637 research outputs found
Computational Methods for Nucleosynthesis and Nuclear Energy Generation
This review concentrates on the two principle methods used to evolve nuclear
abundances within astrophysical simulations, evolution via rate equations and
via equilibria. Because in general the rate equations in nucleosynthetic
applications form an extraordinarily stiff system, implicit methods have proven
mandatory, leading to the need to solve moderately sized matrix equations.
Efforts to improve the performance of such rate equation methods are focused on
efficient solution of these matrix equations, by making best use of the
sparseness of these matrices. Recent work to produce hybrid schemes which use
local equilibria to reduce the computational cost of the rate equations is also
discussed. Such schemes offer significant improvements in the speed of reaction
networks and are accurate under circumstances where calculations with complete
equilibrium fail.Comment: LaTeX2e with graphicx, 40 Pages with 5 embedded figures. To be
published in Computational Astrophysics, The Journal of Computational and
Applied Mathematics, eds. H. Riffert, K. Werne
Silicon Burning I: Neutronization and the Physics of Quasi-Equilibrium
As the ultimate stage of stellar nucleosynthesis, and the source of the iron
peak nuclei, silicon burning is important to our understanding of the evolution
of massive stars and supernovae. Our reexamination of silicon burning, using
results gleaned from simulation work done with a large nuclear network (299
nuclei and more than 3000 reactions) and from independent calculations of
equilibrium abundance distributions, offers new insights into the
quasi-equilibrium mechanism and the approach to nuclear statistical
equilibrium. We find that the degree to which the matter has been neutronized
is of great importance, not only to the final products but also to the rate of
energy generation and the membership of the quasi-equilibrium groups. A small
increase in the global neutronization results in much larger free neutron
fluences, increasing the abundances of more neutron-rich nuclei. As a result,
incomplete silicon burning results in neutron richness among the isotopes of
the iron peak much larger than the global neutronization would indicate.
Finally, we briefly discuss the limitations and pitfalls of models for silicon
burning currently employed within hydrodynamic models. In a forthcoming paper
we will present a new approximation to the full nuclear network which preserves
the most important features of the large nuclear network calculations at a
significant improvement in computational speed. Such improved methods are
ideally suited for hydrodynamic calculations which involve the production of
iron peak nuclei, where the larger network calculation proves unmanageable.Comment: 44 pages of TeX with 25 Postscript figures, uses psfig.sty, To appear
in the The Astrophysical Journal, April 1 1996. Complete PostScript version
of the paper is also available from
http://tycho.as.utexas.edu/~raph/Publications.htm
Atypical Thermonuclear Supernovae from Tidally Crushed White Dwarfs
Suggestive evidence has accumulated that intermediate mass black holes (IMBH)
exist in some globular clusters. As stars diffuse in the cluster, some will
inevitable wander sufficiently close to the hole that they suffer tidal
disruption. An attractive feature of the IMBH hypothesis is its potential to
disrupt not only solar-type stars but also compact white dwarf stars. Attention
is given to the fate of white dwarfs that approach the hole close enough to be
disrupted and compressed to such extent that explosive nuclear burning may be
triggered. Precise modeling of the dynamics of the encounter coupled with a
nuclear network allow for a realistic determination of the explosive energy
release, and it is argued that ignition is a natural outcome for white dwarfs
of all varieties passing well within the tidal radius. Although event rates are
estimated to be significantly less than the rate of normal Type Ia supernovae,
such encounters may be frequent enough in globular clusters harboring an IMBH
to warrant a search for this new class of supernova.Comment: 13 pages, 4 figures, ApJ, accepte
The Effects of Changes in Reaction Rates on Simulations of Nova Explosions
Classical novae participate in the cycle of Galactic chemical evolution in
which grains and metal enriched gas in their ejecta, supplementing those of
supernovae, AGB stars, and Wolf-Rayet stars, are a source of heavy elements for
the ISM. Once in the diffuse gas, this material is mixed with the existing
gases and then incorporated into young stars and planetary systems during star
formation. Infrared observations have confirmed the presence of carbon, SiC,
hydrocarbons, and oxygen-rich silicate grains in nova ejecta, suggesting that
some fraction of the pre-solar grains identified in meteoritic material come
from novae. The mean mass returned by a nova outburst to the ISM probably
exceeds ~2 x 10^{-4} Solar Masses. Using the observed nova rate of 35 per year
in our Galaxy, it follows that novae introduce more than ~7 x 10^{-3} Solar
Masses per year of processed matter into the ISM. Novae are expected to be the
major source of 15N and 17O in the Galaxy and to contribute to the abundances
of other isotopes in this atomic mass range. Here, we report on how changes in
the nuclear reaction rates affect the properties of the outburst and alter the
predictions of the contributions of novae to Galactic chemical evolution. We
also discuss the necessity of including the pep reaction in studies of
thermonuclear runaways in material accreted onto white dwarfs.Comment: 9 pages, 2 figures, as it appeared in the Proceedings of the Tours
2006 Symposium on Nuclear Physic
Neutrinos, Fission Cycling, and the r-process
It has long been suggested that fission cycling may play an important role in
the r-process. Fission cycling can only occur in a very neutron rich
environment. In traditional calculations of the neutrino driven wind of the
core-collapse supernova, the environment is not sufficiently neutron rich to
produce the r-process elements. However, we show that with a reduction of the
electron neutrino flux coming from the supernova, fission cycling does occur
and furthermore it produces an abundance pattern which is consistent with
observed r-process abundance pattern in halo stars. Such a reduction can be
caused by active-sterile neutrino oscillations or other new physics.Comment: Typos corrected. Presented at NIC-IX, International Symposium on
Nuclear Astrophysics - Nuclei in the Cosmos - IX, CERN, Geneva, Switzerland,
25-30 June, 200
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
Ascertaining the Core Collapse Supernova Mechanism: An Emerging Picture?
Here we present the results from two sets of simulations, in two and three
spatial dimensions. In two dimensions, the simulations include multifrequency
flux-limited diffusion neutrino transport in the "ray-by-ray-plus"
approximation, two-dimensional self gravity in the Newtonian limit, and nuclear
burning through a 14-isotope alpha network. The three-dimensional simulations
are model simulations constructed to reflect the post stellar core bounce
conditions during neutrino shock reheating at the onset of explosion. They are
hydrodynamics-only models that focus on critical aspects of the shock stability
and dynamics and their impact on the supernova mechanism and explosion. In two
dimensions, we obtain explosions (although in one case weak) for two
progenitors (11 and 15 Solar mass models). Moreover, in both cases the
explosion is initiated when the inner edge of the oxygen layer accretes through
the shock. Thus, the shock is not revived while in the iron core, as previously
discussed in the literature. The three-dimensional studies of the development
of the stationary accretion shock instability (SASI) demonstrate the
fundamentally new dynamics allowed when simulations are performed in three
spatial dimensions. The predominant l=1 SASI mode gives way to a stable m=1
mode, which in turn has significant ramifications for the distribution of
angular momentum in the region between the shock and proto-neutron star and,
ultimately, for the spin of the remnant neutron star. Moreover, the
three-dimensional simulations make clear, given the increased number of degrees
of freedom, that two-dimensional models are severely limited by artificially
imposed symmetries.Comment: 9 pages, 3 figure
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