132 research outputs found
Introduction to nuclear astrophysics
In the first lecture of this volume, we will present the basic fundamental
ideas regarding nuclear processes occurring in stars. We start from stellar
observations, will then elaborate on some important quantum-mechanical
phenomena governing nuclear reactions, continue with how nuclear reactions
proceed in a hot stellar plasma and, finally, we will provide an overview of
stellar burning stages. At the end, the current knowledge regarding the origin
of the elements is briefly summarized. This lecture is directed towards the
student of nuclear astrophysics. Our intention is to present seemingly
unrelated phenomena of nuclear physics and astrophysics in a coherent
framework.Comment: Proceedings of the 5th European Summer School on Experimental Nuclear
Astrophysics, Santa Tecla, Italy, 2009, 20 pages, 4 figures, 1 tabl
Calculation of resonance energies from Q-values
Resonance energies are frequently derived from precisely measured excitation
energies and reaction Q-values. The latter quantities are usually calculated
from atomic instead of nuclear mass differences. This procedure disregards the
energy shift caused by the difference in the total electron binding energies
before and after the interaction. Assuming that the interacting nuclei in a
stellar plasma are fully ionized, this energy shift can have a significant
effect, considering that the resonance energy enters exponentially into the
expression for the narrow-resonance thermonuclear reaction rates. As an
example, the rate of the Ar(p,)K reaction is discussed,
which, at temperatures below 1 GK, depends only on the contributions of a
single resonance and direct capture. In this case, disregarding the energy
shift caused by the total electron binding energy difference erroneously
enhances the rate by 40\% near temperatures of 70 MK.Comment: 3 pages, 2 figure
Laboratory electron screening in nuclear resonant reactions
Both nonresonant and resonance reaction data are subject to laboratory
electron screening effects. For nonresonant reactions, such effects are well
documented and the measured cross sections can be corrected to find the
unscreened ones. Frequently, the procedure and expression to calculate
laboratory electron screening factors for nonresonant reactions are also
applied to isolated narrow resonances, without much theoretical support or
experimental evidence. A simple model is applied to estimate electron screening
factors, lengths, and potentials for narrow resonances. The corrections to the
measured data result in an enhancement of the unscreened resonance strengths by
less than 0.2%, contrary to published narrow-resonance screening correction
factors, which predict a reduction of the unscreened strengths by up to 25%.
Unless it can be proven otherwise, it is recommended that measured strengths of
isolated narrow resonances not be corrected for laboratory electron screening.
The prospects of investigating laboratory electron screening effects by
measuring almost negligible differences in resonance strengths are not
promising. Instead, the difference of the resonance energy for the unscreened
and screened situation may be measurable. As an example, the case of the E_cm =
956-keV resonance in the 27Al(p,gamma)28Si reaction is discussed. It is also
demonstrated that the claim of a previously reported detection of a resonance
near 800 keV in the 176Lu(p,n)176}Hf reaction is incorrect.Comment: 2 figure
Reaction Rate Uncertainties: NeNa and MgAl in AGB Stars
We study the effect of uncertainties in the proton-capture reaction rates of
the NeNa and MgAl chains on nucleosynthesis due to the operation of hot bottom
burning (HBB) in intermediate-mass asymptotic giant branch (AGB) stars. HBB
nucleosynthesis is associated with the production of sodium, radioactive Al26
and the heavy magnesium isotopes, and it is possibly responsible for the O, Na,
Mg and Al abundance anomalies observed in globular cluster stars.
We model HBB with an analytic code based on full stellar evolution models so
we can quickly cover a large parameter space. The reaction rates are varied
first individually, then all together. This creates a knock-on effect, where an
increase of one reaction rate affects production of an isotope further down the
reaction chain. We find the yields of Ne22, Na23 and Al26 to be the most
susceptible to current nuclear reaction rate uncertainties.Comment: Presented at NIC-IX, International Symposium on Nuclear Astrophysics
- Nuclei in the Cosmos - IX, CERN, Geneva, Switzerland, 25-30 June, 200
Nuclear Thermometers for Classical Novae
Classical novae are stellar explosions occurring in binary systems,
consisting of a white dwarf and a main sequence companion. Thermonuclear
runaways on the surface of massive white dwarfs, consisting of oxygen and neon,
are believed to reach peak temperatures of several hundred million kelvin.
These temperatures are strongly correlated with the underlying white dwarf
mass. The observational counterparts of such models are likely associated with
outbursts that show strong spectral lines of neon in their shells (neon novae).
The goals of this work are to investigate how useful elemental abundances are
for constraining the peak temperatures achieved during these outbursts and
determine how robust "nova thermometers" are with respect to uncertain nuclear
physics input. We present updated observed abundances in neon novae and perform
a series of hydrodynamic simulations for several white dwarf masses. We find
that the most useful thermometers, N/O, N/Al, O/S, S/Al, O/Na, Na/Al, O/P, and
P/Al, are those with the steepest monotonic dependence on peak temperature. The
sensitivity of these thermometers to thermonuclear reaction rate variations is
explored using post-processing nucleosynthesis simulations. The ratios N/O,
N/Al, O/Na, and Na/Al are robust, meaning they are minimally affected by
uncertain rates. However, their dependence on peak temperature is relatively
weak. The ratios O/S, S/Al, O/P, and P/Al reveal strong dependences on
temperature and the poorly known 30P(p,g)31S rate. We compare our model
predictions to neon nova observations and obtain the following estimates for
the underlying white dwarf masses: 1.34-1.35 solar masses (V838 Her), 1.18-1.21
solar masses (V382 Vel), <1.3 solar masses (V693 CrA), <1.2 solar masses (LMC
1990#1), and <1.2 solar masses (QU Vul).Comment: 12 pages, 7 figures, accepted to Ap
On Presolar Stardust Grains from CO Classical Novae
About 30% to 40% of classical novae produce dust 20-100 days after the
outburst, but no presolar stardust grains from classical novae have been
unambiguously identified yet. Although several studies claimed a nova paternity
for certain grains, the measured and simulated isotopic ratios could only be
reconciled assuming that the grains condensed after the nova ejecta mixed with
a much larger amount of close-to-solar matter. However, the source and
mechanism of this potential post-explosion dilution of the ejecta remains a
mystery. A major problem with previous studies is the small number of
simulations performed and the implied poor exploration of the large nova
parameter space. We report the results of a different strategy, based on a
Monte Carlo technique, that involves the random sampling over the most
important nova model parameters: the white dwarf composition; the mixing of the
outer white dwarf layers with the accreted material before the explosion; the
peak temperature and density; the explosion time scales; and the possible
dilution of the ejecta after the outburst. We discuss and take into account the
systematic uncertainties for both the presolar grain measurements and the
simulation results. Only those simulations that are consistent with all
measured isotopic ratios of a given grain are accepted for further analysis. We
also present the numerical results of the model parameters. We identify 18
presolar grains with measured isotopic signatures consistent with a CO nova
origin, without assuming any dilution of the ejecta. Among these, the grains
G270 2, M11-334-2, G278, M11-347-4, M11-151-4, and Ag2 6 have the highest
probability of a CO nova paternity.Comment: 8 figure
Nuclear Mixing Meters for Classical Novae
Classical novae are caused by mass transfer episodes from a main sequence
star onto a white dwarf via Roche lobe overflow. This material forms an
accretion disk around the white dwarf. Ultimately, a fraction of this material
spirals in and piles up on the white dwarf surface under electron-degenerate
conditions. The subsequently occurring thermonuclear runaway reaches hundreds
of megakelvin and explosively ejects matter into the interstellar medium. The
exact peak temperature strongly depends on the underlying white dwarf mass, the
accreted mass and metallicity, and the initial white dwarf luminosity.
Observations of elemental abundance enrichments in these classical nova events
imply that the ejected matter consists not only of processed solar material
from the main sequence partner but also of material from the outer layers of
the underlying white dwarf. This indicates that white dwarf and accreted matter
mix prior to the thermonuclear runaway. The processes by which this mixing
occurs require further investigation to be understood. In this work, we analyze
elemental abundances ejected from hydrodynamic nova models in search of
elemental abundance ratios that are useful indicators of the total amount of
mixing. We identify the abundance ratios CNO/H, Ne/H, Mg/H, Al/H, and
Si/H as useful mixing meters in ONe novae. The impact of thermonuclear reaction
rate uncertainties on the mixing meters is investigated using Monte Carlo
post-processing network calculations with temperature-density evolutions of all
mass zones computed by the hydrodynamic models. We find that the current
uncertainties in the P(,)S rate influence the Si/H
abundance ratio, but overall the mixing meters found here are robust against
nuclear physics uncertainties. A comparison of our results with observations of
ONe novae provides strong constraints for classical nova models
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