6,845 research outputs found
The new Toulouse-Geneva Stellar Evolution Code including radiative accelerations of heavy elements
Atomic diffusion has been recognized as an important process that has to be
considered in any computations of stellar models. In solar-type and cooler
stars, this process is dominated by gravitational settling, which is now
included in most stellar evolution codes. In hotter stars, radiative
accelerations compete with gravity and become the dominant ingredient in the
diffusion flux for most heavy elements. Introducing radiative accelerations
into the computations of stellar models modifies the internal element
distribution and may have major consequences on the stellar structure. Coupling
these processes with hydrodynamical stellar motions has important consequences
that need to be investigated in detail. We aim to include the computations of
radiative accelerations in a stellar evolution code (here the TGEC code) using
a simplified method (SVP) so that it may be coupled with sophisticated
macroscopic motions. We also compare the results with those of the Montreal
code in specific cases for validation and study the consequences of these
coupled processes on accurate models of A- and early-type stars. We implemented
radiative accelerations computations into the Toulouse-Geneva stellar evolution
code following the semi-analytical prescription proposed by Alecian and
LeBlanc. This allows more rapid computations than the full description used in
the Montreal code. We present results for A-type stellar models computed with
this updated version of TGEC and compare them with similar published models
obtained with the Montreal evolution code. We discuss the consequences for the
coupling with macroscopic motions, including thermohaline convection.Comment: 12 pages, 13 figures, published in A&
A low-delay 8 Kb/s backward-adaptive CELP coder
Code excited linear prediction coding is an efficient technique for compressing speech sequences. Communications quality of speech can be obtained at bit rates below 8 Kb/s. However, relatively large coding delays are necessary to buffer the input speech in order to perform the LPC analysis. A low delay 8 Kb/s CELP coder is introduced in which the short term predictor is based on past synthesized speech. A new distortion measure that improves the tracking of the formant filter is discussed. Formal listening tests showed that the performance of the backward adaptive coder is almost as good as the conventional CELP coder
In vivo nuclear magnetic resonance imaging
A number of physiological changes have been demonstrated in bone, muscle and blood after exposure of humans and animals to microgravity. Determining mechanisms and the development of effective countermeasures for long duration space missions is an important NASA goal. The advent of tomographic nuclear magnetic resonance imaging (NMR or MRI) gives NASA a way to greatly extend early studies of this phenomena in ways not previously possible; NMR is also noninvasive and safe. NMR provides both superb anatomical images for volume assessments of individual organs and quantification of chemical/physical changes induced in the examined tissues. The feasibility of NMR as a tool for human physiological research as it is affected by microgravity is demonstrated. The animal studies employed the rear limb suspended rat as a model of mucle atrophy that results from microgravity. And bedrest of normal male subjects was used to simulate the effects of microgravity on bone and muscle
Signatures of superconducting gap inhomogeneities in optical properties
Scanning tunneling spectroscopy applied to the high- cuprates has
revealed significant spatial inhomogeneity on the nanoscale. Regions on the
order of a coherence length in size show variations of the magnitude of the
superconducting gap of order or more. An important unresolved question
is whether or not these variations are also present in the bulk, and how they
influence superconducting properties. As many theories and data analyses for
high- superconductivity assume spatial homogeneity of the gap magnitude,
this is a pressing question. We consider the far-infrared optical conductivity
and evaluate, within an effective medium approximation, what signatures of
spatial variations in gap magnitude are present in various optical quantities.
In addition to the case of d-wave superconductivity, relevant to the high-
cuprates, we have also considered s-wave gap symmetry in order to provide
expected signatures of inhomogeneities for superconductors in general. While
signatures of gap inhomogeneities can be strongly manifested in s-wave
superconductors, we find that the far-infrared optical conductivity in d-wave
is robust against such inhomogeneity.Comment: 8 pages, 7 figure
Symmetry Breaking in the Schr\"odinger Representation for Chern-Simons Theories
This paper discusses the phenomenon of spontaneous symmetry breaking in the
Schr\"odinger representation formulation of quantum field theory. The analysis
is presented for three-dimensional space-time abelian gauge theories with
either Maxwell, Maxwell-Chern-Simons, or pure Chern-Simons terms as the gauge
field contribution to the action, each of which leads to a different form of
mass generation for the gauge fields.Comment: 16pp, LaTeX , UCONN-94-
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