3,920 research outputs found
Development of a Space Vehicle Electromagnetic Interference/compatibility Specification. Volume 3 - System Specification
This specification represents a new approach to controlling electromagnetic interference. In this approach the system integration contractor will be responsible for the development of discrete specification limits to be utilized on the program to which this specification is applied. The specification limits imposed upon subsystem contractors will be developed by the use of a computer program, available from the procuring agency, which is designed to consider the total system electromagnetic environment in the computation of these limits. The integration contractor will be required to mathematically model those circuits which represent required emitters and receptors of electromagnetic energy on the space vehicle. There are contained herein, a list of available computer models into which must be inserted particular parameters of the spacecraft subsystems. These functional models, when inserted into the computer program, will develop discrete specification limits based upon the requirements of the particular system modelled
Time-dependent radiative transfer with PHOENIX
Aims. We present first results and tests of a time-dependent extension to the
general purpose model atmosphere code PHOENIX. We aim to produce light curves
and spectra of hydro models for all types of supernovae. Methods. We extend our
model atmosphere code PHOENIX to solve time-dependent non-grey, NLTE, radiative
transfer in a special relativistic framework. A simple hydrodynamics solver was
implemented to keep track of the energy conservation of the atmosphere during
free expansion. Results. The correct operation of the new additions to PHOENIX
were verified in test calculations. Conclusions. We have shown the correct
operation of our extension to time-dependent radiative transfer and will be
able to calculate supernova light curves and spectra in future work.Comment: 7 pages, 12 figure
Neutrino Trapping in a Supernova and Ion Screening
Neutrino-nucleus elastic scattering is reduced in dense matter because of
correlations between ions. The static structure factor for a plasma of
electrons and ions is calculated from Monte Carlo simulations and parameterized
with a least squares fit. Our results imply a large increase in the neutrino
mean free path. This strongly limits the trapping of neutrinos in a supernova
by coherent neutral current interactions.Comment: 9 pages, 1 postscript figure using epsf.st
Neutrino Interactions in Hot and Dense Matter
We study the charged and neutral current weak interaction rates relevant for
the determination of neutrino opacities in dense matter found in supernovae and
neutron stars. We establish an efficient formalism for calculating differential
cross sections and mean free paths for interacting, asymmetric nuclear matter
at arbitrary degeneracy. The formalism is valid for both charged and neutral
current reactions. Strong interaction corrections are incorporated through the
in-medium single particle energies at the relevant density and temperature. The
effects of strong interactions on the weak interaction rates are investigated
using both potential and effective field-theoretical models of matter. We
investigate the relative importance of charged and neutral currents for
different astrophysical situations, and also examine the influence of
strangeness-bearing hyperons. Our findings show that the mean free paths are
significantly altered by the effects of strong interactions and the
multi-component nature of dense matter. The opacities are then discussed in the
context of the evolution of the core of a protoneutron star.Comment: 41 pages, 25 figure
Electron capture on iron group nuclei
We present Gamow-Teller strength distributions from shell model Monte Carlo
studies of fp-shell nuclei that may play an important role in the pre-collapse
evolution of supernovae. We then use these strength distributions to calculate
the electron-capture cross sections and rates in the zero-momentum transfer
limit. We also discuss the thermal behavior of the cross sections. We find
large differences in these cross sections and rates when compared to the naive
single-particle estimates. These differences need to be taken into account for
improved modeling of the early stages of type II supernova evolution
Neutrino-electron scattering in dense magnetized plasma
We derive exact expressions for the cross section of neutrino scattering on
electrons in dense, hot stellar matter, in the presence of strong magnetic
fields. Numerical calculations of the scattering cross sections at various
densities, temperatures and magnetic fields, are performed. Strong, quantizing
magnetic fields modify significantly the angular and energy dependence of the
scattering cross section.Comment: Physical Review D, to be published, 18 pages, using REVTEX, without
figures. Figures (hardcopy) available upon request from one of the authors
([email protected]
Neutrino Spectroscopy of the Early Phase of Nearby Supernovae
Neutrinos emitted during stellar core collapse up to their trapping phase
carry information about the stage from which the Supernova explosion process
initiates. The dominant emission mechanism is by electron capture on
free protons and f-p shell nuclei and the spectrum of these neutrinos is a
function of the ambient physical conditions within the core as well as the
nuclear equation of state. The number of collapse phase which can be
detected by Super-Kamioka and Sudbury Neutrino Observatory from a Supernova
within 1 kpc, and their generic energy spectra are given.Comment: 9 pages of text and tables plus 2 pages of figures. Accepted for
publication in Phys. Rev. Lett. on 11th Jul., 1997. Please e-mail Comments
etc. to [email protected]
Additive Polynomials for Finite Groups of Lie Type
This paper provides a realization of all classical and most exceptional
finite groups of Lie type as Galois groups over function fields over F_q and
derives explicit additive polynomials for the extensions. Our unified approach
is based on results of Matzat which give bounds for Galois groups of Frobenius
modules and uses the structure and representation theory of the corresponding
connected linear algebraic groups.Comment: 59 pages; v2: added reference, slightly restructured section 6.1, few
small rewordings; v3: completed realization of Steinberg's triality groups
(thanks to P. Mueller for solving the remaining open question); clarified
argument how to use Thm. 3.
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