27,470 research outputs found
Development of a hydrogen-oxygen internal combustion engine space power system
Hydrogen-oxygen internal combustion engine space power syste
Black Holes and Galactic Density Cusps Spherically Symmetric Anisotropic Cusps
Aims: In this paper we study density cusps that may contain central black
holes. The actual co-eval self-similar growth would not distinguish between the
central object and the surroundings. Methods: To study the environment of a
growing black hole we seek descriptions of steady `cusps' that may contain a
black hole and that retain at least a memory of self-similarity. We refer to
the environment in brief as the `bulge' and on smaller scales, the `halo'.
Results: We find simple descriptions of the simulations of collisionless matter
by comparing predicted densities, velocity dispersions and distribution
functions with the simulations. In some cases central point masses may be
included by iteration. We emphasize that the co-eval self-similar growth allows
an explanation of the black hole bulge mass correlation between approximately
similar collisionless systems. Conclusions: We have derived our results from
first principles assuming adiabatic self-similarity and either self-similar
virialisation or normal steady virialisation. We conclude that distribution
functions that retain a memory of self-similar evolution provide an
understanding of collisionless systems. The implied energy relaxation of the
collisionless matter is due to the time dependence. Phase mixing relaxation may
be enhanced by clump-clump interactions.Comment: 9 pp, 3 figs, accepted by A\&
Gypsy moths and American dog ticks: Space partners
An experiment intended for the space shuttle and designed to investigate the effects of weightlessness and total darkness on gypsy moth eggs and engorged American dog ticks is described. The objectives are: (1) to reevaluate the effects of zero gravity on the termination of diapause/hibernation of embryonated gypsy moth eggs, (2) to determine the effect of zero gravity on the ovipositions and subsequent hatch from engorged female American dog ticks that have been induced to diapause in the laboratory, and (3) to determine whether morphological or biochemical changes occur in the insects under examination. Results will be compared with those from a similar experiment conducted on Skylab 4
Parametric down-conversion from a wave-equations approach: geometry and absolute brightness
Using the approach of coupled wave equations, we consider spontaneous
parametric down-conversion (SPDC) in the narrow-band regime and its
relationship to classical nonlinear processes such as sum-frequency generation.
We find simple expressions in terms of mode overlap integrals for the absolute
pair production rate into single spatial modes, and simple relationships
between the efficiencies of the classical and quantum processes. The results,
obtained with Green function techniques, are not specific to any geometry or
nonlinear crystal. The theory is applied to both degenerate and non-degenerate
SPDC. We also find a time-domain expression for the correlation function
between filtered signal and idler fields.Comment: 10 pages, no figure
Testing equality of variances in the analysis of repeated measurements
The problem of comparing the precisions of two instruments using repeated measurements can be cast as an extension of the Pitman-Morgan problem of testing equality of variances of a bivariate normal distribution. Hawkins (1981) decomposes the hypothesis of equal variances in this model into two subhypotheses for which simple tests exist. For the overall hypothesis he proposes to combine the tests of the subhypotheses using Fisher's method and empirically compares the component tests and their combination with the likelihood ratio test. In this paper an attempt is made to resolve some discrepancies and puzzling conclusions in Hawkins's study and to propose simple modifications.\ud
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The new tests are compared to the tests discussed by Hawkins and to each other both in terms of the finite sample power (estimated by Monte Carlo simulation) and theoretically in terms of asymptotic relative efficiencies
The spectrum of hot methane in astronomical objects using a comprehensive computed line list
Hot methane spectra are important in environments ranging from flames to the
atmospheres of cool stars and exoplanets. A new spectroscopic line list,
10to10, for CH containing almost 10 billion transitions is
presented. This comprehensive line list covers a broad spectroscopic range and
is applicable for temperatures up to 1 500 K. Previous methane data are
incomplete leading to underestimated opacities at short wavelengths and
elevated temperatures. Use of 10to10 in models of the bright T4.5 brown dwarf
2MASS 0559-14 leads to significantly better agreement with observations and in
studies of the hot Jupiter exoplanet HD 189733b leads to up to a twentifold
increase in methane abundance. It is demonstrated that proper inclusion of the
huge increase in hot transitions which are important at elevated temperatures
is crucial for accurate characterizations of atmospheres of brown dwarfs and
exoplanets, especially when observed in the near-infrared.Comment: PNAS, Early Edition, June 16, 201
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