740 research outputs found
A semi-analytical model of disk evaporation by thermal conduction
The conditions for disk evaporation by electron thermal conduction are
examined, using a simplified semi--analytical 1-D model. The model is based on
the mechanism proposed by Meyer & Meyer-Hofmeister (1994) in which an advection
dominated accretion flow evaporates the top layers from the underlying disk by
thermal conduction. The evaporation rate is calculated as a function of the
density of the advective flow, and an analysis is made of the time scales and
length scales of the dynamics of the advective flow. It is shown that
evaporation can only completely destroy the disk if the conductive length scale
is of the order of the radius. This implies that radial conduction is an
essential factor in the evaporation process. The heat required for evaporation
is in fact produced at small radii and transported radially towards the
evaporation region.Comment: 9 pages, 4 postscript figures, accepted for publication in A&
Size-sorting dust grains in the surface layers of protoplanetary disks
Aims: We wish to investigate what the effect of dust sedimentation is on the
observed 10 mum feature of protoplanetary disks and how this may affect the
interpretation of the observations.
Methods: Using a combination of modeling tools, we simulated the
sedimentation of a dust grain size distribution in an axisymmetric 2-D model of
a turbulent protoplanetary disk, and we used a radiative transfer program to
compute the resulting spectra.
Results: We find that the sedimentation can turn a flat feature into a pointy
one, but only to a limited degree and for a very limited set of particle size
distributions. Only if we have a bimodal size distribution, i.e. a very small
grain population and a bigger grain population, do we find that the
transformation from a flat to a pointy feature upon dust sedimentation is
strong. However, our model shows that, if sedimentation is the sole reason for
the variety of silicate feature strengths observed in protoplanetary disks,
then we would expect to find a correlation such that disks with weak mid- to
far-infrared excess have a stronger 10 mum silicate feature than disks with a
strong mid- to far-infrared excess. If this is contrary to what is observed,
then this would indicate that sedimentation cannot be the main reason for the
variety of 10 mum silicate features observed in protoplanetary disks.Comment: Astronomy and Astrophysics, in pres
Radiative transfer in very optically thick circumstellar disks
In this paper we present two efficient implementations of the diffusion
approximation to be employed in Monte Carlo computations of radiative transfer
in dusty media of massive circumstellar disks. The aim is to improve the
accuracy of the computed temperature structure and to decrease the computation
time. The accuracy, efficiency and applicability of the methods in various
corners of parameter space are investigated. The effects of using these methods
on the vertical structure of the circumstellar disk as obtained from
hydrostatic equilibrium computations are also addressed. Two methods are
presented. First, an energy diffusion approximation is used to improve the
accuracy of the temperature structure in highly obscured regions of the disk,
where photon counts are low. Second, a modified random walk approximation is
employed to decrease the computation time. This modified random walk ensures
that the photons that end up in the high-density regions can quickly escape to
the lower density regions, while the energy deposited by these photons in the
disk is still computed accurately. A new radiative transfer code, MCMax, is
presented in which both these diffusion approximations are implemented. These
can be used simultaneously to increase both computational speed and decrease
statistical noise. We conclude that the diffusion approximations allow for fast
and accurate computations of the temperature structure, vertical disk structure
and observables of very optically thick circumstellar disks.Comment: Accepted for publication in A&
Interferometer predictions with triangulated images: solving the multi-scale problem
Interferometers play an increasingly important role for spatially resolved
observations. If employed at full potential, interferometry can probe an
enormous dynamic range in spatial scale. Interpretation of the observed
visibilities requires the numerical compu- tation of Fourier integrals over the
synthetic model images. To get the correct values of these integrals, the model
images must have the right size and resolution. Insufficient care in these
choices can lead to wrong results. We present a new general-purpose scheme for
the computation of visibilities of radiative transfer images. Our method
requires a model image that is a list of intensities at arbitrarily placed
positions on the image-plane. It creates a triangulated grid from these
vertices, and assumes that the intensity inside each triangle of the grid is a
linear function. The Fourier integral over each triangle is then evaluated with
an analytic expression and the complex visibility of the entire image is then
the sum of all triangles. The result is a robust Fourier trans- form that does
not suffer from aliasing effects due to grid regularities. The method
automatically ensures that all structure contained in the model gets reflected
in the Fourier transform.Comment: 9 pages, 7 figures, accepted for publication in MNRA
An efficient algorithm for two-dimensional radiative transfer in axisymmetric circumstellar envelopes and disks
We present an algorithm for two-dimensional radiative transfer in
axisymmetric, circumstellar media. The formal integration of the transfer
equation is performed by a generalization of the short characteristics (SC)
method to spherical coordinates. Accelerated Lambda Iteration (ALI) and Ng's
algorithm are used to converge towards a solution. By taking a logarithmically
spaced radial coordinate grid, the method has the natural capability of
treating problems that span several decades in radius, in the most extreme case
from the stellar radius up to parsec scale. Flux conservation is guaranteed in
spherical coordinates by a particular choice of discrete photon directions and
a special treatment of nearly-radially outward propagating radiation. The
algorithm works well from zero up to very high optical depth, and can be used
for a wide variety of transfer problems, including non-LTE line formation, dust
continuum transfer and high temperature processes such as compton scattering.
In this paper we focus on multiple scattering off dust grains and on non-LTE
transfer in molecular and atomic lines. Line transfer is treated according to
an ALI scheme for multi-level atoms/molecules, and includes both random and
systematic velocity fields. The algorithms are implemented in a multi-purpose
user-friendly radiative transfer program named RADICAL. We present two example
computations: one of dust scattering in the Egg Nebula, and one of non-LTE line
formation in rotational transitions of HCO in a flattened protostellar
collapsing cloud.Comment: 18 pages, 32 figure
Flaring and self-shadowed disks around Herbig Ae stars: simulations for 10 micron interferometers
We present simulations of the interferometric visibilities of Herbig Ae star
disks. We investigate whether interferometric measurements in the 10 micrometer
atmospheric window are sensitive to the presence of an increased scale height
at the inner disk edge, predicted by recent models. Furthermore, we investigate
whether such measurements can discriminate between disks with a ``flaring''
geometry and disks with a ``flat'' geometry. We show that both these questions
can be addressed, using measurements at a small number of appropriately chosen
baselines. The classification of Herbig Ae stars in two groups, based on the
appearance of the spectral energy distribution (SED), has been attributed to a
difference in disk geometry. Sources with a group I SED would have a flaring
outer disk geometry, whereas the disk of group II sources is proposed to be
flat (or ``self-shadowed''). We show that this hypothesis can be tested using
long-baseline interferometric measurements in the micrometer atmospheric
window.Comment: 10 pages, 7 figures, accepted for publiction in Astronomy and
Astrophysic
Radiation hydrodynamics including irradiation and adaptive mesh refinement with AZEuS. I. Methods
Aims. The importance of radiation to the physical structure of protoplanetary
disks cannot be understated. However, protoplanetary disks evolve with time,
and so to understand disk evolution and by association, disk structure, one
should solve the combined and time-dependent equations of radiation
hydrodynamics.
Methods. We implement a new implicit radiation solver in the AZEuS adaptive
mesh refinement magnetohydrodynamics fluid code. Based on a hybrid approach
that combines frequency-dependent ray-tracing for stellar irradiation with
non-equilibrium flux limited diffusion, we solve the equations of radiation
hydrodynamics while preserving the directionality of the stellar irradiation.
The implementation permits simulations in Cartesian, cylindrical, and spherical
coordinates, on both uniform and adaptive grids.
Results. We present several hydrostatic and hydrodynamic radiation tests
which validate our implementation on uniform and adaptive grids as appropriate,
including benchmarks specifically designed for protoplanetary disks. Our
results demonstrate that the combination of a hybrid radiation algorithm with
AZEuS is an effective tool for radiation hydrodynamics studies, and produces
results which are competitive with other astrophysical radiation hydrodynamics
codes.Comment: 15 pages, 10 figures, accepted for publication in A&
Gas- and dust evolution in protoplanetary disks
Context. Current models of the size- and radial evolution of dust in
protoplanetary disks generally oversimplify either the radial evolution of the
disk (by focussing at one single radius or by using steady state disk models)
or they assume particle growth to proceed monodispersely or without
fragmentation. Further studies of protoplanetary disks - such as observations,
disk chemistry and structure calculations or planet population synthesis models
- depend on the distribution of dust as a function of grain size and radial
position in the disk.
Aims. We attempt to improve upon current models to be able to investigate how
the initial conditions, the build-up phase, and the evolution of the
protoplanetary disk influence growth and transport of dust.
Methods. We introduce a new version of the model of Brauer et al. (2008) in
which we now include the time-dependent viscous evolution of the gas disk, and
in which more advanced input physics and numerical integration methods are
implemented.
Results. We show that grain properties, the gas pressure gradient, and the
amount of turbulence are much more influencing the evolution of dust than the
initial conditions or the build-up phase of the protoplanetary disk. We
quantify which conditions or environments are favorable for growth beyond the
meter size barrier. High gas surface densities or zonal flows may help to
overcome the problem of radial drift, however already a small amount of
turbulence poses a much stronger obstacle for grain growth.Comment: accepted to A&
Dust-driven viscous ring-instability in protoplanetary disks
Protoplanetary disks often appear as multiple concentric rings in dust
continuum emission maps and scattered light images. These features are often
associated with possible young planets in these disks. Many non-planetary
explanations have also been suggested, including snow lines, dead zones and
secular gravitational instabilities in the dust. In this paper we suggest
another potential origin. The presence of copious amounts of dust tends to
strongly reduce the conductivity of the gas, thereby inhibiting the
magneto-rotational instability, and thus reducing the turbulence in the disk.
From viscous disk theory it is known that a disk tends to increase its surface
density in regions where the viscosity (i.e. turbulence) is low. Local maxima
in the gas pressure tend to attract dust through radial drift, increasing the
dust content even more. We investigate mathematically if this could potentially
lead to a feedback loop in which a perturbation in the dust surface density
could perturb the gas surface density, leading to increased dust drift and thus
amplification of the dust perturbation and, as a consequence, the gas
perturbation. We find that this is indeed possible, even for moderately small
dust grain sizes, which drift less efficiently, but which are more likely to
affect the gas ionization degree. We speculate that this instability could be
triggered by the small dust population initially, and when the local pressure
maxima are strong enough, the larger dust grains get trapped and lead to the
familiar ring-like shapes. We also discuss the many uncertainties and
limitations of this model.Comment: Accepted for publication in A&
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