30 research outputs found
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
Statistical Methods for Thermonuclear Reaction Rates and Nucleosynthesis Simulations
Rigorous statistical methods for estimating thermonuclear reaction rates and
nucleosynthesis are becoming increasingly established in nuclear astrophysics.
The main challenge being faced is that experimental reaction rates are highly
complex quantities derived from a multitude of different measured nuclear
parameters (e.g., astrophysical S-factors, resonance energies and strengths,
particle and gamma-ray partial widths). We discuss the application of the Monte
Carlo method to two distinct, but related, questions. First, given a set of
measured nuclear parameters, how can one best estimate the resulting
thermonuclear reaction rates and associated uncertainties? Second, given a set
of appropriate reaction rates, how can one best estimate the abundances from
nucleosynthesis (i.e., reaction network) calculations? The techniques described
here provide probability density functions that can be used to derive
statistically meaningful reaction rates and final abundances for any desired
coverage probability. Examples are given for applications to s-process neutron
sources, core-collapse supernovae, classical novae, and big bang
nucleosynthesis.Comment: Accepted for publication in J. Phys. G Focus issue "Enhancing the
interaction between nuclear experiment and theory through information and
statistics
Charged-Particle Thermonuclear Reaction Rates: IV. Comparison to Previous Work
We compare our Monte Carlo reaction rates (see Paper II of this series) to
previous results that were obtained by using the classical method of computing
thermonuclear reaction rates. For each reaction, the comparison is presented
using two types of graphs: the first shows the change in reaction rate
uncertainties, while the second displays our new results normalized to the
previously recommended reaction rate. We find that the rates have changed
significantly for almost all reactions considered here. The changes are caused
by (i) our new Monte Carlo method of computing reaction rates (see Paper I of
this series), and (ii) newly available nuclear physics information (see Paper
III of this series).Comment: 67 pages, 62 figure
The Effects of Thermonuclear Reaction Rate Variations on Nova Nucleosynthesis: A Sensitivity Study
We investigate the effects of thermonuclear reaction rate uncertainties on
nova nucleosynthesis. One-zone nucleosynthesis calculations have been performed
by adopting temperature-density-time profiles of the hottest hydrogen-burning
zone (i.e., the region in which most of the nucleosynthesis takes place). We
obtain our profiles from 7 different, recently published, hydrodynamic nova
simulations covering peak temperatures in the range from Tpeak=0.145-0.418 GK.
For each of these profiles, we individually varied the rates of 175 reactions
within their associated errors and analyzed the resulting abundance changes of
142 isotopes in the mass range below A=40. In total, we performed 7350 nuclear
reaction network calculations. We use the most recent thermonuclear reaction
rate evaluations for the mass ranges A=1-20 and A=20-40. For the theoretical
astrophysicist, our results indicate the extent to which nova nucleosynthesis
calculations depend on presently uncertain nuclear physics input, while for the
experimental nuclear physicist our results represent at least a qualitative
guide for future measurements at stable and radioactive ion beam facilities. We
find that present reaction rate estimates are reliable for predictions of Li,
Be, C and N abundances in nova nucleosynthesis. However, rate uncertainties of
several reactions have to be reduced significantly in order to predict more
reliable O, F, Ne, Na, Mg, Al, Si, S, Cl and Ar abundances. Results are
presented in tabular form for each adopted nova simulation.Comment: Accepted for publication in The Astrophysical Journal Suppl. Serie
The Effects of Thermonuclear Reaction-Rate Variations on 26Al Production in Massive Stars: a Sensitivity Study
We investigate the effects of thermonuclear reaction rate variations on 26Al
production in massive stars. The dominant production sites in such events were
recently investigated by using stellar model calculations: explosive
neon-carbon burning, convective shell carbon burning, and convective core
hydrogen burning. Post-processing nucleosynthesis calculations are performed
for each of these sites by adopting temperature-density-time profiles from
recent stellar evolution models. For each profile, we individually multiplied
the rates of all relevant reactions by factors of 10, 2, 0.5 and 0.1, and
analyzed the resulting abundance changes of 26Al. Our simulations are based on
a next-generation nuclear physics library, called STARLIB, which contains a
recent evaluation of Monte Carlo reaction rates. Particular attention is paid
to quantifying the rate uncertainties of those reactions that most sensitively
influence 26Al production. For stellar modelers our results indicate to what
degree predictions of 26Al nucleosynthesis depend on currently uncertain
nuclear physics input, while for nuclear experimentalists our results represent
a guide for future measurements. We tabulate the results of our reaction rate
sensitivity study for each of the three distinct massive star sites referred to
above. It is found that several current reaction rate uncertainties influence
the production of 26Al. Particularly important reactions are 26Al(n,p)26Mg,
25Mg(alpha,n)28Si, 24Mg(n,gamma)25Mg and 23Na(alpha,p)26Mg. These reactions
should be prime targets for future measurements. Overall, we estimate that the
nuclear physics uncertainty of the 26Al yield predicted by the massive star
models explored here amounts to about a factor of 3.Comment: 44 pages, 16 figure
White paper on nuclear astrophysics and low energy nuclear physics Part 1: Nuclear astrophysics
This white paper informs the nuclear astrophysics community and funding agencies about the scientific directions and priorities of the field and provides input from this community for the 2015 Nuclear Science Long Range Plan. It summarizes the outcome of the nuclear astrophysics town meeting that was held on August 21–23, 2014 in College Station at the campus of Texas A&M University in preparation of the NSAC Nuclear Science Long Range Plan. It also reflects the outcome of an earlier town meeting of the nuclear astrophysics community organized by the Joint Institute for Nuclear Astrophysics (JINA) on October 9–10, 2012 Detroit, Michigan, with the purpose of developing a vision for nuclear astrophysics in light of the recent NRC decadal surveys in nuclear physics (NP2010) and astronomy (ASTRO2010). The white paper is furthermore informed by the town meeting of the Association of Research at University Nuclear Accelerators (ARUNA) that took place at the University of Notre Dame on June 12–13, 2014. In summary we find that nuclear astrophysics is a modern and vibrant field addressing fundamental science questions at the intersection of nuclear physics and astrophysics. These questions relate to the origin of the elements, the nuclear engines that drive life and death of stars, and the properties of dense matter. A broad range of nuclear accelerator facilities, astronomical observatories, theory efforts, and computational capabilities are needed. With the developments outlined in this white paper, answers to long standing key questions are well within reach in the coming decade
Catching Element Formation In The Act
Gamma-ray astronomy explores the most energetic photons in nature to address
some of the most pressing puzzles in contemporary astrophysics. It encompasses
a wide range of objects and phenomena: stars, supernovae, novae, neutron stars,
stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays
and relativistic-particle acceleration, and the evolution of galaxies. MeV
gamma-rays provide a unique probe of nuclear processes in astronomy, directly
measuring radioactive decay, nuclear de-excitation, and positron annihilation.
The substantial information carried by gamma-ray photons allows us to see
deeper into these objects, the bulk of the power is often emitted at gamma-ray
energies, and radioactivity provides a natural physical clock that adds unique
information. New science will be driven by time-domain population studies at
gamma-ray energies. This science is enabled by next-generation gamma-ray
instruments with one to two orders of magnitude better sensitivity, larger sky
coverage, and faster cadence than all previous gamma-ray instruments. This
transformative capability permits: (a) the accurate identification of the
gamma-ray emitting objects and correlations with observations taken at other
wavelengths and with other messengers; (b) construction of new gamma-ray maps
of the Milky Way and other nearby galaxies where extended regions are
distinguished from point sources; and (c) considerable serendipitous science of
scarce events -- nearby neutron star mergers, for example. Advances in
technology push the performance of new gamma-ray instruments to address a wide
set of astrophysical questions.Comment: 14 pages including 3 figure
Recommended from our members
Catching Element Formation In The Act
Gamma-ray astronomy explores the most energetic photons in nature to address
some of the most pressing puzzles in contemporary astrophysics. It encompasses
a wide range of objects and phenomena: stars, supernovae, novae, neutron stars,
stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays
and relativistic-particle acceleration, and the evolution of galaxies. MeV
gamma-rays provide a unique probe of nuclear processes in astronomy, directly
measuring radioactive decay, nuclear de-excitation, and positron annihilation.
The substantial information carried by gamma-ray photons allows us to see
deeper into these objects, the bulk of the power is often emitted at gamma-ray
energies, and radioactivity provides a natural physical clock that adds unique
information. New science will be driven by time-domain population studies at
gamma-ray energies. This science is enabled by next-generation gamma-ray
instruments with one to two orders of magnitude better sensitivity, larger sky
coverage, and faster cadence than all previous gamma-ray instruments. This
transformative capability permits: (a) the accurate identification of the
gamma-ray emitting objects and correlations with observations taken at other
wavelengths and with other messengers; (b) construction of new gamma-ray maps
of the Milky Way and other nearby galaxies where extended regions are
distinguished from point sources; and (c) considerable serendipitous science of
scarce events -- nearby neutron star mergers, for example. Advances in
technology push the performance of new gamma-ray instruments to address a wide
set of astrophysical questions