136,343 research outputs found
Radiative transfer models
The purpose of this work was to assist with the development of analytical techniques for the interpretation of infrared observations. We have done the following: (1) helped to develop models for continuum absorption calculations for water vapor in the far infrared spectral region; (2) worked on models for pressure-induced absorption for O2 and N2 and their comparison with available observations; and (3) developed preliminary studies of non-local thermal equilibrium effects in the upper stratosphere and mesosphere for infrared gases. These new techniques were employed for analysis of balloon-borne far infrared data by a group at the Harvard-Smithsonian Center for Astrophysics. The empirical continuum absorption model for water vapor in the far infrared spectral region and the pressure-induced N2 absorption model were found to give satisfactory results in the retrieval of the mixing ratios of a number of stratospheric trace constituents from balloon-borne far infrared observations
Ultraviolet Dust Grain Properties in Starburst Galaxies: Evidence from Radiative Transfer Modeling and Local Group Extinction Curves
This paper summarizes the evidence of the ultraviolet properties of dust
grains found in starburst galaxies. Observations of starburst galaxies clearly
show that the 2175 A feature is weak or absent. This can be the result of
radiative transfer effects (mixing the dust and stars) or due to dust grains
which do not have this feature. Spherical DIRTY radiative transfer models imply
that it is not radiative transfer effects, but other radiative transfer models
with disk/bulge geometries have found cases where it could be radiative
transfer effects. Recent work on the extinction curves in the Magellanic Clouds
and Milky Way has revealed that the traditional explanation of low metallicity
for the absence of the 2175 A feature in the Small Magellanic Cloud is likely
incorrect. The SMC has one sightline with a 2175 A feature and the Milky Way
has sightlines without this feature. In addition, where the 2175 A feature is
found to be weak or absent in both Magellanic Clouds and the Milky Way, there
is evidence for recent star formation. Taking the sum of the radiative transfer
modeling of starburst galaxies and the behavior of Local Group extinction
curves, it is likely that the dust grains in starburst galaxies intrinsically
lack the 2175 A feature.Comment: 7 pages, To appear in the proceedings of: "The Spectral Energy
Distribution of Gas-Rich Galaxies: Confronting Models with Data", Heidelberg,
4-8 Oct. 2004, eds. C.C. Popescu and R.J. Tuffs, AIP Conf. Ser., in press
[fixed typo in title
Time Dependent Monte Carlo Radiative Transfer Calculations For 3-Dimensional Supernova Spectra, Lightcurves, and Polarization
We discuss Monte-Carlo techniques for addressing the 3-dimensional
time-dependent radiative transfer problem in rapidly expanding supernova
atmospheres. The transfer code SEDONA has been developed to calculate the
lightcurves, spectra, and polarization of aspherical supernova models. From the
onset of free-expansion in the supernova ejecta, SEDONA solves the radiative
transfer problem self-consistently, including a detailed treatment of gamma-ray
transfer from radioactive decay and with a radiative equilibrium solution of
the temperature structure. Line fluorescence processes can also be treated
directly. No free parameters need be adjusted in the radiative transfer
calculation, providing a direct link between multi-dimensional hydrodynamical
explosion models and observations. We describe the computational techniques
applied in SEDONA, and verify the code by comparison to existing calculations.
We find that convergence of the Monte Carlo method is rapid and stable even for
complicated multi-dimensional configurations. We also investigate the accuracy
of a few commonly applied approximations in supernova transfer, namely the
stationarity approximation and the two-level atom expansion opacity formalism.Comment: 16 pages, ApJ accepte
SKIRT: the design of a suite of input models for Monte Carlo radiative transfer simulations
The Monte Carlo method is the most popular technique to perform radiative
transfer simulations in a general 3D geometry. The algorithms behind and
acceleration techniques for Monte Carlo radiative transfer are discussed
extensively in the literature, and many different Monte Carlo codes are
publicly available. On the contrary, the design of a suite of components that
can be used for the distribution of sources and sinks in radiative transfer
codes has received very little attention. The availability of such models, with
different degrees of complexity, has many benefits. For example, they can serve
as toy models to test new physical ingredients, or as parameterised models for
inverse radiative transfer fitting. For 3D Monte Carlo codes, this requires
algorithms to efficiently generate random positions from 3D density
distributions. We describe the design of a flexible suite of components for the
Monte Carlo radiative transfer code SKIRT. The design is based on a combination
of basic building blocks (which can be either analytical toy models or
numerical models defined on grids or a set of particles) and the extensive use
of decorators that combine and alter these building blocks to more complex
structures. For a number of decorators, e.g. those that add spiral structure or
clumpiness, we provide a detailed description of the algorithms that can be
used to generate random positions. Advantages of this decorator-based design
include code transparency, the avoidance of code duplication, and an increase
in code maintainability. Moreover, since decorators can be chained without
problems, very complex models can easily be constructed out of simple building
blocks. Finally, based on a number of test simulations, we demonstrate that our
design using customised random position generators is superior to a simpler
design based on a generic black-box random position generator.Comment: 15 pages, 4 figures, accepted for publication in Astronomy and
Computin
Maximal near-field radiative heat transfer between two plates
A parametric study of Drude and Lorentz models performances in maximizing
near-field radiative heat transfer between two semi-infinite planes separated
by nanometric distances at room temperature is presented in this paper. Optimal
parameters of these models that provide optical properties maximizing the
radiative heat flux are reported and compared to real materials usually
considered in similar studies, silicon carbide and heavily doped silicon in
this case. Results are obtained by exact and approximate (in the extreme
near-field regime and the electrostatic limit hypothesis) calculations. The two
methods are compared in terms of accuracy and CPU resources consumption. Their
differences are explained according to a mesoscopic description of near-field
radiative heat transfer. Finally, the frequently assumed hypothesis which
states a maximal radiative heat transfer when the two semi-infinite planes are
of identical materials is numerically confirmed. Its subsequent practical
constraints are then discussed.Comment: 19 pages, 11 figures, submitted to Journal of Physics D : Applied
Physic
Atmospheric dynamics of red supergiant stars and Interferometry
We developed a 3D pure LTE radiative transfer code to derive observables
expected for RSGs, with emphasis on small scale structures, from
radiative-hydrodynamic (RHD) simulations of red supergiant stars (RSGs) carried
out with CO5BOLD (Freytag et al. 2002). We show that the convection-related
surface structures are observable with today's interferometers. Moreover, the
RHD simulations are a great improvement over parametric models for the
interpretation of interferometric observations.Comment: 6 pages, Perspectives in Radiative Transfer and Interferometry, EAS
publication serie
X ray opacity in cluster cooling flows
We have calculated the emergent x-ray properties for a set of spherically symmetric, steady-state cluster cooling flow models including the effects of radiative transfer. Opacity due to resonant x-ray lines, photoelectric absorption, and electron scattering have been included in these calculations, and homogeneous and inhomogeneous gas distributions were considered. The effects of photoionization opacity are small for both types of models. In contrast, resonant line optical depths can be quite high in both homogeneous and inhomogeneous models. The presence of turbulence in the gas can significantly lower the line opacity. We find that integrated x-ray spectra for the flow cooling now are only slightly affected by radiative transfer effects. However x-ray line surface brightness profiles can be dramatically affected by radiative transfer. Line profiles are also strongly affected by transfer effects. The combined effects of opacity and inflow cause many of the lines in optically thick models to be asymmetrical
Radiation Effects on Flow Characteristics in Combustion Chambers
A JANNAF sponsored workshop was held to discuss the importance and role of radiative heat transfer in rocket combustion chambers. The potential impact of radiative transfer on hardware design, reliability, and performance was discussed. The current state of radiative transfer prediction capability in CFD modeling was reviewed and concluded to be substantially lacking in both the physical models used and the radiative property data available. There is a clear need to begin to establish a data base for making radiation calculations in rocket combustion chambers. A natural starting point for this effort would be the NASA thermochemical equilibrium code (CEC)
FitSKIRT: genetic algorithms to automatically fit dusty galaxies with a Monte Carlo radiative transfer code
We present FitSKIRT, a method to efficiently fit radiative transfer models to
UV/optical images of dusty galaxies. These images have the advantage that they
have better spatial resolution compared to FIR/submm data. FitSKIRT uses the
GAlib genetic algorithm library to optimize the output of the SKIRT Monte Carlo
radiative transfer code. Genetic algorithms prove to be a valuable tool in
handling the multi- dimensional search space as well as the noise induced by
the random nature of the Monte Carlo radiative transfer code. FitSKIRT is
tested on artificial images of a simulated edge-on spiral galaxy, where we
gradually increase the number of fitted parameters. We find that we can recover
all model parameters, even if all 11 model parameters are left unconstrained.
Finally, we apply the FitSKIRT code to a V-band image of the edge-on spiral
galaxy NGC4013. This galaxy has been modeled previously by other authors using
different combinations of radiative transfer codes and optimization methods.
Given the different models and techniques and the complexity and degeneracies
in the parameter space, we find reasonable agreement between the different
models. We conclude that the FitSKIRT method allows comparison between
different models and geometries in a quantitative manner and minimizes the need
of human intervention and biasing. The high level of automation makes it an
ideal tool to use on larger sets of observed data.Comment: 14 pages, 10 figures; accepted for publication in Astronomy and
Astrophysic
Radiative transfer models of non-spherical prestellar cores
We present 2D Monte Carlo radiative transfer simulations of prestellar cores.
We consider two types of asymmetry: disk-like asymmetry, in which the core is
denser towards the equatorial plane than towards the poles; and axial
asymmetry, in which the core is denser towards the south pole than the north
pole. We limit our treatment to cores with mild asymmetries, which are exposed
directly to the interstellar radiation field or are embedded inside molecular
clouds.
The isophotal maps of a core depend strongly on the viewing angle. Maps at
wavelengths longer than the peak of the SED (e.g. 850 micron) essentially trace
the column-density. Thus, for instance, cores with disk-like asymmetry appear
elongated when mapped at 850 micron from close to the equatorial plane.
However, at wavelengths near the peak of the SED (e.g. 200 micron), the
emissivity is more strongly dependent on the temperature, and therefore, at
particular viewing angles, there are characteristic features which reflect a
more complicated convolution of the density and temperature fields within the
core.
These characteristic features are on scales 1/5 to 1/3 of the overall core
size, and so high resolution observations are needed to observe them. They are
also weaker if the core is embedded in a molecular cloud (because the range of
temperature within the core is then smaller), and so high sensitivity is needed
to detect them. Herschel, to be launched in 2007, will in principle provide the
necessary resolution and sensitivity at 170 to 250 micron.Comment: 16 pages, 22 figures, accepted by A&A, also available (with high
resolution figures) at
http://www.astro.cf.ac.uk/pub/Dimitrios.Stamatellos/publications
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