13,118 research outputs found
Optimal prediction for moment models: Crescendo diffusion and reordered equations
A direct numerical solution of the radiative transfer equation or any kinetic
equation is typically expensive, since the radiative intensity depends on time,
space and direction. An expansion in the direction variables yields an
equivalent system of infinitely many moments. A fundamental problem is how to
truncate the system. Various closures have been presented in the literature. We
want to study moment closure generally within the framework of optimal
prediction, a strategy to approximate the mean solution of a large system by a
smaller system, for radiation moment systems. We apply this strategy to
radiative transfer and show that several closures can be re-derived within this
framework, e.g. , diffusion, and diffusion correction closures. In
addition, the formalism gives rise to new parabolic systems, the reordered
equations, that are similar to the simplified equations.
Furthermore, we propose a modification to existing closures. Although simple
and with no extra cost, this newly derived crescendo diffusion yields better
approximations in numerical tests.Comment: Revised version: 17 pages, 6 figures, presented at Workshop on Moment
Methods in Kinetic Gas Theory, ETH Zurich, 2008 2 figures added, minor
correction
A 3D radiative transfer framework IX. Time dependence
Context. Time-dependent, 3D radiation transfer calculations are important for
the modeling of a variety of objects, from supernovae and novae to simulations
of stellar variability and activity. Furthermore, time-dependent calculations
can be used to obtain a 3D radiative equilibrium model structure via relaxation
in time. Aims. We extend our 3D radiative transfer framework to include direct
time dependence of the radiation field; i.e., the
terms are fully considered in the solution of radiative transfer problems.
Methods. We build on the framework that we have described in previous papers in
this series and develop a subvoxel method for the
terms. Results. We test the implementation by comparing the 3D results to our
well tested 1D time dependent radiative transfer code in spherical symmetry. A
simple 3D test model is also presented. Conclusions. The 3D time dependent
radiative transfer method is now included in our 3D RT framework and in
PHOENIX/3D.Comment: A&A in press, 7 pages, 14 figure
Milne-Eddington Solutions for Relativistic Plane-Parallel Flows
Radiative transfer in a relativistic plane-parallel flow, e.g., an accretion
disk wind, is examined in the fully special relativistic treatment. Under the
assumption of a constant flow speed, for the relativistically moving atmosphere
we analytically obtain generalized Milne-Eddington solutions of radiative
moment equations; the radiation energy density, the radiative flux, and the
radiation pressure. In the static limit these solutions reduce to the
traditional Milne-Eddington ones for the plane-parallel static atmosphere,
whereas the source function nearly becomes constant as the flow speed
increases. Using the analytical solutions, we analytically integrate the
relativistic transfer equation to obtain the specific intensity. This specific
intensity also reduces to the Milne-Eddinton case in the static limit, while
the emergent intensity is strongly enhanced toward the flow direction due to
the Doppler and aberration effects as the flow speed increases (relativistic
peaking).Comment: 1o pages, 5 figure
Analytical Models of Exoplanetary Atmospheres. II. Radiative Transfer via the Two-stream Approximation
We present a comprehensive analytical study of radiative transfer using the
method of moments and include the effects of non-isotropic scattering in the
coherent limit. Within this unified formalism, we derive the governing
equations and solutions describing two-stream radiative transfer (which
approximates the passage of radiation as a pair of outgoing and incoming
fluxes), flux-limited diffusion (which describes radiative transfer in the deep
interior) and solutions for the temperature-pressure profiles. Generally, the
problem is mathematically under-determined unless a set of closures (Eddington
coefficients) is specified. We demonstrate that the hemispheric (or
hemi-isotropic) closure naturally derives from the radiative transfer equation
if energy conservation is obeyed, while the Eddington closure produces spurious
enhancements of both reflected light and thermal emission. We concoct recipes
for implementing two-stream radiative transfer in stand-alone numerical
calculations and general circulation models. We use our two-stream solutions to
construct toy models of the runaway greenhouse effect. We present a new
solution for temperature-pressure profiles with a non-constant optical opacity
and elucidate the effects of non-isotropic scattering in the optical and
infrared. We derive generalized expressions for the spherical and Bond albedos
and the photon deposition depth. We demonstrate that the value of the optical
depth corresponding to the photosphere is not always 2/3 (Milne's solution) and
depends on a combination of stellar irradiation, internal heat and the
properties of scattering both in optical and infrared. Finally, we derive
generalized expressions for the total, net, outgoing and incoming fluxes in the
convective regime.Comment: Accepted by ApJS. 23 pages, 11 figures, 3 tables, 158 equations. No
change from previous version except for title (to match ApJS convention
Numerical Solution of the Expanding Stellar Atmosphere Problem
In this paper we discuss numerical methods and algorithms for the solution of
NLTE stellar atmosphere problems involving expanding atmospheres, e.g., found
in novae, supernovae and stellar winds. We show how a scheme of nested
iterations can be used to reduce the high dimension of the problem to a number
of problems with smaller dimensions. As examples of these sub-problems, we
discuss the numerical solution of the radiative transfer equation for
relativistically expanding media with spherical symmetry, the solution of the
multi-level non-LTE statistical equilibrium problem for extremely large model
atoms, and our temperature correction procedure. Although modern iteration
schemes are very efficient, parallel algorithms are essential in making large
scale calculations feasible, therefore we discuss some parallelization schemes
that we have developed.Comment: JCAM, in press. 28 pages, also available at
ftp://calvin.physast.uga.edu:/pub/preprints/CompAstro.ps.g
The Classical Stellar Atmosphere Problem
We introduce the classical stellar atmosphere problem and describe in detail
its numerical solution. The problem consists of the solution of the radiation
transfer equations under the constraints of hydrostatic, radiative and
statistical equilibrium (non-LTE). We outline the basic idea of the Accelerated
Lambda Iteration (ALI) technique and statistical methods which finally allow
the construction of non-LTE model atmospheres considering the influence of
millions of metal absorption lines. Some applications of the new models are
presented.Comment: accepted for publication in The Journal of Computational and Applied
Mathematics, Computational Astrophysics, eds. H. Riffert, K. Werne
3D Radiative Transfer with PHOENIX
Using the methods of general relativity Lindquist derived the radiative
transfer equation that is correct to all orders in v/c. Mihalas developed a
method of solution for the important case of monotonic velocity fields with
spherically symmetry. We have developed the generalized atmosphere code
PHOENIX, which in 1-D has used the framework of Mihalas to solve the radiative
transfer equation (RTE) in 1-D moving flows. We describe our recent work
including 3-D radiation transfer in PHOENIX and particularly including moving
flows exactly using a novel affine method. We briefly discuss quantitative
spectroscopy in supernovae.Comment: 13 pages, 9 figures, to appear in Recent Directions in Astrophysical
Quantitative Spectroscopy and Radiation Hydrodynamics, Ed. I. Hubeny,
American Institute of Physics (2009
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