24,404 research outputs found
Diverse Supernova Sources for the r-Process and Abundances in Metal-Poor Stars
The dispersion and mean trends of r-process abundances in metal-poor stars
are discussed based on a model of diverse supernova sources for the r-process.
This model is unique in that its key parameters are inferred from solar system
data independent of stellar observations at low metallicities. It is shown that
this model provides a good explanation for the observed dispersion and mean
trend of Eu abundances over -3 < [Fe/H] < -1. It is also shown that this model
provides a means to discuss r-abundances in general. For example, the Ag
abundance in any metal-poor star with observed Eu and Fe abundances can be
calculated from the model. This approach is demonstrated with success for two
stars and can be further tested by future Ag data. The dispersion and mean
trend of Ag abundances in metal-poor stars are also calculated for comparison
with future observations.Comment: 4 pages, 2 figures, to appear in ApJL (submitted February 1, 2001
Neutrino-Induced Fission and r-Process Nucleosynthesis
An r-process scenario with fission but no fission cycling is considered to
account for the observed abundance patterns of neutron-capture elements in
ultra-metal-poor stars. It is proposed that neutrino reactions play a crucial
role in inducing the fission of the progenitor nuclei after the r-process
freezes out in Type II Supernovae. To facilitate neutrino-induced fission, the
proposed r-process scenario is restricted to occur in a low-density environment
such as the neutrino-driven wind from the neutron star. Further studies to
develop this scenario are emphasized.Comment: 11 pages, 2 figures, to appear in ApJ
Hierarchical Structure Formation and Chemical Evolution of Damped Ly alpha Systems
We present a model for chemical evolution of damped Ly alpha systems
considering production of metals by SNe II and infall associated with
hierarchical structure formation. The growth of metallicity in these systems is
a reflection of the competition between astration and infall. The apparent late
turn-on of these systems is due to the late cut-off of infall. The wide range
in [Fe/H] at a given redshift is explained by the range of the times for onset
of star formation and the range of the times for infall cessation in different
systems. The observed lower bound of [Fe/H] = -3 follows from the very rapid
initial rise of [Fe/H] subsequent to onset of star formation. To reach [Fe/H] =
-3 from a metal-free initial state requires only about 30 Myr so that the
probability of observing lower [Fe/H] values is very small.Comment: 4 pages, 2 figures, to appear in ApJ
Subject-specific finite element modelling of the human hand complex : muscle-driven simulations and experimental validation
This paper aims to develop and validate a subject-specific framework for modelling the human hand. This was achieved by combining medical image-based finite element modelling, individualized muscle force and kinematic measurements. Firstly, a subject-specific human hand finite element (FE) model was developed. The geometries of the phalanges, carpal bones, wrist bones, ligaments, tendons, subcutaneous tissue and skin were all included. The material properties were derived from in-vivo and in-vitro experiment results available in the literature. The boundary and loading conditions were defined based on the kinematic data and muscle forces of a specific subject captured from the in-vivo grasping tests. The predicted contact pressure and contact area were in good agreement with the in-vivo test results of the same subject, with the relative errors for the contact pressures all being below 20%. Finally, sensitivity analysis was performed to investigate the effects of important modelling parameters on the predictions. The results showed that contact pressure and area were sensitive to the material properties and muscle forces. This FE human hand model can be used to make a detailed and quantitative evaluation into biomechanical and neurophysiological aspects of human hand contact during daily perception and manipulation. The findings can be applied to the design of the bionic hands or neuro-prosthetics in the future
Probing r-Process Production of Nuclei Beyond Bi209 with Gamma Rays
We estimate gamma-ray fluxes due to the decay of nuclei beyond Bi209 from a
supernova or a supernova remnant assuming that the r-process occurs in
supernovae. We find that a detector with a sensitivity of about 10**(-7)
photons/cm**2/s at energies of 40 keV to 3 MeV may detect fluxes due to the
decay of Ra226, Th229, Am241, Am243, Cf249, and Cf251 in the newly discovered
supernova remnant near Vela. In addition, such a detector may detect fluxes due
to the decay of Ac227 and Ra228 produced in a future supernova at a distance of
about 1 kpc. As nuclei with mass numbers A > 209 are produced solely by the
r-process, such detections are the best proof for a supernova r-process site.
Further, they provide the most direct information on yields of progenitor
nuclei with A > 209 at r-process freeze-out. Finally, detection of fluxes due
to the decay of r-process nuclei over a range of masses from a supernova or a
supernova remnant provides the opportunity to compare yields in a single
supernova event with the solar r-process abundance pattern.Comment: 24 pages, 3 figures, to appear in the October 10, 1999 issue of Ap
Abundances in the Uranium-Rich Star CS 31082-001
The recent discovery by Cayrel et al. of U in CS 31082-001 along with Os and
Ir at greatly enhanced abundances but with [Fe/H]=-2.9 strongly reinforces the
argument that there are at least two kinds of SNII sources for r-nuclei. One
source is the high-frequency H events responsible for heavy r-nuclei (A>135)
but not Fe. The H-yields calculated from data on other ultra-metal-poor stars
and the sun provide a template for quantitatively predicting the abundances of
all other r-elements. In CS 31082-001 these should show a significant
deficiency at A<135 relative to the solar r-pattern. It is proposed that CS
31082-001 should have had a companion that exploded as an SNII H event. If the
binary survived the explosion, this star should now have a compact companion,
most likely a stellar-mass black hole. Comparison of abundance data with
predicted values and a search for a compact companion should provide a
stringent test of the proposed r-process model. The U-Th age determined by
Cayrel et al. for CS 31082-001 is, to within substantial uncertainties, in
accord with the r-process age determined from solar system data. The time gap
between Big Bang and onset of normal star formation only allows r-process
chronometers to provide a lower limit on the age of the universe.Comment: 5 pages, 1 figur
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