769 research outputs found
The importance of 15O(a,g)19Ne to X-ray bursts and superbursts
One of the two breakout reactions from the hot CNOcycle is 15O(a,g)19Ne,
which at low temperatures depends strongly on the resonance strength of the
4.033 MeV state in 19Ne. An experimental upper limit has been placed on its
strength, but the lower limit on the resonance strength and thereby the
astrophysical reaction rate is unconstrained experimentally. However, this
breakout reaction is crucial to the thermonuclear runaway which causes type I
X-ray bursts on accreting neutron stars. In this paper we exploit astronomical
observations in an attempt to constrain the relevant nuclear physics and deduce
a lower limit on the reaction rate. Our sensitivity study implies that if the
rate were sufficiently small, accreting material would burn stably without
bursts. The existence of type I X-ray bursts and superbursts consequently
suggests a lower limit on the 15O(a,g)19Ne reaction rate at low temperatures.Comment: 10 pages, 4 figures, uses apj.sty, accepted for publ. in Astrophys.
Effects of nuclear molecular configurations on the astrophysical S-factor for O + O
The impact of nuclear molecular configurations on the astrophysical S-factor
for O + O is investigated within the realistic two-center shell
model based on Woods-Saxon potentials. These molecular effects refer to the
formation of a neck between the interacting nuclei and the radial dependent
collective mass parameter. It is demonstrated that the former is crucial to
explain the current experimental data with high accuracy and without any free
parameter, whilst in addition the latter predicts a pronounced maximum in the
S-factor. In contrast to very recent results by Jiang et al., the S-factor does
not decline towards extremely low values as energy decreases.Comment: In press in Physics Letters
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
Astrophysical S-factor for O+O within the adiabatic molecular picture
The astrophysical S-factor for O + O is investigated within the
adiabatic molecular picture. It very well explains the available experimental
data. The collective radial mass causes a pronounced resonant structure in the
S-factor excitation function, providing a motivation for measuring the O
+ O fusion cross section at deep sub-barrier energies.Comment: 5 pages, 2 figures, SOTANCP2008 Conference, Strasbourg, France, May
13-16, 2008, To appear in IJMP
Impact of Nuclear Reaction Uncertainties on AGB Nucleosynthesis Models
Asymptotic giant branch (AGB) stars with low initial mass (1 - 3 Msun) are
responsible for the production of neutron-capture elements through the main
s-process (main slow neutron capture process). The major neutron source is
13C(alpha, n)16O, which burns radiatively during the interpulse periods at
about 8 keV and produces a rather low neutron density (10^7 n/cm^3). The second
neutron source 22Ne(alpha, n)25Mg, partially activated during the convective
thermal pulses when the energy reaches about 23 keV, gives rise to a small
neutron exposure but a peaked neutron density (Nn(peak) > 10^11 n/cm^3). At
metallicities close to solar, it does not substantially change the final
s-process abundances, but mainly affects the isotopic ratios near s-path
branchings sensitive to the neutron density. We examine the effect of the
present uncertainties of the two neutron sources operating in AGB stars, as
well as the competition with the 22Ne(alpha, gamma)26Mg reaction. The analysis
is carried out on AGB the main-s process component (reproduced by an average
between M(AGB; ini) = 1.5 and 3 Msun at half solar metallicity, see Arlandini
et al. 1999), using a set of updated nucleosynthesis models. Major effects are
seen close to the branching points. In particular, 13C(alpha, n)16O mainly
affects 86Kr and 87Rb owing to the branching at 85Kr, while small variations
are shown for heavy isotopes by decreasing or increasing our adopted rate by a
factor of 2 - 3. By changing our 22Ne(alpha, n)25Mg rate within a factor of 2,
a plausible reproduction of solar s-only isotopes is still obtained. We provide
a general overview of the major consequences of these variations on the s-path.
A complete description of each branching will be presented in Bisterzo et al.,
in preparation.Comment: Proceedings of Science 108, XII International Symposium on Nuclei in
the Cosmos 2012 (Cairns, Australia); 6 pages, 2 figure
A simple analytic model for astrophysical S-factors
We propose a physically transparent analytic model of astrophysical S-factors
as a function of a center-of-mass energy E of colliding nuclei (below and above
the Coulomb barrier) for non-resonant fusion reactions. For any given reaction,
the S(E)-model contains four parameters [two of which approximate the barrier
potential, U(r)]. They are easily interpolated along many reactions involving
isotopes of the same elements; they give accurate practical expressions for
S(E) with only several input parameters for many reactions. The model
reproduces the suppression of S(E) at low energies (of astrophysical
importance) due to the shape of the low-r wing of U(r). The model can be used
to reconstruct U(r) from computed or measured S(E). For illustration, we
parameterize our recent calculations of S(E) (using the Sao Paulo potential and
the barrier penetration formalism) for 946 reactions involving stable and
unstable isotopes of C, O, Ne, and Mg (with 9 parameters for all reactions
involving many isotopes of the same elements, e.g., C+O). In addition, we
analyze astrophysically important 12C+12C reaction, compare theoretical models
with experimental data, and discuss the problem of interpolating reliably known
S(E) values to low energies (E <= 2-3 MeV).Comment: 13 pages, 5 figures, Phys. Rev. C, accepte
The rp Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star
We calculate the nucleosynthesis during stable nuclear burning on an
accreting neutron star. This is appropriate for weakly magnetic neutron stars
accreting at near-Eddington rates in low mass X-ray binaries, and for most
accreting X-ray pulsars. We show that the nuclear burning proceeds via the
rapid proton capture process (rp process), and makes nuclei far beyond the iron
group. The final mixture of nuclei consists of elements with a range of masses
between approximately A=60 and A=100. The average nuclear mass of the ashes is
set by the extent of helium burning via (alpha,p) reactions, and depends on the
local accretion rate.
Our results imply that the crust of these accreting neutron stars is made
from a complex mixture of heavy nuclei, with important implications for its
thermal, electrical and structural properties. A crustal lattice as impure as
our results suggest will have a conductivity set mostly by impurity scattering,
allowing more rapid Ohmic diffusion of magnetic fields than previously
estimated.Comment: To appear in the Astrophysical Journal (33 pages, LaTeX, including 11
postscript figures
CEMP-s and CEMP-s/r stars: last update
We provide an updated discussion of the sample of CEMP-s and CEMP-s/r stars
collected from the literature. Observations are compared with the theoretical
nucleosynthesis models of asymptotic giant branch (AGB) stars presented by
Bisterzo et al. (2010, 2011, 2012), in the light of the most recent
spectroscopic results.Comment: 10 pages, 2 figures, New advances in stellar physics: from
microscopic to macroscopic processes, May 27-31 2013, Roscoff, France, EDP
Science, EAS Publications Series, in pres
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