62 research outputs found
Nucleosynthesis in Type Ia Supernovae
Among the major uncertainties involved in the Chandrasekhar mass models for
Type Ia supernovae are the companion star of the accreting white dwarf (or the
accretion rate that determines the carbon ignition density) and the flame speed
after ignition. We present nucleosynthesis results from relatively slow
deflagration (1.5 - 3 % of the sound speed) to constrain the rate of accretion
from the companion star. Because of electron capture, a significant amount of
neutron-rich species such as ^{54}Cr, ^{50}Ti, ^{58}Fe, ^{62}Ni, etc. are
synthesized in the central region. To avoid the too large ratios of
^{54}Cr/^{56}Fe and ^{50}Ti/^{56}Fe, the central density of the white dwarf at
thermonuclear runaway must be as low as \ltsim 2 \e9 \gmc. Such a low central
density can be realized by the accretion as fast as \dot M \gtsim 1 \times
10^{-7} M_\odot yr^{-1}. These rapidly accreting white dwarfs might correspond
to the super-soft X-ray sources.Comment: 10 page LaTeX, 7 PostScript figures, to appear in Nuclear Physics A,
Vol. A621 (1997
Nucleosynthesis in multi-dimensional SNIa explosions
We present the results of nucleosynthesis calculations based on
multidimensional (2D and 3D) hydrodynamical simulations of the thermonuclear
burning phase in SNIa. The detailed nucleosynthetic yields of our explosion
models are calculated by post-processing the ejecta, using passively advected
tracer particles. The nuclear reaction network employed in computing the
explosive nucleosynthesis contains 383 nuclear species. We analyzed two
different choices of ignition conditions (centrally ignited, in which the
spherical initial flame geometry is perturbated with toroidal rings, and
bubbles, in which multi-point ignition conditions are simulated). We show that
unburned C and O varies typically from ~40% to ~50% of the total ejected
material.The main differences between all our models and standard 1D
computations are, besides the higher mass fraction of unburned C and O, the C/O
ratio (in our case is typically a factor of 2.5 higher than in 1D
computations), and somewhat lower abundances of certain intermediate mass
nuclei such as S, Cl, Ar, K, and Ca, and of 56Ni. Because explosive C and O
burning may produce the iron-group elements and their isotopes in rather
different proportions one can get different 56Ni-fractions (and thus supernova
luminosities) without changing the kinetic energy of the explosion. Finally, we
show that we need the high resolution multi-point ignition (bubbles) model to
burn most of the material in the center (demonstrating that high resolution
coupled with a large number of ignition spots is crucial to get rid of unburned
material in a pure deflagration SNIa model).Comment: Accepted for A&A, 14 pages, 11 Figures, 2 Table
Charged-Particle and Neutron-Capture Processes in the High-Entropy Wind of Core-Collapse Supernovae
The astrophysical site of the r-process is still uncertain, and a full
exploration of the systematics of this process in terms of its dependence on
nuclear properties from stability to the neutron drip-line within realistic
stellar environments has still to be undertaken. Sufficiently high neutron to
seed ratios can only be obtained either in very neutron-rich low-entropy
environments or moderately neutron-rich high-entropy environments, related to
neutron star mergers (or jets of neutron star matter) and the high-entropy wind
of core-collapse supernova explosions. As chemical evolution models seem to
disfavor neutron star mergers, we focus here on high-entropy environments
characterized by entropy , electron abundance and expansion velocity
. We investigate the termination point of charged-particle reactions,
and we define a maximum entropy for a given and ,
beyond which the seed production of heavy elements fails due to the very small
matter density. We then investigate whether an r-process subsequent to the
charged-particle freeze-out can in principle be understood on the basis of the
classical approach, which assumes a chemical equilibrium between neutron
captures and photodisintegrations, possibly followed by a -flow
equilibrium. In particular, we illustrate how long such a chemical equilibrium
approximation holds, how the freeze-out from such conditions affects the
abundance pattern, and which role the late capture of neutrons originating from
-delayed neutron emission can play.Comment: 52 pages, 31 figure
The consequences of nuclear electron capture in core collapse supernovae
The most important weak nuclear interaction to the dynamics of stellar core
collapse is electron capture, primarily on nuclei with masses larger than 60.
In prior simulations of core collapse, electron capture on these nuclei has
been treated in a highly parameterized fashion, if not ignored. With realistic
treatment of electron capture on heavy nuclei come significant changes in the
hydrodynamics of core collapse and bounce. We discuss these as well as the
ramifications for the post-bounce evolution in core collapse supernovae.Comment: Accepted by PRL, 5 pages, 2 figure
Nucleosynthesis in Two-Dimensional Delayed Detonation Models of Type Ia Supernova Explosions
The nucleosynthetic characteristics of various explosion mechanisms of Type
Ia supernovae (SNe Ia) is explored based on three two-dimensional explosion
simulations representing extreme cases: a pure turbulent deflagration, a
delayed detonation following an approximately spherical ignition of the initial
deflagration, and a delayed detonation arising from a highly asymmetric
deflagration ignition. Apart from this initial condition, the deflagration
stage is treated in a parameter-free approach. The detonation is initiated when
the turbulent burning enters the distributed burning regime. This occurs at
densities around g cm -- relatively low as compared to existing
nucleosynthesis studies for one-dimensional spherically symmetric models. The
burning in these multidimensional models is different from that in
one-dimensional simulations as the detonation wave propagates both into
unburned material in the high density region near the center of a white dwarf
and into the low density region near the surface. Thus, the resulting yield is
a mixture of different explosive burning products, from carbon-burning products
at low densities to complete silicon-burning products at the highest densities,
as well as electron-capture products synthesized at the deflagration stage. In
contrast to the deflagration model, the delayed detonations produce a
characteristic layered structure and the yields largely satisfy constraints
from Galactic chemical evolution. In the asymmetric delayed detonation model,
the region filled with electron capture species (e.g., Ni, Fe) is
within a shell, showing a large off-set, above the bulk of Ni
distribution, while species produced by the detonation are distributed more
spherically (abridged).Comment: Accepted by the Astrophysical Journal. 15 pages, 14 figures, 4 table
The Role of Electron Captures in Chandrasekhar Mass Models for Type Ia Supernovae
The Chandrasekhar mass model for Type Ia Supernovae (SNe Ia) has received
increasing support from recent comparisons of observations with light curve
predictions and modeling of synthetic spectra. It explains SN Ia events via
thermonuclear explosions of accreting white dwarfs in binary stellar systems,
being caused by central carbon ignition when the white dwarf approaches the
Chandrasekhar mass. As the electron gas in white dwarfs is degenerate,
characterized by high Fermi energies for the high density regions in the
center, electron capture on intermediate mass and Fe-group nuclei plays an
important role in explosive burning. Electron capture affects the central
electron fraction Y_e, which determines the composition of the ejecta from such
explosions. Up to the present, astrophysical tabulations based on shell model
matrix elements were only available for light nuclei in the sd-shell. Recently
new Shell Model Monte Carlo (SMMC) and large-scale shell model diagonalization
calculations have also been performed for pf-shell nuclei. These lead in
general to a reduction of electron capture rates in comparison with previous,
more phenomenological, approaches. Making use of these new shell model based
rates, we present the first results for the composition of Fe-group nuclei
produced in the central regions of SNe Ia and possible changes in the
constraints on model parameters like ignition densities and burning front
speeds.Comment: 26 pages, 8 figures, submitted to Ap
Supernova Remnants as Clues to Their Progenitors
Supernovae shape the interstellar medium, chemically enrich their host
galaxies, and generate powerful interstellar shocks that drive future
generations of star formation. The shock produced by a supernova event acts as
a type of time machine, probing the mass loss history of the progenitor system
back to ages of 10 000 years before the explosion, whereas supernova
remnants probe a much earlier stage of stellar evolution, interacting with
material expelled during the progenitor's much earlier evolution. In this
chapter we will review how observations of supernova remnants allow us to infer
fundamental properties of the progenitor system. We will provide detailed
examples of how bulk characteristics of a remnant, such as its chemical
composition and dynamics, allow us to infer properties of the progenitor
evolution. In the latter half of this chapter, we will show how this exercise
may be extended from individual objects to SNR as classes of objects, and how
there are clear bifurcations in the dynamics and spectral characteristics of
core collapse and thermonuclear supernova remnants. We will finish the chapter
by touching on recent advances in the modeling of massive stars, and the
implications for observable properties of supernovae and their remnants.Comment: A chapter in "Handbook of Supernovae" edited by Athem W. Alsabti and
Paul Murdin (18 pages, 6 figures
Possible Resonances in the 12C + 12C Fusion Rate and Superburst Ignition
Observationally inferred superburst ignition depths are shallower than models
predict. We address this discrepancy by reexamining the superburst trigger
mechanism. We first explore the hypothesis of Kuulkers et al. that exothermic
electron captures trigger superbursts. We find that all electron capture
reactions are thermally stable in accreting neutron star oceans and thus are
not a viable trigger mechanism. Fusion reactions other than 12C + 12C are
infeasible as well since the possible reactants either deplete at much
shallower depths or have prohibitively large Coulomb barriers. Thus we confirm
the proposal of Cumming & Bildsten and Strohmayer & Brown that 12C + 12C
triggers superbursts. We then examine the 12C + 12C fusion rate. The reaction
cross-section is experimentally unknown at astrophysically relevant energies,
but resonances exist in the 12C + 12C system throughout the entire measured
energy range. Thus it is likely, and in fact has been predicted, that a
resonance exists near the Gamow peak energy ~ 1.5 MeV. For such a hypothetical
1.5 MeV resonance, we derive both a fiducial value and upper limit to the
resonance strength and find that such a resonance could decrease the
theoretically predicted superburst ignition depth by up to a factor of 4; in
this case, observationally inferred superburst ignition depths would accord
with model predictions for a range of plausible neutron star parameters. Said
differently, such a resonance would decrease the temperature required for
unstable 12C ignition at a column depth 10^12 g/cm^2 from 6 x 10^8 K to 5 x
10^8 K. Determining the existence of a strong resonance in the Gamow window
requires measurements of the 12C + 12C cross-section down to a center-of-mass
energy near 1.5 MeV, which is within reach of the proposed DUSEL facility.Comment: 13 pages, 4 figures; minor improvements, results and conclusions
unchanged, accepted to Ap
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