14,057 research outputs found
Synsedimentary deformation and thrusting on the eatern margin of the Barberton Greenstone belt, Swaziland
Mapping on the eastern margin of the 3.6 to 3.3 Ga Barberton Greenstone Belt NW Swaziland has revealed a tectonic complex which is more than 5 km thick. The area consists of fault bound units made up of three lithological associations. Some of these were affected by four phases of deformation. Fold structures, foliations, and lineations are associated with the deformation. Sedimentation, geological structures, and evolutionary history of the area are explained
Hypersonic reentry vehicle Patent
Aerodynamic configuration of reentry vehicle heat shield to provide longitudinal and directional stability at hypersonic velocitie
Von K\'arm\'an vortex street in a Bose-Einstein condensate
Vortex shedding from an obstacle potential moving in a Bose-Einstein
condensate is investigated. Long-lived alternately aligned vortex pairs are
found to form in the wake, as for the von K\'arm\'an vortex street in classical
viscous fluids. Various patterns of vortex shedding are systematically studied
and the drag force on the obstacle is calculated. It is shown that the
phenomenon can be observed in a trapped system.Comment: 4 pages, 5 figure
Two-dimensional Stokes flow driven by elliptical paddles
A fast and accurate numerical technique is developed for solving the biharmonic equation in a multiply connected domain, in two dimensions. We apply the technique to the computation of slow viscous flow (Stokes flow) driven by multiple stirring rods. Previously, the technique has been
restricted to stirring rods of circular cross section; we show here how the prior method fails for noncircular rods and how it may be adapted to accommodate general rod cross sections, provided only that for each there exists a conformal mapping to a circle. Corresponding simulations of the flow are described, and their stirring properties and energy requirements are discussed briefly. In particular the method allows an accurate calculation of the flow when flat paddles are used to stir a fluid chaotically
Evidence Against an Association Between Gamma-Ray Bursts and Type I Supernovae
We present a rigorous method, based on Bayesian inference, for calculating
the odds favoring the hypothesis that any particular class of astronomical
transients produce gamma-ray bursts over the hypothesis that they do not. We
then apply this method to a sample of 83 Type Ia supernovae and a sample of 20
Type Ib-Ic supernovae. We find overwhelming odds against the hypothesis that
all Type Ia supernovae produce gamma-ray bursts, whether at low redshift
() or high-redshift (), and very large odds ()
against the hypothesis that all Type Ib, Ib/c, and Ic supernovae produce
observable gamma-ray bursts. We find large odds () against the hypothesis
that a fraction of Type Ia supernovae produce observable gamma-ray bursts, and
moderate odds () against the hypothesis that a fraction of Type Ib-Ic
supernovae produce observable bursts. We have also re-analyzed both a corrected
version of the Wang & Wheeler sample of Type Ib-Ic SNe and our larger sample of
20 Type Ib-Ic SNe, using a generalization of their frequentist method. We find
no significant evidence in either case of a correlation between Type Ib-Ic SNe
and GRBs, consistent with the very strong evidence against such a correlation
that we find from our Bayesian analysis.Comment: 45 pages, 2 PostScript figures. Uses AASTEX macros. Submitted to The
Astrophysical Journa
The Poisson geometry of SU(1,1)
We study the natural Poisson structure on the Lie group SU(1,1) and related
questions. In particular, we give an explicit description of the
Ginzburg-Weinstein isomorphism for the sets of admissible elements. We also
establish an analogue of Thompson's conjecture for this group.Comment: 11 pages, minor correction
Bounds on the Compactness of Neutron Stars from Brightness Oscillations
The discovery of high-amplitude brightness oscillations at the spin frequency
or its first overtone in six neutron stars in low-mass X-ray binaries during
type~1 X-ray bursts provides a powerful new way to constrain the compactness of
these stars, and hence to constrain the equation of state of the dense matter
in all neutron stars. Here we present the results of general relativistic
calculations of the maximum fractional rms amplitudes that can be observed
during bursts. In particular, we determine the dependence of the amplitude on
the compactness of the star, the angular dependence of the emission from the
surface, the rotational velocity at the stellar surface, and whether there are
one or two emitting poles. We show that if two poles are emitting, as is
strongly indicated by independent evidence in 4U 1636-536 and KS 1731-26, the
resulting limits on the compactness of the star can be extremely restrictive.
We also discuss the expected amplitudes of X-ray color oscillations and the
observational signatures necessary to derive convincing constraints on neutron
star compactness from the amplitudes of burst oscillations.Comment: 8 pages plus one figure, AASTeX v. 4.0, submitted to The
Astrophysical Journal Letter
Coherent vibrations of submicron spherical gold shells in a photonic crystal
Coherent acoustic radial oscillations of thin spherical gold shells of
submicron diameter excited by an ultrashort optical pulse are observed in the
form of pronounced modulations of the transient reflectivity on a subnanosecond
time scale. Strong acousto-optical coupling in a photonic crystal enhances the
modulation of the transient reflectivity up to 4%. The frequency of these
oscillations is demonstrated to be in good agreement with Lamb theory of free
gold shells.Comment: Error in Eqs.2 and 3 corrected; Tabl. I corrected; Fig.1 revised; a
model that explains the dependence of the oscillation amplitude of the
transient reflectivity with wavelength adde
- …