Plasmoid impacts on neutron stars and highest energy cosmic rays

Particle acceleration by electrostatic polarization fields that arise in plasmas streaming across magnetic fields is discussed as a possible acceleration mechanism of highest-energy cosmic rays. Specifically, plasmoids arising in planetoid impacts onto neutron star magnetospheres are considered. We find that such impacts at plausible rates may account for the observed flux and energy spectrum of the highest energy cosmic rays.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Lett., uses REVTE

Formation of standing shocks in stellar winds and related astrophysical flows

Stellar winds and other analogous astrophysical flows can be described, to lowest order, by the familiar one dimensional hydrodynamic equations which, being nonlinear, admit in some instances discontinuous as well as continuous transonic solutions for identical inner boundary conditions. The characteristics of the time dependent differential equations of motion are described to show how a perturbation changes profile in time and, under well defined conditions, develops into a stationary shock discontinuity. The formation of standing shocks in wind type astrophysical flows depends on the fulfillment of appropriate necessary conditions, which are determined by the conservation of mass, momentum and energy across the discontinuity, and certain sufficient conditions, which are determined by the flow's history

The Effect of Ru substitution for Ni on the superconductivity in MgCNi3-xRux

The superconductor MgCNi3 has been chemically doped by partial substitution of Ru for Ni in the solid solution MgCNi3-xRux for 0<x<0.5. Magnetic and specific heat measurements show that the Sommerfeld parameter (gamma_exp) and TC decrease immediately on Ru substitution, but that a TC above 2K is maintained even for a relatively large decrease in gamma_exp. Ferromagnetism is not observed to develop through Ru substitution, and the normal state magnetic susceptibility is suppressed.Comment: 18 pages, 13 figure

On the miscible Rayleigh-Taylor instability: two and three dimensions

We investigate the miscible Rayleigh-Taylor (RT) instability in both 2 and 3 dimensions using direct numerical simulations, where the working fluid is assumed incompressible under the Boussinesq approximation. We first consider the case of randomly perturbed interfaces. With a variety of diagnostics, we develop a physical picture for the detailed temporal development of the mixed layer: We identify three distinct evolutionary phases in the development of the mixed layer, which can be related to detailed variations in the growth of the mixing zone. Our analysis provides an explanation for the observed differences between two and three-dimensional RT instability; the analysis also leads us to concentrate on the RT models which (1) work equally well for both laminar and turbulent flows, and (2) do not depend on turbulent scaling within the mixing layer between fluids. These candidate RT models are based on point sources within bubbles (or plumes) and interaction with each other (or the background flow). With this motivation, we examine the evolution of single plumes, and relate our numerical results (of single plumes) to a simple analytical model for plume evolution.Comment: 31 pages, 27 figures, to appear in November issue of JFM, 2001. For better figures: http://astro.uchicago.edu/~young/ps/jfmtry08.ps.

Generation of flux tube waves in stellar convection zones. 1: Longitudinal tube waves

The source functions and the energy fluxes are derived for wave generation in magnetic flux tubes embedded in an otherwise magnetic- field free, turbulent, and compressible fluid. Specific results for the generation of longitudinal tube waves are presented