
In Fermionic Molecular Dynamics antisymmetrized products of Gaussian wave
packets are projected on angular momentum, linear momentum, and parity. An
appropriately chosen set of these states span the many-body Hilbert space in
which the Hamiltonian is diagonalized. The wave packet parameters - position,
momentum, width and spin - are obtained by variation under constraints. The
great flexibility of this basis allows to describe not only shell-model like
states but also exotic states like halos, e.g. the two-proton halo in 17Ne, or
cluster states as they appear for example in 12C close to the \alpha-breakup
threshold where the Hoyle state is located. Even a fully microscopic
calculation of the 3He(\alpha,\gamma)7Be capture reaction is possible and
yields an astrophysical S-factor that compares very well with newer data. As
representatives of numerous results these cases will be discussed in this
contribution, some of them not published so far. The Hamiltonian is based on
the realistic Argonne V18 nucleon-nucleon interaction.Comment: Presented at HIAS 2013, 8.-12. April 2013, Canberr
Mean-field instability of trapped dilute boson-fermion mixtures
The influence of boson-boson and boson-fermion interactions on the stability
of a binary mixture of bosonic and fermionic atoms is investigated. The density
profiles of the trapped mixture are obtained from direct numerical solution of
a modified Gross-Pitaevskii equation that is self-consistently coupled to the
mean-field generated by the interaction with the fermionic species. The
fermions which in turn feel the mean-field created by the bosons are treated in
Thomas-Fermi approximation. We study the effects of different combinations of
signs of the boson-boson and the boson-fermion scattering lengths and determine
explicit expressions for critical particle numbers as function of these
scattering lengths.Comment: 4 pages, 2 figures (using RevTeX4
Cluster structures within Fermionic Molecular Dynamics
The many-body states in an extended Fermionic Molecular Dynamics approach are
flexible enough to allow the description of nuclei with shell model nature as
well as nuclei with cluster and halo structures. Different many-body
configurations are obtained by minimizing the energy under constraints on
collective variables like radius, dipole, quadrupole and octupole deformations.
In the sense of the Generator Coordinate Method we perform variation after
projection and multiconfiguration calculations. The same effective interaction
derived from realistic interactions by means of the Unitary Correlation
Operator Method is used for all nuclei. Aspects of the shell model and cluster
nature of the ground and excited states of C12 are discussed. To understand
energies and radii of neutron-rich He isotopes the soft-dipole mode is found to
be important.Comment: 5 pages, proceedings of the 8th International conference on
Clustering Aspects of Nuclear Structure and Dynamics, Nov. 2003, Nara, Japan,
to be published in Nucl. Phys.
Nonlocal radiative coupling in non monotonic stellar winds
There is strong observational evidence of shocks and clumping in
radiation-driven stellar winds from hot, luminous stars. The resulting non
monotonic velocity law allows for radiative coupling between distant locations,
which is so far not accounted for in hydrodynamic wind simulations. In the
present paper, we determine the Sobolev source function and radiative line
force in the presence of radiative coupling in spherically symmetric flows,
extending the geometry-free formalism of Rybicki and Hummer (1978) to the case
of three-point coupling, which can result from, e.g., corotating interaction
regions, wind shocks, or mass overloading. For a simple model of an overloaded
wind, we find that, surprisingly, the flow decelerates at all radii above a
certain height when nonlocal radiative coupling is accounted for. We discuss
whether radiation-driven winds might in general not be able to re-accelerate
after a non monotonicity has occurred in the velocity law.Comment: accepted by A&A, 8 pages, 4 figure
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