Symmetry analysis of the 1+1 dimensional relativistic imperfect fluid dynamics

The flow of the relativistic imperfect fluid in two dimensions is discussed. We calculate the symmetry group of the energy-momentum tensor conservation equation in the ultrarelativistic limit. Group-invariant solutions for the incompressible fluid are obtainedComment: 11 pages PS format at http://theor1.ifa.ro/~alexa/iop.p

Comprehensive rate coefficients for electron collision induced transitions in hydrogen

Energy-changing electron-hydrogen atom collisions are crucial to regulating the energy balance in astrophysical and laboratory plasmas and relevant to the formation of stellar atmospheres, recombination in H-II clouds, primordial recombination, three-body recombination and heating in ultracold and fusion plasmas. Computational modeling of electron-hydrogen collision has been attempted through quantum mechanical scattering state-to-state calculations of transitions involving low-lying energy levels in hydrogen (with principal quantum number n < 7) and at large principal quantum numbers using classical trajectory techniques. Analytical expressions are proposed which interpolates the current quantum mechanical and classical trajectory results for electron-hydrogen scattering in the entire range of energy levels, for nearly all temperature range of interest in astrophysical environments. An asymptotic expression for the Born cross-section is interpolated with a modified expression derived previously for electron-hydrogen scattering in the Rydberg regime using classical trajectory Monte Carlo simulations. The derived formula is compared to existing numerical data for transitions involving low principal quantum numbers, and the dependence of the deviations upon temperature is discussed.Comment: To appear in The Astrophysical Journa