2,349 research outputs found
The Influence of Nuclear Composition on the Electron Fraction in the Post-Core-Bounce Supernova Environment
We study the early evolution of the electron fraction (or, alternatively, the
neutron-to-proton ratio) in the region above the hot proto-neutron star formed
after a supernova explosion. We study the way in which the electron fraction in
this environment is set by a competition between lepton (electron, positron,
neutrino, and antineutrino) capture processes on free neutrons and protons and
nuclei. Our calculations take explicit account of the effect of nuclear
composition changes, such as formation of alpha particles (the alpha effect)
and the shifting of nuclear abundances in nuclear statistical equilibrium
associated with cooling in near-adiabatic outflow. We take detailed account of
the process of weak interaction freeze-out in conjunction with these nuclear
composition changes. Our detailed treatment shows that the alpha effect can
cause significant increases in the electron fraction, while neutrino and
antineutrino capture on heavy nuclei tends to have a buffering effect on this
quantity. We also examine the effect on weak rates and the electron fraction of
fluctuations in time in the neutrino and antineutrino energy spectra arising
from hydrodynamic waves. Our analysis is guided by the Mayle & Wilson supernova
code numerical results for the neutrino energy spectra and density and velocity
profiles.Comment: 38 pages, AAS LaTeX, 8 figure
A single case study of articulatory adaptation during acoustic mimicry
The distribution of fine-grained phonetic variation
can be observed in the speech of members of welldefined
social groups. It is evident that such
variation must somehow be able to propagate
through a speech community from speaker to
hearer. However, technological barriers have
meant that close and direct study of the articulatory
links of this speaker-hearer chain has not, to date,
been possible. We present the results of a singlecase
study using an ultrasound-based method to
investigate temporal and configurational lingual
adaptation during mimicry. Our study focuses on
allophonic variants of postvocalic /r/ found in
speech from Central Scotland. Our results show
that our informant was able to adjust tongue
gesture timing towards that of the stimulus, but did
not alter tongue configuration
Atoms in boxes: from confined atoms to electron-atom scattering
We show that both confined atoms and electron-atom scattering can be
described by a unified basis set method. The central idea behind this method is
to place the atom inside a hard potential sphere, enforced by a standard Slater
type basis set multiplied by a cutoff factor. For confined atoms, where the
wall is placed close to the atomic nucleus, we show how the energy of the
highest occupied atomic orbital and the static polarizability of helium and
neon atoms evolve with the confinement radius. To our knowledge, these are the
first confined atom polarizability calculations that include correlation,
through the use of time-dependent density-functional theory. By placing the
atom in a large spherical box, with a wall outside the electron density, we
obtain scattering phase shifts using a recently developed method [M. van
Faassen, A. Wasserman, E. Engel, F. Zhang, and K. Burke, Phys. Rev. Lett. {\bf
99}, 043005 (2007)]. We show that the basis set method gives identical results
to previously obtained phase shifts for -H and -He scattering.Comment: 8 pages, 6 figures, submitted to Journal of Chemical Physic
Sharpening the predictions of big-bang nucleosynthesis
Motivated by the recent measurement of the primeval abundance of deuterium,
we re-examine the nuclear inputs to big-bang nucleosynthesis (BBN). Using
Monte-Carlo realization of the nuclear cross-section data to directly estimate
the theoretical uncertainties for the yields of D, 3-He and 7-Li, we show that
previous estimates were a factor of 2 too large. We sharpen the BBN
determination of the baryon density based upon deuterium, rho_B = (3.6 +/- 0.4)
* 10^{-31} g/cm^3 (Omega_B h^2 = 0.019 +/- 0.0024), which leads to a predicted
4-He abundance, Y_P = 0.246 +/- 0.0014 and a stringent limit to the equivalent
number of light neutrino species: N_nu < 3.20 (all at 95% cl). The predicted
7-Li abundance, 7-Li/H = (3.5 + 1.1 - 0.9) * 10^{-10}, is higher than that
observed in pop II stars, (1.7 +/- 0.3) * 10^{-10} (both, 95% cl). We identify
key reactions and the energies where further work is needed.Comment: 5 pages, 4 figures (epsfig), REVTeX; submitted to Phys. Rev. Let
Building oriented surface generation through IGES specification
This thesis focuses the use of Initial Graphics Exchange Specification to generate automatically three dimensional surfaces. Currently, different CAD systems utilize their own data formats to generate graphic databases which are not compatible with one another. Besides, most of the existing CAD systems require an interactive data input process which is very much time consuming. In order to solve above problems, an efficient algorithm has been developed in this research to automatically generate portable graphic database following ICES specifications. This portable graphic database can be adopted by different commercial CAD/CAM systems. Using the proposed method, the current design process can be upgraded from the interactive data input method to automatic modelling method that accelerates design process in surface generation significantly. The main objectives of this research are: (1) curtail the surface generation time, (2) reduce the memory space for data storage and communication. A user-friendly program has been written in FORTRAN 77 to apply the proposed algorithm. A case study illustrates the effectiveness of the proposed method
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