We describe RAGE, the "Radiation Adaptive Grid Eulerian" radiation-hydrodynamics code, including its data structures, its parallelization strategy and performance, its hydrodynamic algorithm(s), its (gray) radiation diffusion algorithm, and some of the considerable amount of verification and validation efforts. The hydrodynamics is a basic Godunov solver, to which we have made significant improvements to increase the advection algorithm's robustness and to converge stiffnesses in the equation of state. Similarly, the radiation transport is a basic gray diffusion, but our treatment of the radiation-material coupling, wherein we converge nonlinearities in a novel manner to allow larger timesteps and more robust behavior, can be applied to any multi-group transport algorithm.

We present the first hydrodynamic, multidimensional simulations of He shell flash convection. We investigate the properties of shell convection immediately before the He luminosity peak during the 15th thermal pulse of a stellar evolution track with initially 2 solar masses and metallicity Z ¼ 0:01. This choice is a representative example of a low-mass asymptotic giant branch thermal pulse. We construct the initial vertical stratification with a set of polytropes to resemble the stellar evolution structure. Convection is driven by a constant volume heating in a thin layer at the bottom of the unstable layer. We calculate a grid of two-dimensional simulations with different resolutions and heating rates, plus one low-resolution three-dimensional run. The flow field is dominated by large convective cells that are centered in the lower half of the convection zone. It generates a rich spectrum of gravity waves in the stable layers both above and beneath the convective shell. The magnitude of the convective velocities from our one-dimensional mixing-length theory model and the rms-averaged vertical velocities from the hydrodynamic model are consistent within a factor of a few. However, the velocity profile in the hydrodynamic simulation is more asymmetric and decays exponentially inside the convection zone. Both g-modes and convective motions cross the formal convective boundaries, which leads to mixing across the boundaries. Our resolution study shows consistent flow structures among the higher resolution runs, and we see indications for convergence of the vertical velocity profile inside the convection zone for the highest resolution simulations. Many of the convective properties, in particular the exponential decay of the velocities, depend only weakly on the heating rate. However, the amplitudes of the gravity waves increase with both the heating rate and the resolution.

Ionizing UV radiation and supernova flows amidst clustered minihalos at high redshift regulated the rise of the first stellar populations in the universe. Previous studies have addressed the effects of very massive primordial stars on the collapse of nearby halos into new stars, but the absence of the odd-even nucleosynthetic signature of pair-instability supernovae in ancient metal-poor stars suggests that Population III stars may have been less than 100 M ⊙ . We extend our earlier survey of local UV feedback on star formation to 25 -80 M ⊙ stars and include kinetic feedback by supernovae for 25 -40 M ⊙ stars. We find radiative feedback to be relatively uniform over this mass range, primarily because the larger fluxes of more massive stars are offset by their shorter lifetimes. Our models demonstrate that prior to the rise of global UV backgrounds, Lyman-Werner photons from nearby stars cannot prevent halos from forming new stars. These calculations also reveal that violent dynamical instabilities can erupt in the UV radiation front enveloping a primordial halo but that they ultimately have no effect on the formation of a star. Finally, our simulations suggest that relic H II regions surrounding partially evaporated halos may expel Lyman-Werner backgrounds at lower redshifts, allowing stars to form that were previously suppressed. We provide fits to radiative and kinetic feedback on star formation for use in both semianalytic models and numerical simulations.

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Abstract. -Water vapor in the Earth's troposphere produces fluctuations in the phase of millimeter-wavelength radiation from astronomical sources. Such fluctuations seriously limit the spatial resolution achievable with current millimeter interferometers. Since water vapor is also a source of atmospheric opacity at these wavelengths, radiometric measurements of sky brightness may be used to monitor the fluctuating water vapor content of the atmosphere and thereby the fluctuations in the interferometric phase. The atmospheric opacity depends on the frequency and on the physical conditions of those atmospheric regions in which the water vapor is located. Atmospheric temperature influences the strengths of the various absorption lines, and pressure influences the degree of line broadening. The magnitude of the phase fluctuations relative to the brightness fluctuations is therefore also dependent on frequency, temperature, and pressure. The frequency of a radiometric monitoring system may be chosen to minimize the dependence of this ratio on the atmospheric parameters.