541 research outputs found
The equation of state and symmetry energy of low density nuclear matter
The symmetry energy of nuclear matter is a fundamental ingredient in the
investigation of exotic nuclei, heavy-ion collisions and astrophysical
phenomena. A recently developed quantum statistical (QS) approach that takes
the formation of clusters into account predicts low density symmetry energies
far above the usually quoted mean field limits. A consistent description of the
symmetry energy has been developed that joins the correct low-density limit
with values calculated from quasi-particle approaches valid near the saturation
density. The results are confronted with experimental values for free symmetry
energies and internal symmetry energies, determined at sub-saturation densities
and temperatures below 10 MeV using data from heavy-ion collisions. There is
very good agreement between the experimental symmetry energy values and those
calculated in the QS approachComment: 16 pages, 10 figures. arXiv admin note: text overlap with
arXiv:0908.234
Towards an understanding of Type Ia supernovae from a synthesis of theory and observations
Motivated by the fact that calibrated light curves of Type Ia supernovae (SNe
Ia) have become a major tool to determine the expansion history of the
Universe, considerable attention has been given to, both, observations and
models of these events over the past 15 years. Here, we summarize new
observational constraints, address recent progress in modeling Type Ia
supernovae by means of three-dimensional hydrodynamic simulations, and discuss
several of the still open questions. It will be be shown that the new models
have considerable predictive power which allows us to study observable
properties such as light curves and spectra without adjustable non-physical
parameters. This is a necessary requisite to improve our understanding of the
explosion mechanism and to settle the question of the applicability of SNe Ia
as distance indicators for cosmology. We explore the capabilities of the models
by comparing them with observations and we show how such models can be applied
to study the origin of the diversity of SNe Ia.Comment: 26 pages, 13 figures, Frontiers of Physics, in prin
Three-dimensional modeling of Type Ia supernovae - The power of late time spectra
Late time synthetic spectra of Type Ia supernovae, based on three-dimensional
deflagration models, are presented. We mainly focus on one
model,"c3_3d_256_10s", for which the hydrodynamics (Roepke 2005) and
nucleosynthesis (Travaglio et al. 2004) was calculated up to the homologous
phase of the explosion. Other models with different ignition conditions and
different resolution are also briefly discussed. The synthetic spectra are
compared to observed late time spectra. We find that while the model spectra
after 300 to 500 days show a good agreement with the observed Fe II-III
features, they also show too strong O I and C I lines compared to the observed
late time spectra. The oxygen and carbon emission originates from the
low-velocity unburned material in the central regions of these models. To get
agreement between the models and observations we find that only a small mass of
unburned material may be left in the center after the explosion. This may be a
problem for pure deflagration models, although improved initial conditions, as
well as higher resolution decrease the discrepancy. The relative intensity from
the different ionization stages of iron is sensitive to the density of the
emitting iron-rich material. We find that clumping, with the presence of low
density regions, is needed to reproduce the observed iron emission, especially
in the range between 4000 and 6000 AA. Both temperature and ionization depend
sensitively on density, abundances and radioactive content. This work therefore
illustrates the importance of including the inhomogeneous nature of realistic
three-dimensional explosion models. We briefly discuss the implications of the
spectral modeling for the nature of the explosion.Comment: 20 pages, 9 figures, resolution of Fig 1 is reduced to meet astro-ph
file size restriction, submitted to A&
Delayed detonations in full-star models of Type Ia supernova explosions
Aims: We present the first full-star three-dimensional explosion simulations
of thermonuclear supernovae including parameterized deflagration-to-detonation
transitions that occur once the flame enters the distributed burning regime.
Methods: Treating the propagation of both the deflagration and the detonation
waves in a common front-tracking approach, the detonation is prevented from
crossing ash regions. Results: Our criterion triggers the detonation wave at
the outer edge of the deflagration flame and consequently it has to sweep
around the complex structure and to compete with expansion. Despite the impeded
detonation propagation, the obtained explosions show reasonable agreement with
global quantities of observed type Ia supernovae. By igniting the flame in
different numbers of kernels around the center of the exploding white dwarf, we
set up three different models shifting the emphasis from the deflagration phase
to the detonation phase. The resulting explosion energies and iron group
element productions cover a large part of the diversity of type Ia supernovae.
Conclusions: Flame-driven deflagration-to-detonation transitions, if
hypothetical, remain a possibility deserving further investigation.Comment: 4 pages, 1 figur
Turbulence in a three-dimensional deflagration model for Type Ia supernovae: I. Scaling properties
We analyze the statistical properties of the turbulent velocity field in the
deflagration model for Type Ia supernovae. In particular, we consider the
question of whether turbulence is isotropic and consistent with the Kolmogorov
theory at small length scales. Using numerical data from a high-resolution
simulation of a thermonuclear supernova explosion, spectra of the turbulence
energy and velocity structure functions are computed. We show that the
turbulent velocity field is isotropic at small length scales and follows a
scaling law that is consistent with the Kolmogorov theory until most of the
nuclear fuel is burned. At length scales greater than a certain characteristic
scale, turbulence becomes anisotropic. Here, the radial velocity fluctuations
follow the scaling law of the Rayleigh-Taylor instability, whereas the angular
component still obeys Kolmogorov scaling. In the late phase of the explosion,
this characteristic scale drops below the numerical resolution of the
simulation. The analysis confirms that a subgrid-scale model for the unresolved
turbulence energy is required for the consistent calculation of the flame speed
in deflagration models of Type Ia supernovae, and that the assumption of
isotropy on these scales is appropriate.Comment: 7 pages with 16 figures, submitted to Ap
Pair Fluctuations in Ultra-small Fermi Systems within Self-Consistent RPA at Finite Temperature
A self-consistent version of the Thermal Random Phase Approximation (TSCRPA)
is developed within the Matsubara Green's Function (GF) formalism. The TSCRPA
is applied to the many level pairing model. The normal phase of the system is
considered. The TSCRPA results are compared with the exact ones calculated for
the Grand Canonical Ensemble. Advantages of the TSCRPA over the Thermal Mean
Field Approximation (TMFA) and the standard Thermal Random Phase Approximation
(TRPA) are demonstrated. Results for correlation functions, excitation
energies, single particle level densities, etc., as a function of temperature
are presented.Comment: 22 pages, 13 figers and 3 table
Dielectric function of a two-component plasma including collisions
A multiple-moment approach to the dielectric function of a dense non-ideal
plasma is treated beyond RPA including collisions in Born approximation. The
results are compared with the perturbation expansion of the Kubo formula. Sum
rules as well as Ward identities are considered. The relations to optical
properties as well as to the dc electrical conductivity are pointed out.Comment: latex, 10 pages, 7 figures in ps forma
Deuteron life-time in hot and dense nuclear matter near equilibrium
We consider deuteron formation in hot and dense nuclear matter close to
equilibrium and evaluate the life-time of the deuteron fluctuations within the
linear response theory. To this end we derive a generalized linear Boltzmann
equation where the collision integral is related to equilibrium correlation
functions. In this framework we then utilize finite temperature Green functions
to evaluate the collision integrals. The elementary reaction cross section is
evaluated within the Faddeev approach that is suitably modified to reflect the
properties of the surrounding hot and dense matter.Comment: 15 pages, 5 figure
Medium corrections in the formation of light charged particles in heavy ion reactions
Within a microscopic statistical description of heavy ion collisions, we
investigate the effect of the medium on the formation of light clusters. The
dominant medium effects are self-energy corrections and Pauli blocking that
produce the Mott effect for composite particles and enhanced reaction rates in
the collision integrals. Microscopic description of composites in the medium
follows the Dyson equation approach combined with the cluster mean-field
expansion. The resulting effective few-body problem is solved within a properly
modified Alt-Grassberger-Sandhas formalism. The results are incorporated in a
Boltzmann-Uehling-Uhlenbeck simulation for heavy ion collisions. The number and
spectra of light charged particles emerging from a heavy ion collision changes
in a significant manner in effect of the medium modification of production and
absorption processes.Comment: 16 pages, 6 figure
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