Distribution of Gamma-Ray Bursts in Halo Neutron Star-Comet Models

The motions of comets and neutron stars have been integrated over five billion years in the Galactic potential to determine a gamma-ray burst distribution, presuming that bursts are the result of interactions between these two families of objects. The comets originate in two distinct populations - one from ejection by stars in the Galactic disk, and the other from ejection by stars in globular clusters. No choice of the free parameters resulted in agreement with both the isotropy data and the $\log(N>F) - \log(F)$ data.Comment: 4 pages LaTex and two style files, tarred, compressed, and uuencoded. One postscript figure. To appear in Astrophysics and Space Science as part of the proceedings of the 29th ESLAB Symposium 'Toward the Source of Gamma-Ray Bursts' held in Noordwijk, 1995. A postscript version can be found at http://astro.queensu.ca/~mark/preprints.htm

Plastic Motion of a Flux-line Lattice Driven by Alternating Current

We have measured the response of the flux-line lattice in the low T$_c$ superconductor, 2H-NbSe$_2$, to finite frequency drives. In a well-defined range of fields, temperatures, and driving amplitudes the system exhibits a variety of novel non-linear phenomena. Most strikingly, the flux-lines can move easily in response to currents that are significantly lower than the DC critical current if the direction of the current is reversed periodically while the amplitude, I, is kept fixed, but they do not respond to a current that periodically switches between zero and I, while the direction is kept fixed. Pronounced memory effects associated with these phenomena indicate the presence of dynamically generated structural changes in the flux lattice.Comment: 5 pages, 4 figures

Galvanic coupling between D6AC steel, 6061-T6 aluminum, Inconel 718 and graphite-epoxy composite material: Corrosion occurrence and prevention

The effects of galvanic coupling between D6AC steel, 6061-T6 aluminum, Inconel 718, and graphite-epoxy composite material (G/E) in 3.5% NaCl were studied. Measurements of corrosion potentials, galvanic currents and corrosion rates of the bare metals using weight-loss methods served to establish the need for corrosion protection in cases where D6AC steel and 6061-T6 aluminum are galvanically coupled to G/E in salt water while Inconel 718 was shown to be compatible with G/E. Six tests were made to study corrosion protective methods for eliminating galvanic corrosion in the cases of D6AC steel and 6061-T6 aluminum coupled to G/E. These results indicate that, when the G/E is completely coated with paint or a paint/polyurethane resin combination, satisfactory protection of the D6AC steel is achieved with either a coat of zinc-rich primer or a primer/topcoat combination. Likewise, satisfactory corrosion protection of the aluminum is achieved by coating it with an epoxy coating system

Discrimination and Aggregative Patterns among and between populations of Entamoeba spp.

Kin recognition facilitates the evolution of cooperation in animals, but its relevance in microorganisms and their behavior toward relatives remains unclear. The Entamoeba lineage constitutes an ideal model to determine the behavioral and signaling cues needed for aggregative preference. Chemical cues have been reported for E. histolytica but not E. dispar, suggesting cell-communication between con-specifics and behavioral differences with relevance for disease. Entamoeba varieties have been reported as ‘morphologically undistinguishable’. Two strains of E. invadens have been isolated from different hosts (VK-1: NS - Varanus komodoensis and IP1- serpentes) but were classified within the same ‘species’. Trophozoites of each strain aggregate only with members of their own variety, suggesting they are able to associate based on behavioral and chemical communication. Adaptations to different environments and horizontal gene exchange could have influenced diversification of each lineage. Measurable aggregation and behavioral cues in fluorescence micrographs of Entamoeba varieties suggest that these characteristics should be included in phylogenetic studies

Wave packet dynamics of potassium dimers attached to helium nanodroplets

The dynamics of vibrational wave packets excited in K$_2$ dimers attached to superfluid helium nanodroplets is investigated by means of femtosecond pump-probe spectroscopy. The employed resonant three-photon-ionization scheme is studied in a wide wavelength range and different pathways leading to K$^+_2$-formation are identified. While the wave packet dynamics of the electronic ground state is not influenced by the helium environment, perturbations of the electronically excited states are observed. The latter reveal a strong time dependence on the timescale 3-8 ps which directly reflects the dynamics of desorption of K$_2$ off the helium droplets

An electrochemical study of the corrosion behavior of primer coated 2219-T87 aluminum

The corrosion behavior for 2219-T87 aluminum coated with various primers, including those used for the external tank and solid rocket boosters of the Space Shuttle Transportation System, were investigated using electrochemical techniques. Corrosion potential time, polarization resistance time, electrical resistance time, and corrosion rate time measurements were all investigated. It was found that electrical resistance time and corrosion rate time measurement were most useful for studying the corrosion behavior of painted aluminum. Electrical resistance time determination give useful information concerning the porosity of paint films, while corrosion rate time curves give important information concerning overall corrosion rates and corrosion mechanisms. In general, the corrosion rate time curves all exhibited at least one peak during the 30 day test period, which was attributed, according to the proposed mechanisms, to the onset of the hydrogen evolution reaction and the beginning of destruction of the protective properties of the paint film

Stochastic geometry and topology of non-Gaussian fields

Gaussian random fields pervade all areas of science. However, it is often the departures from Gaussianity that carry the crucial signature of the nonlinear mechanisms at the heart of diverse phenomena, ranging from structure formation in condensed matter and cosmology to biomedical imaging. The standard test of non-Gaussianity is to measure higher order correlation functions. In the present work, we take a different route. We show how geometric and topological properties of Gaussian fields, such as the statistics of extrema, are modified by the presence of a non-Gaussian perturbation. The resulting discrepancies give an independent way to detect and quantify non-Gaussianities. In our treatment, we consider both local and nonlocal mechanisms that generate non-Gaussian fields, both statically and dynamically through nonlinear diffusion.Comment: 8 pages, 4 figure