13,432 research outputs found
Radiative equilibrium in Monte Carlo radiative transfer using frequency distribution adjustment
The Monte Carlo method is a powerful tool for performing radiative
equilibrium calculations, even in complex geometries. The main drawback of the
standard Monte Carlo radiative equilibrium methods is that they require
iteration, which makes them numerically very demanding. Bjorkman & Wood
recently proposed a frequency distribution adjustment scheme, which allows
radiative equilibrium Monte Carlo calculations to be performed without
iteration, by choosing the frequency of each re-emitted photon such that it
corrects for the incorrect spectrum of the previously re-emitted photons.
Although the method appears to yield correct results, we argue that its
theoretical basis is not completely transparent, and that it is not completely
clear whether this technique is an exact rigorous method, or whether it is just
a good and convenient approximation. We critically study the general problem of
how an already sampled distribution can be adjusted to a new distribution by
adding data points sampled from an adjustment distribution. We show that this
adjustment is not always possible, and that it depends on the shape of the
original and desired distributions, as well as on the relative number of data
points that can be added. Applying this theorem to radiative equilibrium Monte
Carlo calculations, we provide a firm theoretical basis for the frequency
distribution adjustment method of Bjorkman & Wood, and we demonstrate that this
method provides the correct frequency distribution through the additional
requirement of radiative equilibrium. We discuss the advantages and limitations
of this approach, and show that it can easily be combined with the presence of
additional heating sources and the concept of photon weighting. However, the
method may fail if small dust grains are included... (abridged)Comment: 17 pages, 2 figures, accepted for publication in New Astronom
Monte Carlo transition probabilities
Transition probabilities governing the interaction of energy packets and
matter are derived that allow Monte Carlo NLTE transfer codes to be constructed
without simplifying the treatment of line formation. These probabilities are
such that the Monte Carlo calculation asymptotically recovers the local
emissivity of a gas in statistical equilibrium. Numerical experiments with
one-point statistical equilibrium problems for Fe II and Hydrogen confirm this
asymptotic behaviour. In addition, the resulting Monte Carlo emissivities are
shown to be far less sensitive to errors in the populations of the emitting
levels than are the values obtained with the basic emissivity formula.Comment: Improved text. Accepted for publication in A&
Time Dependent Radiative Transfer Calculations for Supernovae
In previous papers we discussed results from fully time-dependent radiative
transfer models for core-collapse supernova (SN) ejecta, including the Type
II-peculiar SN 1987A, the more "generic" SN II-Plateau, and more recently Type
IIb/Ib/Ic SNe. Here we describe the modifications to our radiative modeling
code, CMFGEN, which allowed those studies to be undertaken. The changes allow
for time-dependent radiative transfer of SN ejecta in homologous expansion. In
the modeling we treat the entire SN ejecta, from the innermost layer that does
not fall back on the compact remnant out to the progenitor surface layers. From
our non-LTE time-dependent line-blanketed synthetic spectra, we compute the
bolometric and multi-band light curves: light curves and spectra are thus
calculated simultaneously using the same physical processes and numerics. These
upgrades, in conjunction with our previous modifications which allow the
solution of the time dependent rate equations, will improve the modeling of SN
spectra and light curves, and hence facilitate new insights into SN ejecta
properties, the SN progenitors and the explosion mechanism(s). CMFGEN can now
be applied to the modeling of all SN typesComment: 20 pages, 10 figures, to appear in MNRA
The effect of scattering on the structure and SED of protoplanetary disks
In this paper we investigate how the inclusion of scattering of the stellar
radiation into a passive flaring disk model affects its structure and spectral
energy distribution, and whether neglecting it could significantly decrease the
model reliability. In order to address these questions we construct a detailed
1+1D vertical structure model in which the scattering properties of the dust
can be varied. Models are presented with and without dust scattering, and for
different albedos and phase functions. It is found that scattering has the
effect of reducing the disk temperature at all heights, so that the disk
"shrinks", i.e., the the density at all intermediate heights decreases.
However, this effect in most cases is more than compensated by the increase of
the total extinction (absorption + scattering) cross section, so that the
surface scale height increases, and images in scattered light will see a
slightly thicker disk. The integrated infrared emission decreases as the albedo
increases, because an increasing part of the flux captured by the disk is
reflected away instead of absorbed and reprocessed. The reduction of the
infrared thermal emission of the disk is stronger at short wavelengths (near
infrared) and practically negligible at millimeter wavelengths. For relatively
low albedo (alb <~ 0.5), or for strongly forward-peaked scattering (g roughly
>0.8), the infrared flux reduction is relatively small.Comment: Accepted for publication in Astronomy & Astrophysic
Monte-Carlo methods for NLTE spectral synthesis of supernovae
We present JEKYLL, a new code for modelling of supernova (SN) spectra and
lightcurves based on Monte-Carlo (MC) techniques for the radiative transfer.
The code assumes spherical symmetry, homologous expansion and steady state for
the matter, but is otherwise capable of solving the time-dependent radiative
transfer problem in non-local-thermodynamic-equilibrium (NLTE). The method used
was introduced in a series of papers by Lucy, but the full time-dependent NLTE
capabilities of it have never been tested. Here, we have extended the method to
include non-thermal excitation and ionization as well as charge-transfer and
two-photon processes. Based on earlier work, the non-thermal rates are
calculated by solving the Spencer-Fano equation. Using a method previously
developed for the SUMO code, macroscopic mixing of the material is taken into
account in a statistical sense. In addition, a statistical Markov-chain model
is used to sample the emission frequency, and we introduce a method to control
the sampling of the radiation field. Except for a description of JEKYLL, we
provide comparisons with the ARTIS, SUMO and CMFGEN codes, which show good
agreement in the calculated spectra as well as the state of the gas. In
particular, the comparison with CMFGEN, which is similar in terms of physics
but uses a different technique, shows that the Lucy method does indeed converge
in the time-dependent NLTE case. Finally, as an example of the time-dependent
NLTE capabilities of JEKYLL, we present a model of a Type IIb SN, taken from a
set of models presented and discussed in detail in an accompanying paper. Based
on this model we investigate the effects of NLTE, in particular those arising
from non-thermal excitation and ionization, and find strong effects even on the
bolometric lightcurve. This highlights the need for full NLTE calculations when
simulating the spectra and lightcurves of SNe.Comment: Accepted for publication by Astronomy & Astrophysic
Monte Carlo transient phonons transport in silicon and germanium at nanoscales
Heat transport at nanoscales in semiconductors is investigated with a
statistical method. The Boltzmann Transport Equation (BTE) which characterize
phonons motion and interaction within the crystal lattice has been simulated
with a Monte Carlo technique. Our model takes into account media frequency
properties through the dispersion curves for longitudinal and transverse
acoustic branches. The BTE collisional term involving phonons scattering
processes is simulated with the Relaxation Times Approximation theory. A new
distribution function accounting for the collisional processes has been
developed in order to respect energy conservation during phonons scattering
events. This non deterministic approach provides satisfactory results in what
concerns phonons transport in both ballistic and diffusion regimes. The
simulation code has been tested with silicon and germanium thin films;
temperature propagation within samples is presented and compared to analytical
solutions (in the diffusion regime). The two materials bulk thermal
conductivity is retrieved for temperature ranging between 100 K and 500 K. Heat
transfer within a plane wall with a large thermal gradient (250 K-500 K) is
proposed in order to expose the model ability to simulate conductivity thermal
dependence on heat exchange at nanoscales. Finally, size effects and validity
of heat conduction law are investigated for several slab thicknesses
Monte Carlo transition probabilities. II
The macroscopic quantizations of matter into macro-atoms and radiant and
thermal energies into r- and k-energy packets initiated in Paper I is completed
with the definition of transition probabilities governing energy flows to and
from the thermal pool. The resulting Monte Carlo method is then applied to the
problem of computing the hydrogen spectrum of a Type II supernova. This test
problem is used to demonstrate the scheme's consistency as the number of energy
packets N -> infinity, to investigate the accuracy of Monte Carlo estimators of
radiative rates, and to illustrate the convergence characteristics of the
geometry-independent, constrained Lambda-iteration method employed to obtain
the NLTE stratifications of temperature and level populations. In addition, the
method's potential, when combined with analytic ionization and excitation
formulae, for obtaining useful approximate NLTE solutions is emphasized.Comment: 17 pages, 4 figure
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