2,388 research outputs found
Astrophysical relevance of transition energies
The relevant gamma energy range is explicitly identified where additional
gamma$ strength has to be located for having an impact on astrophysically
relevant reactions. It is shown that folding the energy dependences of the
transmission coefficients and the level density leads to maximal contributions
for gamma energies of 2<=E_gamma<=4 MeV unless quantum selection rules allow
isolated states to contribute. Under this condition, electric dipole
transitions dominate. These findings allow to more accurately judge the
relevance of modifications of the \gamma strength for astrophysics.Comment: 5 pages, 11 figures, version accepted as a Rapid Communication in
Phys. Rev.
Nucleosynthesis in Type I X-ray Bursts
Type I X-ray bursts are thermonuclear explosions that occur in the envelopes
of accreting neutron stars. Detailed observations of these phenomena have
prompted numerous studies in theoretical astrophysics and experimental nuclear
physics since their discovery over 35 years ago. In this review, we begin by
discussing key observational features of these phenomena that may be sensitive
to the particular patterns of nucleosynthesis from the associated thermonuclear
burning. We then summarize efforts to model type I X-ray bursts, with emphasis
on determining the nuclear physics processes involved throughout these bursts.
We discuss and evaluate limitations in the models, particularly with regard to
key uncertainties in the nuclear physics input. Finally, we examine recent,
relevant experimental measurements and outline future prospects to improve our
understanding of these unique environments from observational, theoretical and
experimental perspectives.Comment: Accepted by Prog. Part. Nucl. Phys., 45 pages, 14 figure
Properties of Carbon-Oxygen White Dwarfs From Monte Carlo Stellar Models
We investigate properties of carbon-oxygen white dwarfs with respect to the
composite uncertainties in the reaction rates using the stellar evolution
toolkit, Modules for Experiments in Stellar Astrophysics (MESA) and the
probability density functions in the reaction rate library STARLIB. These are
the first Monte Carlo stellar evolution studies that use complete stellar
models. Focusing on 3 M models evolved from the pre main-sequence to
the first thermal pulse, we survey the remnant core mass, composition, and
structure properties as a function of 26 STARLIB reaction rates covering
hydrogen and helium burning using a Principal Component Analysis and Spearman
Rank-Order Correlation. Relative to the arithmetic mean value, we find the
width of the 95\% confidence interval to be
0.019 M for the core mass at the first thermal pulse,
12.50 Myr for the age, 0.013 for the central temperature, 0.060 for the central density,
2.610 for the central electron
fraction, 5.810,
0.392, and 0.392. Uncertainties in the experimental
C(, triple-, and
N( reaction rates dominate these variations. We
also consider a grid of 1 to 6 M models evolved from the pre
main-sequence to the final white dwarf to probe the sensitivity of the
initial-final mass relation to experimental uncertainties in the hydrogen and
helium reaction rates.Comment: Accepted for publication in The Astrophysical Journal; 19 Pages, 23
Figures, 5 Table
Mean proton and alpha-particle reduced widths of the Porter-Thomas distribution and astrophysical applications
The Porter-Thomas distribution is a key prediction of the Gaussian orthogonal ensemble in random matrix theory. It is routinely used to provide a measure for the number of levels that are missing in a given resonance analysis. The Porter-Thomas distribution is also of crucial importance for estimates of thermonuclear reaction
rates where the contributions of certain unobserved resonances to the total reaction rate need to be taken into account. In order to estimate such contributions by randomly sampling over the Porter-Thomas distribution,
the mean value of the reduced width must be known. We present mean reduced width values for protons and α particles of compound nuclei in the A = 28–67 mass range. The values are extracted from charged-particle
elastic scattering and reaction data that weremeasured at the riangle Universities Nuclear Laboratory over several decades. Our new values differ significantly from those previously reported that were based on a preliminary analysis of a smaller data set. As an example for the application of our results, we present new thermonuclear rates for the 40Ca(α,γ)44Ti reaction, which is important for 44Ti production in core-collapse supernovae, and compare with previously reported results.Peer ReviewedPostprint (published version
Reaction rate uncertainties and the operation of the NeNa and MgAl chains during HBB in intermediate-mass AGB stars
We test the effect of proton-capture reaction rate uncertainties on the
abundances of the Ne, Na, Mg and Al isotopes processed by the NeNa and MgAl
chains during hot bottom burning (HBB) in asymptotic giant branch (AGB) stars
of intermediate mass between 4 and 6 solar masses and metallicities between
Z=0.0001 and 0.02. We provide uncertainty ranges for the AGB stellar yields,
for inclusion in galactic chemical evolution models, and indicate which
reaction rates are most important and should be better determined. We use a
fast synthetic algorithm based on detailed AGB models. We run a large number of
stellar models, varying one reaction per time for a very fine grid of values,
as well as all reactions simultaneously. We show that there are uncertainties
in the yields of all the Ne, Na, Mg and Al isotopes due to uncertain
proton-capture reaction rates. The most uncertain yields are those of 26Al and
23Na (variations of two orders of magnitude), 24Mg and 27Al (variations of more
than one order of magnitude), 20Ne and 22Ne (variations between factors 2 and
7). In order to obtain more reliable Ne, Na, Mg and Al yields from IM-AGB stars
the rates that require more accurate determination are: 22Ne(p,g)23Na,
23Na(p,g)24Mg, 25Mg(p,g)26Al, 26Mg(p,g)27Al and 26Al(p,g)27Si. Detailed
galactic chemical evolution models should be constructed to address the impact
of our uncertainty ranges on the observational constraints related to HBB
nucleosynthesis, such as globular cluster chemical anomalies.Comment: accepted for publication on Astronomy & Astrophysic
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
Direct measurement of the 14N(p,g)15O S-factor
We have measured the 14N(p,g)15O excitation function for energies in the
range E_p = 155--524 keV. Fits of these data using R-matrix theory yield a
value for the S-factor at zero energy of 1.64(17) keV b, which is significantly
smaller than the result of a previous direct measurement. The corresponding
reduction in the stellar reaction rate for 14N(p,g)15O has a number of
interesting consequences, including an impact on estimates for the age of the
Galaxy derived from globular clusters.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Let
Theoretical Studies of Accretion of Matter onto White Dwarfs and the Single Degenerate Scenario for Supernovae of Type Ia
We present a brief summary of the Single Degenerate Scenario for the
progenitors of Type Ia Supernovae in which it is assumed that a low mass
carbon-oxygen white dwarf is growing in mass as a result of accretion from a
secondary star in a close binary system. Recent hydrodynamic simulations of
accretion of solar material onto white dwarfs without mixing always produce a
thermonuclear runaway and steady burning does not occur. For a broad range in
WD mass (0.4 Solar masses to 1.35 Solar Masses), the maximum ejected material
occurs for the 1.25 Solar Mass sequences and then decreases as the white dwarf
mass decreases. Therefore, the white dwarfs are growing in mass as a
consequence of the accretion of solar material and as long as there is no
mixing of accreted material with core material. In contrast, a thermonuclear
runaway in the accreted hydrogen-rich layers on the low luminosity WDs in close
binary systems where mixing of core matter with accreted material has occurred
is the outburst mechanism for Classical, Recurrent, and Symbiotic novae. The
differences in characteristics of these systems is likely the WD mass and mass
accretion rate. The high levels of enrichment of CN ejecta in elements ranging
from carbon to sulfur confirm that there is dredge-up of matter from the core
of the WD and enable them to contribute to the chemical enrichment of the
interstellar medium. Therefore, studies of CNe can lead to an improved
understanding of Galactic nucleosynthesis, some sources of pre-solar grains,
and the Extragalactic distance scale. The characteristics of the outburst
depend on the white dwarf mass, luminosity, mass accretion rate, and the
chemical composition of both the accreting material and WD material. The
properties of the outburst also depends on when, how, and if the accreted
layers are mixed with the WD core and the mixing mechanism is still unknown.Comment: 25 Pages, Bulletin of the Astronomical Society of India (BASI) in
pres
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