1,072 research outputs found
A 3D radiative transfer framework: IV. spherical & cylindrical coordinate systems
We extend our framework for 3D radiative transfer calculations with a
non-local operator splitting methods along (full) characteristics to spherical
and cylindrical coordinate systems. These coordinate systems are better suited
to a number of physical problems than Cartesian coordinates. The scattering
problem for line transfer is solved via means of an operator splitting (OS)
technique. The formal solution is based on a full characteristics method. The
approximate operator is constructed considering nearest neighbors
exactly. The code is parallelized over both wavelength and solid angle using
the MPI library. We present the results of several test cases with different
values of the thermalization parameter for the different coordinate systems.
The results are directly compared to 1D plane parallel tests. The 3D results
agree very well with the well-tested 1D calculations.Comment: A&A, in pres
A 3D radiative transfer framework: VII. Arbitrary velocity fields in the Eulerian frame
A solution of the radiative-transfer problem in 3D with arbitrary velocity
fields in the Eulerian frame is presented. The method is implemented in our 3D
radiative transfer framework and used in the PHOENIX/3D code. It is tested by
comparison to our well- tested 1D co-moving frame radiative transfer code,
where the treatment of a monotonic velocity field is implemented in the
Lagrangian frame. The Eulerian formulation does not need much additional memory
and is useable on state-of-the-art computers, even large-scale applications
with 1000's of wavelength points are feasible
A 3D radiative transfer framework: III. periodic boundary conditions
We present a general method to solve radiative transfer problems including
scattering in the continuum as well as in lines in 3D configurations with
periodic boundary conditions. he scattering problem for line transfer is solved
via means of an operator splitting (OS) technique. The formal solution is based
on a full characteristics method. The approximate operator is
constructed considering nearest neighbors exactly. The code is parallelized
over both wavelength and solid angle using the MPI library. We present the
results of several test cases with different values of the thermalization
parameter and two choices for the temperature structure. The results are
directly compared to 1D plane parallel tests. The 3D results agree very well
with the well-tested 1D calculations.Comment: A&A, in press, visualization figure omitted due to size, available at
ftp://phoenix.hs.uni-hamburg.de/preprints/3DRT_paper3.pd
A 3D radiative transfer framework: XI. multi-level NLTE
Multi-level non-local thermodynamic equilibrium (NLTE) radiation transfer
calculations have become standard throughout the stellar atmospheres community
and are applied to all types of stars as well as dynamical systems such as
novae and supernovae. Even today spherically symmetric 1D calculations with
full physics are computationally intensive. We show that full NLTE calculations
can be done with fully 3 dimensional (3D) radiative transfer. With modern
computational techniques and current massive parallel computational resources,
full detailed solution of the multi-level NLTE problem coupled to the solution
of the radiative transfer scattering problem can be solved without sacrificing
the micro physics description. We extend the use of a rate operator developed
to solve the coupled NLTE problem in spherically symmetric 1D systems. In order
to spread memory among processors we have implemented the NLTE/3D module with a
hierarchical domain decomposition method that distributes the NLTE levels,
radiative rates, and rate operator data over a group of processes so that each
process only holds the data for a fraction of the voxels. Each process in a
group holds all the relevant data to participate in the solution of the 3DRT
problem so that the 3DRT solution is parallelized within a domain decomposition
group. We solve a spherically symmetric system in 3D spherical coordinates in
order to directly compare our well-tested 1D code to the 3D case. We compare
three levels of tests: a) a simple H+He test calculation, b) H+He+CNO+Mg, c)
H+He+Fe. The last test is computationally large and shows that realistic
astrophysical problems are solvable now, but they do require significant
computational resources. With presently available computational resources it is
possible to solve the full 3D multi-level problem with the same detailed
micro-physics as included in 1D modeling.Comment: 20 pages, 14 figures, A&A, in pres
Non-LTE Treatment of Fe II in Astrophysical Plasmas
We describe our implementation of an extremely detailed model atom of singly
ionized iron for NLTE computations in static and moving astrophysical plasmas.
Our model atom includes 617 levels, 13675 primary permitted transitions and up
to 1.2 million secondary transitions. Our approach guarantees that the total
iron opacity is included at the correct wavelength with reasonable memory and
CPU requirements. We find that the lines saturate the wavelength space, such
that special wavelength points inserted along the detailed profile functions
may be replaced with a statistical sampling method. We describe the results of
various test calculations for novae and supernovae.Comment: 17 pages, latex, aip style, no figures included, full text with
figures available at ftp://brian.la.asu.edu/pub/preprint/FeII-NLTE.ps.Z or at
http://brian.la.asu.edu
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
General Relativistic Radiative Transfer
We present a general method to calculate radiative transfer including
scattering in the continuum as well as in lines in spherically symmetric
systems that are influenced by the effects of general relativity (GR). We
utilize a comoving wavelength ansatz that allows to resolve spectral lines
throughout the atmosphere. The used numerical solution is an operator splitting
(OS) technique that uses a characteristic formal solution. The bending of
photon paths and the wavelength shifts due to the effects of GR are fully taken
into account, as is the treatment of image generation in a curved spacetime. We
describe the algorithm we use and demonstrate the effects of GR on the
radiative transport of a two level atom line in a neutron star like atmosphere
for various combinations of continuous and line scattering coefficients. In
addition, we present grey continuum models and discuss the effects of different
scattering albedos on the emergent spectra and the determination of effective
temperatures and radii of neutron star atmospheres
Improved discretization of the wavelength derivative term in CMF operator splitting numerical radiative transfer
We describe two separate wavelength discretization schemes that can be used
in the numerical solution of the comoving frame radiative transfer equation. We
present an improved second order discretization scheme and show that it leads
to significantly less numerical diffusion than previous scheme. We also show
that due to the nature of the second order term in some extreme cases it can
become numerically unstable. We stabilize the scheme by introducing a mixed
discretization scheme and present the results from several test calculations.Comment: 9 pages, 9 figures, A&A, in pres
Time-dependent radiative transfer with PHOENIX
Aims. We present first results and tests of a time-dependent extension to the
general purpose model atmosphere code PHOENIX. We aim to produce light curves
and spectra of hydro models for all types of supernovae. Methods. We extend our
model atmosphere code PHOENIX to solve time-dependent non-grey, NLTE, radiative
transfer in a special relativistic framework. A simple hydrodynamics solver was
implemented to keep track of the energy conservation of the atmosphere during
free expansion. Results. The correct operation of the new additions to PHOENIX
were verified in test calculations. Conclusions. We have shown the correct
operation of our extension to time-dependent radiative transfer and will be
able to calculate supernova light curves and spectra in future work.Comment: 7 pages, 12 figure
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