8,343 research outputs found
Smoke and Shadows: Rendering and Light Interaction of Smoke in Real-Time Rendered Virtual Environments
Realism in computer graphics depends upon digitally representing what we see in the world with careful attention to detail, which usually requires a high degree of complexity in modelling the scene. The inevitable trade-off between realism and performance means that new techniques that aim to improve the visual fidelity of a scene must do so without compromising the real-time rendering performance. We describe and discuss a simple method for realistically casting shadows from an opaque solid object through a GPU (graphics processing unit) based particle system representing natural phenomena, such as smoke
AREPO-RT: Radiation hydrodynamics on a moving mesh
We introduce AREPO-RT, a novel radiation hydrodynamic (RHD) solver for the
unstructured moving-mesh code AREPO. Our method solves the moment-based
radiative transfer equations using the M1 closure relation. We achieve second
order convergence by using a slope limited linear spatial extrapolation and a
first order time prediction step to obtain the values of the primitive
variables on both sides of the cell interface. A Harten-Lax-Van Leer flux
function, suitably modified for moving meshes, is then used to solve the
Riemann problem at the interface. The implementation is fully conservative and
compatible with the individual timestepping scheme of AREPO. It incorporates
atomic Hydrogen (H) and Helium (He) thermochemistry, which is used to couple
the ultra-violet (UV) radiation field to the gas. Additionally, infrared
radiation is coupled to the gas under the assumption of local thermodynamic
equilibrium between the gas and the dust. We successfully apply our code to a
large number of test problems, including applications such as the expansion of
regions, radiation pressure driven outflows and the levitation
of optically thick layer of gas by trapped IR radiation. The new implementation
is suitable for studying various important astrophysical phenomena, such as the
effect of radiative feedback in driving galactic scale outflows, radiation
driven dusty winds in high redshift quasars, or simulating the reionisation
history of the Universe in a self consistent manner.Comment: v2, accepted for publication in MNRAS, changed to a Strang split
scheme to achieve second order convergenc
Mapping CO Gas in the GG Tauri A Triple System with 50 AU Spatial Resolution
We aim to unveil the observational imprint of physical mechanisms that govern
planetary formation in the young, multiple system GG Tau A. We present ALMA
observations of CO and CO 3-2 and 0.9 mm continuum emission with
0.35" resolution. The CO 3-2 emission, found within the cavity of the
circumternary dust ring (at radius AU) where no CO emission is
detected, confirms the presence of CO gas near the circumstellar disk of GG Tau
Aa. The outer disk and the recently detected hot spot lying at the outer edge
of the dust ring are mapped both in CO and CO. The gas emission
in the outer disk can be radially decomposed as a series of slightly
overlapping Gaussian rings, suggesting the presence of unresolved gaps or dips.
The dip closest to the disk center lies at a radius very close to the hot spot
location at ~AU. The CO excitation conditions indicate that the
outer disk remains in the shadow of the ring. The hot spot probably results
from local heating processes. The two latter points reinforce the hypothesis
that the hot spot is created by an embedded proto-planet shepherding the outer
disk.Comment: 8 pages, 4 figures. Accepted by Ap
Shadows cast on the transition disk of HD 135344B. Multiwavelength VLT/SPHERE polarimetric differential imaging
The protoplanetary disk around the F-type star HD 135344B (SAO 206462) is in
a transition stage and shows many intriguing structures both in scattered light
and thermal (sub-)millimeter emission which are possibly related to planet
formation processes. We study the morphology and surface brightness of the disk
in scattered light to gain insight into the innermost disk regions, the
formation of protoplanets, planet-disk interactions traced in the surface and
midplane layers, and the dust grain properties of the disk surface. We have
carried out high-contrast polarimetric differential imaging (PDI) observations
with VLT/SPHERE and obtained polarized scattered light images with ZIMPOL in R-
and I-band and with IRDIS in Y- and J-band. The scattered light images reveal
with unprecedented angular resolution and sensitivity the spiral arms as well
as the 25 au cavity of the disk. Multiple shadow features are discovered on the
outer disk with one shadow only being present during the second observation
epoch. A positive surface brightness gradient is observed in the stellar
irradiation corrected images in southwest direction possibly due to an
azimuthally asymmetric perturbation of the temperature and/or surface density
by the passing spiral arms. The disk integrated polarized flux, normalized to
the stellar flux, shows a positive trend towards longer wavelengths which we
attribute to large aggregate dust grains in the disk surface. Part of the the
non-azimuthal polarization signal in the Uphi image of the J-band observation
could be the result of multiple scattering in the disk. The detected shadow
features and their possible variability have the potential to provide insight
into the structure of and processes occurring in the innermost disk regions.Comment: Accepted for publication in A&A, 20 pages, 15 figure
Hier ist wahrhaftig ein Loch im Himmel: The NGC1999 dark globule is not a globule
The NGC1999 reflection nebula features a dark patch with a size of ~10 000 AU, which has been interpreted as a small, dense foreground globule and possible site of imminent star formation. We present Herschel PACS far-infrared 70 and 160 μmmaps, which reveal a flux deficit at the location of the globule. We estimate the globule mass needed to produce such an absorption feature to be a few tenths to a few M_⊙. Inspired by this Herschel observation, we obtained APEX LABOCA and SABOCA submillimeter continuum maps, and Magellan PANIC near-infrared images of the region. We do not detect a submillimer source at the location of the Herschel flux decrement; furthermore our observations place an upper limit on the mass of the globule of ~2.4×10^(−2) M_⊙. Indeed, the submillimeter maps appear to show a flux depression as well. Furthermore, the near–infrared images detect faint background stars that are less affected by extinction inside the dark patch than in its surroundings. We suggest that the dark patch is in fact a hole or
cavity in the material producing the NGC1999 reflection nebula, excavated by protostellar jets from the V380 Ori multiple system
Hier ist wahrhaftig ein Loch im Himmel - The NGC 1999 dark globule is not a globule
The NGC 1999 reflection nebula features a dark patch with a size of ~10,000
AU, which has been interpreted as a small, dense foreground globule and
possible site of imminent star formation. We present Herschel PACS far-infrared
70 and 160mum maps, which reveal a flux deficit at the location of the globule.
We estimate the globule mass needed to produce such an absorption feature to be
a few tenths to a few Msun. Inspired by this Herschel observation, we obtained
APEX LABOCA and SABOCA submillimeter continuum maps, and Magellan PANIC
near-infrared images of the region. We do not detect a submillimer source at
the location of the Herschel flux decrement; furthermore our observations place
an upper limit on the mass of the globule of ~2.4x10^-2 Msun. Indeed, the
submillimeter maps appear to show a flux depression as well. Furthermore, the
near-infrared images detect faint background stars that are less affected by
extinction inside the dark patch than in its surroundings. We suggest that the
dark patch is in fact a hole or cavity in the material producing the NGC 1999
reflection nebula, excavated by protostellar jets from the V 380 Ori multiple
system.Comment: accepted for the A&A Herschel issue; 7 page
Drishti, a volume exploration and presentation tool
Among several rendering techniques for volumetric data, direct volume rendering is a powerful visualization tool for a wide variety of applications. This paper describes the major features of hardware based volume exploration and presentation tool - Drishti. The word, Drishti, stands for vision or insight in Sanskrit, an ancient Indian language. Drishti is a cross-platform open-source volume rendering system that delivers high quality, state of the art renderings. The features in Drishti include, though not limited to, production quality rendering, volume sculpting, multi-resolution zooming, transfer function blending, profile generation, measurement tools, mesh generation, stereo/anaglyph/crosseye renderings. Ultimately, Drishti provides an intuitive and powerful interface for choreographing animations
A Deep HST Search for Escaping Lyman Continuum Flux at z~1.3: Evidence for an Evolving Ionizing Emissivity
We have obtained deep Hubble Space Telescope far-UV images of 15 starburst
galaxies at z~1.3 in the GOODS fields to search for escaping Lyman continuum
photons. These are the deepest far-UV images m_{AB}=28.7, 3\sigma, 1" diameter)
over this large an area (4.83 arcmin^2) and provide the best escape fraction
constraints for any galaxy at any redshift. We do not detect any individual
galaxies, with 3\sigma limits to the Lyman Continuum (~700 \AA) flux 50--149
times fainter (in f_nu) than the rest-frame UV (1500 \AA) continuum fluxes.
Correcting for the mean IGM attenuation (factor ~2), as well as an intrinsic
stellar Lyman Break (~3), these limits translate to relative escape fraction
limits of f_{esc,rel}<[0.03,0.21]. The stacked limit is
f_{esc,rel}(3\sigma)<0.02. We use a Monte Carlo simulation to properly account
for the expected distribution of IGM opacities. When including constraints from
previous surveys at z~1.3 we find that, at the 95% confidence level, no more
than 8% of star--forming galaxies at z~1.3 can have relative escape fractions
greater than 0.50. Alternatively, if the majority of galaxies have low, but
non-zero, escaping Lyman Continuum, the escape fraction can not be more than
0.04. Both the stacked limits, and the limits from the Monte Carlo simulation
suggest that the average ionizing emissivity (relative to non-ionizing UV
emissivity) at z~1.3 is significantly lower than has been observed in Lyman
Break Galaxies (LBGs) at z~3. If the ionizing emissivity of star-forming
galaxies is in fact increasing with redshift, it would help to explain the high
photoionization rates seen in the IGM at z>4 and reionization of the
intergalactic medium at z>6. [Abridged]Comment: Submitted to ApJ (Nov. 6) Comments Welcome. 11 pages, 8 figure
Dusty spirals triggered by shadows in transition discs
Context. Despite the recent discovery of spiral-shaped features in
protoplanetary discs in the near-infrared and millimetric wavelengths, there is
still an active discussion to understand how they formed. In fact, the spiral
waves observed in discs around young stars can be due to different physical
mechanisms: planet/companion torques, gravitational perturbations or
illumination effects. Aims. We study the spirals formed in the gaseous phase
due to two diametrically opposed shadows cast at fixed disc locations. The
shadows are created by an inclined non-precessing disc inside the cavity, which
is assumed to be optically thick. In particular, we analyse the effect of these
spirals on the dynamics of the dust particles and discuss their detectability
in transition discs. Methods. We perform gaseous hydrodynamical simulations
with shadows, then we compute the dust evolution on top of the gaseous
distribution, and finally we produce synthetic ALMA observations of the dust
emission based on radiative transfer calculations. Results. Our main finding is
that mm- to cm-sized dust particles are efficiently trapped inside the
shadow-triggered spirals. We also observe that particles of various sizes
starting at different stellocentric distances are well mixed inside these
pressure maxima. This dynamical effect would favour grain growth and affect the
resulting composition of planetesimals in the disc. In addition, our radiative
transfer calculations show spiral patterns in the disc at 1.6 {\mu}m and 1.3
mm. Due to their faint thermal emission (compared to the bright inner regions
of the disc) the spirals cannot be detected with ALMA. Our synthetic
observations prove however that shadows are observable as dips in the thermal
emission.Comment: 15 pages, 11 figures, accepted for publication in A&
Cooling in the shade of warped transition disks
The mass of the gaseous reservoir in young circumstellar disks is a crucial
initial condition for the formation of planetary systems, but estimates vary by
orders of magnitude. In some disks with resolvable cavities, sharp inner disk
warps cast two-sided shadows on the outer rings; can the cooling of the gas as
it crosses the shadows bring constraints on its mass? The finite cooling
timescale should result in dust temperature decrements shifted ahead of the
optical/IR shadows in the direction of rotation. However, some systems show
temperature drops, while others do not. The depth of the drops and the
amplitude of the shift depend on the outer disk surface density Sigma through
the extent of cooling during the shadow crossing time, and also on the
efficiency of radiative diffusion. These phenomena may bear observational
counterparts, which we describe with a simple one-dimensional model. An
application to the HD142527 disk suggests an asymmetry in its shadows, and
predicts a >~10deg shift for a massive gaseous disk, with peak Sigma > 8.3
g/cm2. Another application to the DoAr44 disk limits the peak surface density
to Sigma < 13g/cm2Comment: accepted to MNRAS Letter
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