2,500 research outputs found
Measurements of r-process nuclei
Progress in the astrophysical understanding of r-process nucleosynthesis also
depends on the knowledge of nuclear-physics quantities of extremely
neutron-rich isotopes. In this context, experiments at CERN-ISOLDE have played
a pioneering role in exploring new shell-structure far from stability. Possible
implications of new nuclear-data input on the reproduction of r-abundance
observations are presented.Comment: 10 pages, 3 figures; Proc. "Nuclei in the Cosmos 2000", Nucl. Phys.
Stellar and nuclear-physics constraints on two r-process components in the early Galaxy
Proceedings of "Nuclei in the Cosmos 2000", Aarhus, DanmarkComment: 3 pages, 2 figures; to be publ. in Nucl. Phys.
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
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
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
A tentative 4- isomeric state in Sr-98
Annual Report 2001, Institut fuer Kernchemie,
Johannes-Gutenberg-Universitaet, Mainz, GermanyComment: 3 pages, 1 figur
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
The Ubiquity of the Rapid Neutron-Capture Process
To better characterize the abundance patterns produced by the r-process, we
have derived new abundances or upper limits for the heavy elements zinc (Zn),
yttrium (Y), lanthanum (La), europium (Eu), and lead (Pb). Our sample of 161
metal-poor stars includes new measurements from 88 high resolution and high
signal-to-noise spectra obtained with the Tull Spectrograph on the 2.7m Smith
Telescope at McDonald Observatory, and other abundances are adopted from the
literature. We use models of the s-process in AGB stars to characterize the
high Pb/Eu ratios produced in the s-process at low metallicity, and our new
observations then allow us to identify a sample of stars with no detectable
s-process material. In these stars, we find no significant increase in the
Pb/Eu ratios with increasing metallicity. This suggests that s-process material
was not widely dispersed until the overall Galactic metallicity grew
considerably, perhaps even as high as [Fe/H]=-1.4. We identify a dispersion of
at least 0.5 dex in [La/Eu] in metal-poor stars with [Eu/Fe]<+0.6 attributable
to the r-process, suggesting that there is no unique "pure" r-process elemental
ratio among pairs of rare earth elements. We confirm earlier detections of an
anti-correlation between Y/Eu and Eu/Fe bookended by stars strongly enriched in
the r-process (e.g., CS 22892-052) and those with deficiencies of the heavy
elements (e.g., HD 122563). We can reproduce the range of Y/Eu ratios using
simulations of high-entropy neutrino winds of core-collapse supernovae that
include charged-particle and neutron-capture components of r-process
nucleosynthesis. The heavy element abundance patterns in most metal-poor stars
do not resemble that of CS 22892-052, but the presence of heavy elements such
as Ba in nearly all metal-poor stars without s-process enrichment suggests that
the r-process is a common phenomenon.Comment: Accepted for publication in the Astrophysical Journal. 25 pages, 13
figure
Cure path dependency of mode i fracture toughness in thermoplastic particle interleaf toughened prepreg laminates
AbstractThe effect of cure cycle on fracture behaviour of a commercial thermoplastic particle interleaved prepreg system was investigated. Laminates were manufactured at 700kPa in an autoclave using eight different thermal cycles that included both raising the cure temperature above the standard 180°C cure cycle and incorporating an intermediate dwell stage between 150 and 170°C prior to reaching the 180°C cure temperature. Double cantilever beam tests were conducted on specimens from the cured laminates. The stick–slip crack behaviour, observed in samples manufactured using the standard cure cycle, changed to stable crack growth when processing deviated by 10°C. The mode I fracture toughness values were reduced by 11–22% when incorporating an intermediate dwell stage before the final cure temperature. Scanning electron microscopy inspection of the fracture surfaces showed differences between samples made by standard cure cycles and those made using process deviations
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