2,656 research outputs found
Modelling ripples in Orion with coupled dust dynamics and radiative transfer
In light of the recent detection of direct evidence for the formation of
Kelvin-Helmholtz instabilities in the Orion nebula, we expand upon previous
modelling efforts by numerically simulating the shear-flow driven gas and dust
dynamics in locations where the H region and the molecular cloud
interact. We aim to directly confront the simulation results with the infrared
observations. Methods: To numerically model the onset and full nonlinear
development of the Kelvin-Helmholtz instability we take the setup proposed to
interpret the observations, and adjust it to a full 3D hydrodynamical
simulation that includes the dynamics of gas as well as dust. A dust grain
distribution with sizes between 5-250 nm is used, exploiting the gas+dust
module of the MPI-AMRVAC code, in which the dust species are represented by
several pressureless dust fluids. The evolution of the model is followed well
into the nonlinear phase. The output of these simulations is then used as input
for the SKIRT dust radiative transfer code to obtain infrared images at several
stages of the evolution, which can be compared to the observations. Results: We
confirm that a 3D Kelvin-Helmholtz instability is able to develop in the
proposed setup, and that the formation of the instability is not inhibited by
the addition of dust. Kelvin-Helmholtz billows form at the end of the linear
phase, and synthetic observations of the billows show striking similarities to
the infrared observations. It is pointed out that the high density dust regions
preferentially collect on the flanks of the billows. To get agreement with the
observed Kelvin-Helmholtz ripples, the assumed geometry between the background
radiation, the billows and the observer is seen to be of critical importance.Comment: 8 pages, 10 figure
Hierarchical octree and k-d tree grids for 3D radiative transfer simulations
A crucial ingredient for numerically solving the 3D radiative transfer
problem is the choice of the grid that discretizes the transfer medium. Many
modern radiative transfer codes, whether using Monte Carlo or ray tracing
techniques, are equipped with hierarchical octree-based grids to accommodate a
wide dynamic range in densities. We critically investigate two different
aspects of octree grids in the framework of Monte Carlo dust radiative
transfer. Inspired by their common use in computer graphics applications, we
test hierarchical k-d tree grids as an alternative for octree grids. On the
other hand, we investigate which node subdivision-stopping criteria are optimal
for constructing of hierarchical grids. We implemented a k-d tree grid in the
3D radiative transfer code SKIRT and compared it with the previously
implemented octree grid. We also considered three different node
subdivision-stopping criteria (based on mass, optical depth, and density
gradient thresholds). Based on a small suite of test models, we compared the
efficiency and accuracy of the different grids, according to various quality
metrics. For a given set of requirements, the k-d tree grids only require half
the number of cells of the corresponding octree. Moreover, for the same number
of grid cells, the k-d tree is characterized by higher discretization accuracy.
Concerning the subdivision stopping criteria, we find that an optical depth
criterion is not a useful alternative to the more standard mass threshold,
since the resulting grids show a poor accuracy. Both criteria can be combined;
however, in the optimal combination, for which we provide a simple approximate
recipe, this can lead to a 20% reduction in the number of cells needed to reach
a certain grid quality. An additional density gradient threshold criterion can
be added that solves the problem of poorly resolving sharp edges and...
(abridged).Comment: 10 pages, 6 figures. Accepted for publication in A&
Trasplante de Pelo en la Alopecia Fibrosante Frontal: ¿Parte de la Solución?
info:eu-repo/semantics/publishedVersio
New directional results and determination of absolute archaeointensity using both the classical Thellier and the multi-specimen procedures for two kilns excavated at Osterietta, Italy
Archaeointensity determinations from Italy: new data and the Earth's magnetic field strength variation over the past three millennia.
Large and small-scale structures and the dust energy balance problem in spiral galaxies
The interstellar dust content in galaxies can be traced in extinction at
optical wavelengths, or in emission in the far-infrared. Several studies have
found that radiative transfer models that successfully explain the optical
extinction in edge-on spiral galaxies generally underestimate the observed
FIR/submm fluxes by a factor of about three. In order to investigate this
so-called dust energy balance problem, we use two Milky Way-like galaxies
produced by high-resolution hydrodynamical simulations. We create mock optical
edge-on views of these simulated galaxies (using the radiative transfer code
SKIRT), and we then fit the parameters of a basic spiral galaxy model to these
images (using the fitting code FitSKIRT). The basic model includes smooth
axisymmetric distributions along a S\'ersic bulge and exponential disc for the
stars, and a second exponential disc for the dust. We find that the dust mass
recovered by the fitted models is about three times smaller than the known dust
mass of the hydrodynamical input models. This factor is in agreement with
previous energy balance studies of real edge-on spiral galaxies. On the other
hand, fitting the same basic model to less complex input models (e.g. a smooth
exponential disc with a spiral perturbation or with random clumps), does
recover the dust mass of the input model almost perfectly. Thus it seems that
the complex asymmetries and the inhomogeneous structure of real and
hydrodynamically simulated galaxies are a lot more efficient at hiding dust
than the rather contrived geometries in typical quasi-analytical models. This
effect may help explain the discrepancy between the dust emission predicted by
radiative transfer models and the observed emission in energy balance studies
for edge-on spiral galaxies.Comment: 9 pages, 5 figures, accepted for publication in A&
Mutual coupling effects on antenna radiation pattern: An experimental study applied to interferometric radiometers
New archaeointensity results from Italian kilns and archaeomagnetic dating based on the full geomagnetic field vector
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