1,297 research outputs found
Nuclear Structure Studies at ISOLDE and their Impact on the Astrophysical r-Process
The focus of the present review is the production of the heaviest elements in
nature via the r-process. A correct understanding and modeling requires the
knowledge of nuclear properties far from stability and a detailed prescription
of the astrophysical environment. Experiments at CERN/ISOLDE have played a
pioneering role in exploring the characteristics of nuclear structure in terms
of masses and beta-decay properties. Initial examinations paid attention to far
unstable nuclei with magic neutron numbers related to r-process peaks, while
present activities are centered on the evolution of shell effects with the
distance from the valley of stability. We first show in site-independent
applications the effect of both types of nuclear properties on r-process
abundances. Then, we explore the results of calculations related to two
different `realistic' astrophysical sites, (i) the supernova neutrino wind and
(ii) neutron star mergers. We close with a list of remaining theoretical and
experimental challenges needed to overcome for a full understanding of the
nature of the r-process, and the role CERN/ISOLDE can play in this process.Comment: LATEX, 38 pages, 16 figures, submitted to Hyperfine Interaction
Nucleosynthesis Basics and Applications to Supernovae
This review concentrates on nucleosynthesis processes in general and their
applications to massive stars and supernovae. A brief initial introduction is
given to the physics in astrophysical plasmas which governs composition
changes. We present the basic equations for thermonuclear reaction rates and
nuclear reaction networks. The required nuclear physics input for reaction
rates is discussed, i.e. cross sections for nuclear reactions,
photodisintegrations, electron and positron captures, neutrino captures,
inelastic neutrino scattering, and beta-decay half-lives. We examine especially
the present state of uncertainties in predicting thermonuclear reaction rates,
while the status of experiments is discussed by others in this volume (see M.
Wiescher). It follows a brief review of hydrostatic burning stages in stellar
evolution before discussing the fate of massive stars, i.e. the nucleosynthesis
in type II supernova explosions (SNe II). Except for SNe Ia, which are
explained by exploding white dwarfs in binary stellar systems (which will not
be discussed here), all other supernova types seem to be linked to the
gravitational collapse of massive stars (M8M) at the end of their
hydrostatic evolution. SN1987A, the first type II supernova for which the
progenitor star was known, is used as an example for nucleosynthesis
calculations. Finally, we discuss the production of heavy elements in the
r-process up to Th and U and its possible connection to supernovae.Comment: 52 pages, 20 figures, uses cupconf.sty (included); to appear in
"Nuclear and Particle Astrophysics", eds. J. Hirsch., D. Page, Cambridge
University Pres
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
Correlations of r-process elements in very metal-poor stars as clues to their nucleosynthesis sites
Aims. Various nucleosynthesis studies have pointed out that the r-process elements in very metal-poor (VMP) halo stars might have different origins. By means of familiar concepts from statistics (correlations, cluster analysis, and rank tests of elemental abundances), we look for causally correlated elemental abundance patterns and attempt to link them to astrophysical events. Some of these events produce the r-process elements jointly with iron, while others do not have any significant iron contribution. We try to (a) characterize these different types of events by their abundance patterns and (b) identify them among the existing set of suggested r-process sites.
Methods. The Pearson and Spearman correlation coefficients were used in order to investigate correlations among r-process elements (X,Y) as well as their relation to iron (Fe) in VMP halo stars. We gradually tracked the evolution of those coefficients in terms of the element enrichments [X/Fe] or [X/Y] and the metallicity [Fe/H]. This approach, aided by cluster analysis to find different structures of abundance patterns and rank tests to identify whether several events contributed to the observed pattern, is new and provides deeper insights into the abundances of VMP stars.
Results. In the early stage of our Galaxy, at least three r-process nucleosynthesis sites have been active. The first two produce and eject iron and the majority of the lighter r-process elements. We assign them to two different types of core-collapse events, not identical to regular core-collapse supernovae (CCSNe), which produce only light trans-Fe elements. The third category is characterized by a strong r-process and is responsible for the major fraction of the heavy main r-process elements without a significant coproduction of Fe. It does not appear to be connected to CCSNe, in fact most of the Fe found in the related r-process enriched stars must come from previously occurring CCSNe. The existence of actinide boost stars indicates a further division among strong r-process sites. We assign these two strong r-process sites to neutron star mergers without fast black hole formation and to events where the ejecta are dominated by black hole accretion disk outflows. Indications from the lowest-metallicity stars hint at a connection with massive single stars (collapsars) forming black holes in the early Galaxy
Nucleosynthesis Modes in the High-Entropy-Wind of Type II Supernovae: Comparison of Calculations with Halo-Star Observations
While the high-entropy wind (HEW) of Type II supernovae remains one of the
more promising sites for the rapid neutron-capture (r-) process, hydrodynamic
simulations have yet to reproduce the astrophysical conditions under which the
latter occurs. We have performed large-scale network calculations within an
extended parameter range of the HEW, seeking to identify or to constrain the
necessary conditions for a full reproduction of all r-process residuals
N_{r,\odot}=N_{\odot}-N_{s,\odot} by comparing the results with recent
astronomical observations. A superposition of weighted entropy trajectories
results in an excellent reproduction of the overall N_{r,\odot}-pattern beyond
Sn. For the lighter elements, from the Fe-group via Sr-Y-Zr to Ag, our HEW
calculations indicate a transition from the need for clearly different sources
(conditions/sites) to a possible co-production with r-process elements,
provided that a range of entropies are contributing. This explains recent
halo-star observations of a clear non-correlation of Zn and Ge and a weak
correlation of Sr - Zr with heavier r-process elements. Moreover, new
observational data on Ru and Pd seem to confirm also a partial correlation with
Sr as well as the main r-process elements (e.g. Eu).Comment: 15 pages, 1 table, 4 figures; To be published in the Astrophysical
Journal Letter
Direct Neutron Capture for Magic-Shell Nuclei
In neutron capture for magic--shell nuclei the direct reaction mechanism can
be important and may even dominate. As an example we investigated the reaction
Ca(n,Ca for projectile energies below 250\,keV in a direct
capture model using the folding procedure for optical and bound state
potentials. The obtained theoretical cross sections are in agreement with the
experimental data showing the dominance of the direct reaction mechanism in
this case. The above method was also used to calculate the cross section for
Ca(n,Ca.Comment: REVTeX, 7 pages plus 3 uuencoded figures, the complete uuencoded
postscript file is available at ftp://is1.kph.tuwien.ac.at/pub/ohu/calcium.u
On three topical aspects of the N=28 isotonic chain
The evolution of single-particle orbits along the N=28 isotonic chain is
studied within the framework of a relativistic mean-field approximation. We
focus on three topical aspects of the N=28 chain: (a) the emergence of a new
magic number at Z=14; (b) the possible erosion of the N=28 shell; and (c) the
weakening of the spin-orbit splitting among low-j neutron orbits. The present
model supports the emergence of a robust Z=14 subshell gap in 48Ca, that
persists as one reaches the neutron-rich isotone 42Si. Yet the proton removal
from 48Ca results in a significant erosion of the N=28 shell in 42Si. Finally,
the removal of s1/2 protons from 48Ca causes a ~50% reduction of the spin-orbit
splitting among neutron p-orbitals in 42Si.Comment: 12 pages with 5 color figure
Beta-decay half-lives and beta-delayed neutron emission probabilities of nuclei in the region below A=110, relevant for the r-process
Measurements of the beta-decay properties of r-process nuclei below A=110
have been completed at the National Superconducting Cyclotron Laboratory, at
Michigan State University. Beta-decay half-lives for Y-105, Zr-106,107 and
Mo-111, along with beta-delayed neutron emission probabilities of Y-104,
Mo-109,110 and upper limits for Y-105, Zr-103,104,105,106,107 and Mo-108,111
have been measured for the first time. Studies on the basis of the quasi-random
phase approximation are used to analyze the ground-state deformation of these
nuclei.Comment: 21 pages, 10 figures, article accepted for publication in Physical
Review
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