Robert P Weaver

Interstellar bubbles. II - Structure and evolution

The detailed structure of the interaction of a strong stellar wind with the interstellar medium is presented. First, an adiabatic similarity solution is given which is applicable at early times. Second, a similarity solution is derived which includes the effects of thermal conduction between the hot (about 1 million K) interior and the cold shell of swept-up interstellar matter. This solution is then modified to include the effects of radiative energy losses. The evolution of an interstellar bubble is calculated, including the radiative losses. The quantitative results for the outer-shell radius and velocity and the column density of highly ionized species such as O VI are within a factor 2 of the approximate results of Castor, McCray, and Weaver (1975). The effect of stellar motion on the structure of a bubble, the hydrodynamic stability of the outer shell, and the observable properties of the hot region and the outer shell are discussed.

Richtmyer–Meshkov instability growth: experiment, simulation and theory

Richtmyer–Meshkov instability is investigated for negative Atwood number and two-dimensional sinusoidal perturbations by comparing experiments, numerical simulations and analytic theories. The experiments were conducted on the NOVA laser with strong radiatively driven shocks with Mach numbers greater than 10. Three different hydrodynamics codes (RAGE, PROMETHEUS and FronTier) reproduce the amplitude evolution and the gross features in the experiment while the fine-scale features differ in the different numerical techniques. Linearized theories correctly calculate the growth rates at small amplitude and early time, but fail at large amplitude and late time. A nonlinear theory using asymptotic matching between the linear theory and a potential flow model shows much better agreement with the late-time and large-amplitude growth rates found in the experiments and simulations. We vary the incident shock strength and initial perturbation amplitude to study the behaviour of the simulations and theory and to study the effects of compression and nonlinearity.

THE ORIGIN AND KINEMATICS OF COLD GAS IN GALACTIC WINDS: INSIGHT FROM NUMERICAL SIMULATIONS

We study the origin of Na?I-absorbing gas in ultraluminous infrared galaxies motivated by the recent observations by Martin of extremely superthermal linewidths in this cool gas. We model the effects of repeated supernova explosions driving supershells in the central regions of molecular disks with Md = 1010 M ?, using cylindrically symmetric gas dynamical simulations run with ZEUS-3D. The shocked swept-up shells quickly cool and fragment by Rayleigh-Taylor (R-T) instability as they accelerate out of the dense, stratified disks. The numerical resolution of the cooling and compression at the shock fronts determines the peak shell density, and so the speed of R-T fragmentation. We identify cooled shells and shell fragments as Na?I-absorbing gas and study its kinematics along various sightlines across the grid. We find that simulations with a numerical resolution of ?0.2?pc produce multiple R-T fragmented shells in a given line of sight that appear to explain the observed kinematics. We suggest that the observed wide Na?I absorption lines, v = 320 ? 120 km s?1, are produced by these multiple fragmented shells traveling at different velocities. We also suggest that some shell fragments can be accelerated above the observed average terminal velocity of 750 km s?1 by the same energy-driven wind with an instantaneous starburst of ~109 M ?. The mass carried by these fragments is only a small fraction of the total shell mass, while the bulk of mass is traveling with velocities consistent with the observed average shell velocity 330 ? 100 km s?1. Our results show that an energy-driven bubble causing R-T instabilities can explain the kinematics of cool gas seen in the Na?I observations without invoking additional physics relying primarily on momentum conservation, such as entrainment of gas by Kelvin-Helmholtz instabilities, ram pressure driving of cold clouds by a hot wind, or radiation pressure acting on dust.

The Comptonization of iron X-ray features in compact X-ray sources

We describe the formation of X-ray spectral features due to iron in a relatively cool cloud of gas with Thomson depth tau/sub T/>1 surrounding a compact source of continuum X-rays. Coupled equations are solved for the ionization structure of the cloud and for the radiative transfer of the X-rays. Photoionization suppresses the strength of emission lines and absorption edges. Comptonization of the radiation broadens emission lines, fills in absorption edges, and produces a high-energy cutoff. In order to describe multiple scattering, we derive a Fokker-Planck equation which includes an important modification of the Kompaneets equation. Narrow resonance lines are treated with an escape probability formalism.

Time-dependent radiative transfer with automatic flux limiting

Abstract We discuss a simple method for solving the time-dependent transfer problem. This scheme is automatically flux-limited and affords physical insight into how flux limitation occurs. We then develop a second-order, time-dependent radiation energy equation that is similar in form to the diffusion limit radiation energy equation. This time-dependent energy equation approaches physically reasonable equations in optically thick and thin regions. Computational aspects of solving this energy equation are discussed.

Radiation transfer in the fluid frame: A covariant formulation: Part I: Radiation hydrodynamics

Abstract The equations of radiation hydrodynamics are developed with a tensor formalism. This development includes a review of the current literature in this field, written in a common notation and having the underlying assumptions of each result explicitly stated.

Massively Parallel Simulations with DOE?s ASCI Supercomputers: An Overview of the Los Alamos Crestone Project

The Los Alamos Crestone Project is part of the Department of Energy’s (DOE) Accelerated Strategic Computing Initiative, or ASCI Program. The main goal of this software development project is to investigate the use of continuous adaptive mesh refinement (CAMR) techniques for application to problems of interest to the Laboratory. There are many code development efforts in the Crestone Project, both unclassified and classified codes. In this overview I will discuss the unclassified SAGE and the RAGE codes. The SAGE (SAIC adaptive grid Eulerian) code is a one-, two-, and three-dimensional, multimaterial, Eulerian, massively parallel hydrodynamics code for use in solving a variety of high-deformation flow problems. The RAGE CAMR code is built from the SAGE code by adding various radiation packages, improved setup utilities, and graphics packages. It is used for problems in which radiation transport of energy is important. The goal of these massively-parallel versions of the SAGE and RAGE codes is to run extremely large problems in a reasonable amount of calendar time. Our target is scalable performance to ∼10,000 processors on a 1 billion CAMR computational cell problem that requires hundreds of variables per cell, multiple physics packages (e.g., radiation and hydrodynamics), and implicit matrix solves for each cycle. A general description of the RAGE code has been published in [1], [2], [3] and [4].

On iterative solutions of the LTE model atmosphere problem

Abstract We discuss iterative methods for solving the coupled radiative-transfer and energy-balance equations in the LTE model atmospheres problem including isotropic coherent scattering. We show that iterative solution (e.g. by SOR techniques) of the grand matrix encountered in such problems is vastly more efficient than a direct solution, and is easily vectorized. The final computational effort is linear in the number of depths and frequencies considered, and thus this approach opens the door for the computation of both static and dynamic line-blanketed models using large numbers of depth-points and huge numbers of frequencies. The iterative methods discussed here can be applied to line-formation problems with complete redistribution and to certain classes of problems with partial redistribution (e.g. Compton scattering problems in the Fokker-Planck approximation).

Multi-organization — Multi-discipline effort developing a mitigation concept for planetary defense

There have been significant recent efforts in addressing mitigation approaches to neutralize Potentially Hazardous Asteroids (PHA). One such research effort was performed in 2015 by an integrated, inter-disciplinary team of asteroid scientists, energy deposition modeling scientists, payload engineers, orbital dynamicist engineers, spacecraft discipline engineers, and systems / architecture engineers from NASAs Goddard Space Flight Center (GSFC) and the Department of Energy (DoE) / National Nuclear Security Administration (NNSA) laboratories (Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratories (LLNL) and Sandia National Laboratories). The study team collaborated with GSFCs Integrated Design Centers Mission Design Lab (MDL) which engaged a team of GSFC flight hardware discipline engineers to work with GSFC, LANL, and LLNL Near-Earth Asteroid (NEA)-related subject matter experts during a one-week intensive concept formulation study in an integrated concurrent engineering environment. This team has analyzed the first of several distinct study cases for a multi-year NASA research grant. This Case 1 study references the NEA named Bennu as the notional target due to the availability of a very detailed Design Reference Asteroid (DRA) model for its orbit and physical characteristics (courtesy of the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer [OSIRIS-REx] mission team). The research involved the formulation and optimization of spacecraft trajectories to intercept Bennu, overall mission and architecture concepts, and high-fidelity modeling of both kinetic impact (spacecraft collision to change a NEAs momentum and orbit) and nuclear detonation effects on Bennu, for purposes of deflecting Bennu.

Calculations of Tsunamis from Submarine Landslides

Great underwater landslides like Storegga off the Norwegian coast leave massive deposits on the seafloor and probably produce enormous tsunamis. We have performed a numerical study of such landslides using the multi-material compressible hydrocode Sage in order to understand the relationship between the rheology of the slide material, the configuration of the resulting deposits on the seafloor, and the tsunami that is produced. Instabilities in the fluid-fluid mixing between slide material and seawater produce vortices and swirls with sizes that depend on the rheology of the slide material. These dynamical features of the flow may be preserved as ridges when the sliding material finally stops. Thus studying the configuration of the morphology of prehistoric slide relics on the abyssal plain may help us understand the circumstances under which the slide was initiated.