550 research outputs found
Silicon Burning II: Quasi-Equilibrium and Explosive Burning
Having examined the application of quasi-equilibrium to hydrostatic silicon
burning in Paper I of this series, Hix & Thielemann (1996), we now turn our
attention to explosive silicon burning. Previous authors have shown that for
material which is heated to high temperature by a passing shock and then cooled
by adiabatic expansion, the results can be divided into three broad categories;
\emph{incomplete burning}, \emph{normal freezeout} and \emph{-rich
freezeout}, with the outcome depending on the temperature, density and cooling
timescale. In all three cases, we find that the important abundances obey
quasi-equilibrium for temperatures greater than approximately 3 GK, with
relatively little nucleosynthesis occurring following the breakdown of
quasi-equilibrium. We will show that quasi-equilibrium provides better
abundance estimates than global nuclear statistical equilibrium, even for
normal freezeout and particularly for -rich freezeout. We will also
examine the accuracy with which the final nuclear abundances can be estimated
from quasi-equilibrium.Comment: 27 pages, including 15 inline figures. LaTeX 2e with aaspp4 and
graphicx packages. Accepted to Ap
The Innermost Ejecta of Core Collapse Supernovae
We ensure successful explosions (of otherwise non-explosive models) by
enhancing the neutrino luminosity via reducing the neutrino scattering cross
sections or by increasing the heating efficiency via enhancing the neutrino
absorption cross sections in the heating region. Our investigations show that
the resulting electron fraction Ye in the innermost ejecta is close to 0.5, in
some areas even exceeding 0.5. We present the effects of the resulting values
for Ye on the nucleosynthesis yields of the innermost zones of core collapse
supernovae.Comment: 4pages, 2figures; contribution to Nuclei In The Cosmos VIII, to
appear in Nucl. Phys.
Simulation of the Spherically Symmetric Stellar Core Collapse, Bounce, and Postbounce Evolution of a 13 Solar Mass Star with Boltzmann Neutrino Transport, and Its Implications for the Supernova Mechanism
With exact three-flavor Boltzmann neutrino transport, we simulate the stellar
core collapse, bounce, and postbounce evolution of a 13 solar mass star in
spherical symmetry, the Newtonian limit, without invoking convection. In the
absence of convection, prior spherically symmetric models, which implemented
approximations to Boltzmann transport, failed to produce explosions. We are
motivated to consider exact transport to determine if these failures were due
to the transport approximations made and to answer remaining fundamental
questions in supernova theory. The model presented here is the first in a
sequence of models beginning with different progenitors. In this model, a
supernova explosion is not obtained. We discuss the ramifications of our
results for the supernova mechanism.Comment: 5 pages, 3 figures, Submitted to Physical Review Letter
Low-lying dipole response in the Relativistic Quasiparticle Time Blocking Approximation and its influence on neutron capture cross sections
We have computed dipole strength distributions for nickel and tin isotopes
within the Relativistic Quasiparticle Time Blocking approximation (RQTBA).
These calculations provide a good description of data, including the
neutron-rich tin isotopes Sn. The resulting dipole strengths have
been implemented in Hauser-Feshbach calculations of astrophysical neutron
capture rates relevant for r-process nucleosynthesis studies. The RQTBA
calculations show the presence of enhanced dipole strength at energies around
the neutron threshold for neutron rich nuclei. The computed neutron capture
rates are sensitive to the fine structure of the low lying dipole strength,
which emphasizes the importance of a reliable knowledge of this excitation
mode.Comment: 15 pages, 4 figures, Accepted in Nucl. Phys.
Inclusive 12C(ΜΌ,Ό)12N reaction in the continuum random phase approximation
Motivated by a recent experiment at LAMPF we calculate cross sections for muon-neutrino and muon-antineutrino charged-current reactions on 12C within the continuum RPA model. We also determine the branching ratios for the main decay channels of these reactions by application of a statistical model
Extracting the rp-process from X-ray burst light curves
The light curves of type I X-ray bursts (XRBs) result from energy released
from the atmosphere of a neutron star when accreted hydrogen and helium ignite
and burn explosively via the rp-process. Since charged particle reaction rates
are both density and very temperature dependent, a simulation model must
provide accurate values of these variables to predict the reaction flow. This
paper uses a self-consistent one-dimensional model calculation with a constant
accretion rate of dM/dt=5e16g/s (0.045 Eddington) and reports on the detailed
rp-process reaction flow of a given burst.Comment: 4 pages, submitted to Nucl. Phys. A as part of the Nuclei in Cosmos 8
proceeding
Two-Dimensional Hydrodynamics of Pre-Core Collapse: Oxygen Shell Burning
By direct hydrodynamic simulation, using the Piecewise Parabolic Method (PPM)
code PROMETHEUS, we study the properties of a convective oxygen burning shell
in a SN 1987A progenitor star prior to collapse. The convection is too
heterogeneous and dynamic to be well approximated by one-dimensional
diffusion-like algorithms which have previously been used for this epoch.
Qualitatively new phenomena are seen.
The simulations are two-dimensional, with good resolution in radius and
angle, and use a large (90-degree) slice centered at the equator. The
microphysics and the initial model were carefully treated. Many of the
qualitative features of previous multi-dimensional simulations of convection
are seen, including large kinetic and acoustic energy fluxes, which are not
accounted for by mixing length theory. Small but significant amounts of
carbon-12 are mixed non-uniformly into the oxygen burning convection zone,
resulting in hot spots of nuclear energy production which are more than an
order of magnitude more energetic than the oxygen flame itself. Density
perturbations (up to 8%) occur at the `edges' of the convective zone and are
the result of gravity waves generated by interaction of penetrating flows into
the stable region. Perturbations of temperature and electron fraction at the
base of the convective zone are of sufficient magnitude to create angular
inhomogeneities in explosive nucleosynthesis products, and need to be included
in quantitative estimates of yields. Combined with the plume-like velocity
structure arising from convection, the perturbations will contribute to the
mixing of nickel-56 throughout supernovae envelopes. Runs of different
resolution, and angular extent, were performed to test the robustness of theseComment: For mpeg movies of these simulations, see
http://www.astrophysics.arizona.edu/movies.html Submitted to the
Astrophysical Journa
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