22,659 research outputs found
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
A Functional Approach to FBSDEs and Its Application in Optimal Portfolios
In Liang et al (2009), the current authors demonstrated that BSDEs can be
reformulated as functional differential equations, and as an application, they
solved BSDEs on general filtered probability spaces. In this paper the authors
continue the study of functional differential equations and demonstrate how
such approach can be used to solve FBSDEs. By this approach the equations can
be solved in one direction altogether rather than in a forward and backward
way. The solutions of FBSDEs are then employed to construct the weak solutions
to a class of BSDE systems (not necessarily scalar) with quadratic growth, by a
nonlinear version of Girsanov's transformation. As the solving procedure is
constructive, the authors not only obtain the existence and uniqueness theorem,
but also really work out the solutions to such class of BSDE systems with
quadratic growth. Finally an optimal portfolio problem in incomplete markets is
solved based on the functional differential equation approach and the nonlinear
Girsanov's transformation.Comment: 26 page
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
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