1,767 research outputs found
Fully-Coupled Simulation of Cosmic Reionization. I: Numerical Methods and Tests
We describe an extension of the Enzo code to enable fully-coupled radiation
hydrodynamical simulation of inhomogeneous reionization in large cosmological volumes with thousands to millions of point sources. We
solve all dynamical, radiative transfer, thermal, and ionization processes
self-consistently on the same mesh, as opposed to a postprocessing approach
which coarse-grains the radiative transfer. We do, however, employ a simple
subgrid model for star formation which we calibrate to observations. Radiation
transport is done in the grey flux-limited diffusion (FLD) approximation, which
is solved by implicit time integration split off from the gas energy and
ionization equations, which are solved separately. This results in a faster and
more robust scheme for cosmological applications compared to the earlier
method. The FLD equation is solved using the hypre optimally scalable geometric
multigrid solver from LLNL. By treating the ionizing radiation as a grid field
as opposed to rays, our method is scalable with respect to the number of
ionizing sources, limited only by the parallel scaling properties of the
radiation solver. We test the speed and accuracy of our approach on a number of
standard verification and validation tests. We show by direct comparison with
Enzo's adaptive ray tracing method Moray that the well-known inability of FLD
to cast a shadow behind opaque clouds has a minor effect on the evolution of
ionized volume and mass fractions in a reionization simulation validation test.
We illustrate an application of our method to the problem of inhomogeneous
reionization in a 80 Mpc comoving box resolved with Eulerian grid
cells and dark matter particles.Comment: 32 pages, 23 figures. ApJ Supp accepted. New title and substantial
revisions re. v
A Generalized Model of Nonlinear Diffusive Shock Acceleration Coupled to an Evolving Supernova Remnant
To better model the efficient production of cosmic rays (CRs) in supernova
remnants (SNRs) with the associated coupling between CR production and SNR
dynamics, we have generalized an existing cr-hydro-NEI code (i.e., Ellison et
al. 2012) to include the following processes: (1) an explicit calculation of
the upstream precursor structure including the position dependent flow speed,
density, temperature, and magnetic field strength; (2) a momentum and space
dependent CR diffusion coefficient; (3) an explicit calculation of magnetic
field amplification (MFA); (4) calculation of the maximum CR momentum using the
amplified magnetic field; (5) a finite Alfven speed for the particle scattering
centers; and (6) the ability to accelerate a superthermal seed population of
CRs as well as the ambient thermal plasma. While a great deal of work has been
done modeling SNRs, most work has concentrated on either the continuum emission
from relativistic electrons or ions, or the thermal emission from the shock
heated plasma. Our generalized code combines these elements and describes the
interplay between CR production and SNR evolution, including the nonlinear
coupling of efficient diffusive shock acceleration (DSA), based mainly on the
work of P. Blasi and co-workers, and a non-equilibrium ionization (NEI)
calculation of thermal X-ray line emission. We believe our generalized model
will provide a consistent modeling platform for SNRs, including those
interacting with molecular clouds, and improve the interpretation of current
and future observations, including the high-quality spectra expected from
Astro-H. SNR RX J1713.7-3946 is modeled as an example.Comment: 13 pages, 5 figures, accepted by the Astrophysical Journa
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