James H. Hunter

Multidimensional similarity models for exploding foils

A general derivation of the Gaussian, isothermal similarity equations for multidimensional hydrodynamic expansions is presented. Analytical solutions for the cases of constant heating and adiabatic flow for planar, cylindrical, and spherical expansions are given. An energy integral is derived for general multidimensional adiabatic flows. For two‐dimensional adiabatic expansions, a second invariant of the motion is demonstrated. It is shown that this invariant gives the asymptotic shape of a Gaussian object in terms of its initial shape. A simple model for laser heating of an exploding foil is described and the asymptotic form of the corresponding solutions are given for two and three dimensions. The similarity solutions are compared with two‐dimensional numerical hydrodynamic simulations for an example of an exploding foil, designed to produce a recombination x‐ray laser.

Model Gas Flows in Selected Barred spiral Galaxies

Method for deriving the potentials of various stellar components of barred spiral galaxies are applied here to model gas flows in such galaxies. The results show that the pattern speeds of perturbations must be rapid, placing corotation just beyond the ends of the bars. For galaxies possessing central H I depressions, physical bars having masses greater than about 10 percent of the disk mass must be present. When rotated rapidly, triaxial model bars evacuate much of the gas from the bar regions. For a model bar of fixed mass and pattern speed, flattening of the triaxial figure does not change the gas response significantly. 20 refs.

Self‐Gravity Driven Instabilities at Accelerated Interfaces

Abstract: Nonlinear hydrodynamic flows are ubiquitous in the interstellar medium (ISM). Such flows play an important role in shaping atomic and molecular clouds and determining the initial conditions for star formation. One mechanism by which nonlinear flows arise is the onset and growth of interfacial instabilities. Any interface of discontinuous density is subject to a host of instabilities, including Rayleigh‐Taylor, Kelvin‐Helmholtz, and Richtmyer‐Meshkov. As part of an ongoing study of structure formation in the ISM, Hunter, Whitaker, and Lovelace discovered an additional density interface instability. This instability is driven by self‐gravity and termed the self‐gravity interfacial instability (SGI). The SGI causes any displacement of the interface to grow on roughly a free‐fall time scale, even when the perturbation wavelength is much less than the Jeans length. Numerical simulations have confirmed the expectations of linear theory, including the near scale invariance of the growth rate. Here, we build upon previous work by considering an initial condition in which the acceleration due to self‐gravity is non‐zero at the interface.

Satellites as Probes of the Masses of Spiral Galaxies

ABSTRACT: We present atomic hydrogen (HI) observations and analyses of the kinematics of satellite‐primary galaxy pairs. Two estimates for the masses of the primaries are available, one from their rotation curves and one from the orbital properties of the satellites. Defining χ as the ratio of these two mass estimates, it is a measure of the presence, or absence, of a significant halo. The χ distribution is presented and the selection effects are discussed. We show that our data, compared with the more numerous pairs identified by Zaritsky et al. [11], [12] , have similar distributions for projected separations of less than 200 kpc, even though the selection criteria employed were quite different. Observational biases have a negligible effect; the biased and unbiased distributions are essentially identical. N‐body calculations were executed to simulate the dynamical behavior of relatively low mass satellites orbiting primary disk galaxies with and without extended halos. In addition, we made a partially analytical analysis of the behavior of orbits in a logarithmic potential. We find that a “generic” model, characterized by a single disk‐halo combination, cannot reproduce the observed P(χ) distribution. However, a simple two‐component population of galaxies, composed of not more than 60% with halos and 40% without halos, is successful, if galaxies have dimensions of order 200 kpc. If galaxies are considerably larger with sizes extending to 400 kpc or more, no generic model can describe the full range of the observed P(χ), particularly if the distribution for rp < 200 kpc is compared with that for rp > 200 kpc. Regardless of the mix of orbital eccentricities, neither pure halo, nor canonical models (disk and halo masses are comparable within the disk radius) will work. A multicomponent approximation can be constructed; the canonical model must be mixed with a small fraction of systems essentially devoid of a massive dark halo. Only by including these complexities can the full range of P(χ) be modeled with any degree of success over all radial extents. We show that dynamical friction cannot be ignored in these explorations and that the average mass of a galaxy is in the range of (1‐5) × 1012 M˙o, with a mass‐to‐luminosity ratio of at most a few hundred. This is insufficient to close the Universe.

The Development of Structure in Shearing Viscous Media. Flat Disksa

1. Density waves and vorticity may be excited, and grow spectacularly, in Chandrasekhars singular modes, having wave vectors parallel to the axis of rotation. 2. In viscous media, such as protostellar disks, the familiar density waves may be strongly damped, or overdamped, with only the slowly propagating vorticity modes surviving. 3. Shear viscosity breaks Kelvins circulation theorem by requiring that spin be exchanged between adjacent fluid elements. Consequently, shear viscous forces can effectively cancel the stabilizing Coriolis terms, thereby allowing gravitational instabilities to develop at roughly the Jeans length. 4. By assuming exponential solutions for the time dependences of the linear modes, the resulting approximate dispersion relation may be used to predict when self-gravitating structures can form. In the present communication, this approach is reformulated for the more realistic limiting case of an arbitrarily flat disk.