646 research outputs found
Double-detonation supernovae of sub-Chandrasekhar mass white dwarfs
In the "double-detonation sub-Chandrasekhar" model for type Ia supernovae, a
carbon-oxygen (C + O) white dwarf accumulates sufficient amounts of helium such
that a detonation ignites in that layer before the Chandrasekhar mass is
reached. This detonation is thought to trigger a secondary detonation in the C
+ O core. By means of one- and two-dimensional hydrodynamic simulations, we
investigate the robustness of this explosion mechanism for generic 1-M_sun
models and analyze its observable predictions. Also a resolution dependence in
numerical simulations is analyzed. The propagation of thermonuclear detonation
fronts, both in helium and in the carbon-oxygen mixture, is computed by means
of both a level-set function and a simplified description for nuclear
reactions. The decision whether a secondary detonation is triggered in the
white dwarf's core or not is made based on criteria given in the literature. In
a parameter study involving different initial flame geometries for He-shell
masses of 0.2 and 0.1 M_sun, we find that a secondary detonation ignition is a
very robust process. Converging shock waves originating from the detonation in
the He shell generate the conditions for a detonation near the center of the
white dwarf in most of the cases considered. Finally, we follow the complete
evolution of three selected models with 0.2 M_sun of He through the
C/O-detonation phase and obtain nickel-masses of about 0.40 to 0.45 M_sun.
Although we have not done a complete scan of the possible parameter space, our
results show that sub-Chandrasekhar models are not good candidates for normal
or sub-luminous type Ia supernovae. The chemical composition of the ejecta
features significant amounts of nickel in the outer layers at high expansion
velocities, which is inconsistent with near-maximum spectra. (abbreviated)Comment: 11 pages, 10 figures, PDFLaTeX, accepted for publication in A&
Quasiparticle light elements and quantum condensates in nuclear matter
Nuclei in dense matter are influenced by the medium. In the cluster mean
field approximation, an effective Schr\"odinger equation for the -particle
cluster is obtained accounting for the effects of the surrounding medium, such
as self-energy and Pauli blocking. Similar to the single-baryon states (free
neutrons and protons), the light elements (, internal quantum
state ) are treated as quasiparticles with energies that depend on the center of mass momentum , the temperature
, and the total densities of neutrons and protons, respectively.
We consider the composition and thermodynamic properties of nuclear matter at
low densities. At low temperatures, quartetting is expected to occur.
Consequences for different physical properties of nuclear matter and finite
nuclei are discussed.Comment: 5 pages, 1 figure, 2 table
Deuteron formation in nuclear matter
We investigate deuteron formation in nuclear matter at finite temperatures
within a systematic quantum statistical approach. We consider formation through
three-body collisions relevant already at rather moderate densities because of
the strong correlations. The three-body in-medium reaction rates driven by the
break-up cross section are calculated using exact three-body equations
(Alt-Grassberger-Sandhas type) that have been suitably modified to consistently
include the energy shift and the Pauli blocking. Important quantities are the
lifetime of deuteron fluctuations and the chemical relaxation time. We find
that the respective times differ substantially while using in-medium or
isolated cross sections. We expect implications for the description of heavy
ion collisions in particular for the formation of light charged particles at
low to intermediate energies.Comment: 19 pages, 5 figure
Nucleosynthesis in thermonuclear supernovae with tracers: convergence and variable mass particles
Nucleosynthetic yield predictions for multi-dimensional simulations of
thermonuclear supernovae generally rely on the tracer particle method to obtain
isotopic information of the ejected material for a given supernova simulation.
We investigate how many tracer particles are required to determine converged
integrated total nucleosynthetic yields. For this purpose, we conduct a
resolution study in the number of tracer particles for different hydrodynamical
explosion models at fixed spatial resolution. We perform hydrodynamic
simulations on a co-expanding Eulerian grid in two dimensions assuming
rotational symmetry for both pure deflagration and delayed detonation Type Ia
supernova explosions. Within a given explosion model, we vary the number of
tracer particles to determine the minimum needed for the method to give a
robust prediction of the integrated yields of the most abundant nuclides. For
the first time, we relax the usual assumption of constant tracer particle mass
and introduce a radially vary- ing distribution of tracer particle masses. We
find that the nucleosynthetic yields of the most abundant species (mass
fraction > 10E-5) are reasonably well predicted for a tracer number as small as
32 per axis and direction - more or less independent of the explosion model. We
conclude that the number of tracer particles that were used in extant published
works appear to have been sufficient as far as integrated yields are concerned
for the most copiously produced nuclides. Additionally we find that a suitably
chosen tracer mass distribution can improve convergence for nuclei produced in
the outer layer of the supernova where the constant tracer mass prescription
suffers from poor spatial resolution.Comment: 9 pages, 5 figures, accepted for publication in MNRA
Quantum Condensates in Nuclear Matter: Problems
In connection with the contribution "Quantum Condensates in Nuclear Matter"
some problems are given to become more familiar with the techniques of
many-particle physics.Comment: 8 pages, 1 figur
The alpha-particle in nuclear matter
Among the light nuclear clusters the alpha-particle is by far the strongest
bound system and therefore expected to play a significant role in the dynamics
of nuclei and the phases of nuclear matter. To systematically study the
properties of the alpha-particle we have derived an effective four-body
equation of the Alt-Grassberger-Sandhas (AGS) type that includes the dominant
medium effects, i.e. self energy corrections and Pauli-blocking in a consistent
way. The equation is solved utilizing the energy dependent pole expansion for
the sub system amplitudes. We find that the Mott transition of an
alpha-particle at rest differs from that expected from perturbation theory and
occurs at approximately 1/10 of nuclear matter densities.Comment: 9 pages RevTex file, 1 figure, submitted to Phys. Lett.
On Unconstrained SU(2) Gluodynamics with Theta Angle
The Hamiltonian reduction of classical SU(2) Yang-Mills field theory to the
equivalent unconstrained theory of gauge invariant local dynamical variables is
generalized to the case of nonvanishing theta angle. It is shown that for any
theta angle the elimination of the pure gauge degrees of freedom leads to a
corresponding unconstrained nonlocal theory of self-interacting second rank
symmetric tensor fields, and that the obtained classical unconstrained
gluodynamics with different theta angles are canonically equivalent as on the
original constrained level.Comment: 13 pages Revtex, no figures; several misprints corrected; version to
appear in Eur. Phys. J.
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
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