8,038 research outputs found
Axisymmetric circumstellar interaction in supernovae
Multiwavelength observations of Type II supernovae have shown evidence for
the interaction of supernovae with the dense slow winds from the red supergiant
progenitor stars. Observations of planetary nebulae and the nebula around SN
1987A show that the slow winds from extended stars frequently have an axisymme-
tric structure with a high density in the equatorial plane. We have carried out
numerical calculations of the interaction of a supernova with such an axisymme-
tric density distribution. For small values of the angular density gradient at
the pole, the asymmetry in the interaction shell is greater than, but close to,
that expected from purely radial motion. If the angular density gradient is
above a moderate value, the flow qualitatively changes and a protrusion emerges
along the axis. For a power-law supernova density profile, the flow approaches
a self-similar state in which the protrusion length is times the radius
of the main shell. The critical density gradient is larger for steeper density
profiles of the ejecta. Most of our calculations are axisymmetric, but we have
carried out a 3-dimensional calculation to show that the protrusion is not a
numerical artifact along the symmetry axis. For typical supernova parameters,
the protrusions take several years to develop. The appearance of the
shell with protrusions is similar to that observed in VLBI radio images of the
remnant 41.9 +58 in M82 and, possibly, of SN 1986J. We also considered the
possibility of asymmetric ejecta and found that it had a relatively small
effect on the asymmetry of the interaction region.Comment: 22 page postscript file (gzipped and uuencoded), 10 gzipped
postscript figures may be retrieved from
ftp://www.astro.su.se/pub/supernova/blc96_asym/ Submitted to Ap
Recovering facial shape using a statistical model of surface normal direction
In this paper, we show how a statistical model of facial shape can be embedded within a shape-from-shading algorithm. We describe how facial shape can be captured using a statistical model of variations in surface normal direction. To construct this model, we make use of the azimuthal equidistant projection to map the distribution of surface normals from the polar representation on a unit sphere to Cartesian points on a local tangent plane. The distribution of surface normal directions is captured using the covariance matrix for the projected point positions. The eigenvectors of the covariance matrix define the modes of shape-variation in the fields of transformed surface normals. We show how this model can be trained using surface normal data acquired from range images and how to fit the model to intensity images of faces using constraints on the surface normal direction provided by Lambert's law. We demonstrate that the combination of a global statistical constraint and local irradiance constraint yields an efficient and accurate approach to facial shape recovery and is capable of recovering fine local surface details. We assess the accuracy of the technique on a variety of images with ground truth and real-world images
CNN-based Real-time Dense Face Reconstruction with Inverse-rendered Photo-realistic Face Images
With the powerfulness of convolution neural networks (CNN), CNN based face
reconstruction has recently shown promising performance in reconstructing
detailed face shape from 2D face images. The success of CNN-based methods
relies on a large number of labeled data. The state-of-the-art synthesizes such
data using a coarse morphable face model, which however has difficulty to
generate detailed photo-realistic images of faces (with wrinkles). This paper
presents a novel face data generation method. Specifically, we render a large
number of photo-realistic face images with different attributes based on
inverse rendering. Furthermore, we construct a fine-detailed face image dataset
by transferring different scales of details from one image to another. We also
construct a large number of video-type adjacent frame pairs by simulating the
distribution of real video data. With these nicely constructed datasets, we
propose a coarse-to-fine learning framework consisting of three convolutional
networks. The networks are trained for real-time detailed 3D face
reconstruction from monocular video as well as from a single image. Extensive
experimental results demonstrate that our framework can produce high-quality
reconstruction but with much less computation time compared to the
state-of-the-art. Moreover, our method is robust to pose, expression and
lighting due to the diversity of data.Comment: Accepted by IEEE Transactions on Pattern Analysis and Machine
Intelligence, 201
Hydrodynamical Non-radiative Accretion Flows in Two-Dimensions
Two-dimensional (axially symmetric) numerical hydrodynamical calculations of
accretion flows which cannot cool through emission of radiation are presented.
The calculations begin from an equilibrium configuration consisting of a thick
torus with constant specific angular momentum. Accretion is induced by the
addition of a small anomalous azimuthal shear stress which is characterized by
a function \nu. We study the flows generated as the amplitude and form of \nu
are varied. A spherical polar grid which spans more than two orders of
magnitude in radius is used to resolve the flow over a wide range of spatial
scales. We find that convection in the inner regions produces significant
outward mass motions that carry away both the energy liberated by, and a large
fraction of the mass participating in, the accretion flow. Although the
instantaneous structure of the flow is complex and dominated by convective
eddies, long time averages of the dynamical variables show remarkable
correspondence to certain steady-state solutions. Near the equatorial plane,
the radial profiles of the time-averaged variables are power-laws with an index
that depends on the radial scaling of the shear stress. We find that regardless
of the adiabatic index of the gas, or the form or magnitude of the shear
stress, the mass inflow rate is a strongly increasing function of radius, and
is everywhere nearly exactly balanced by mass outflow. The net mass accretion
rate through the disc is only a fraction of the rate at which mass is supplied
to the inflow at large radii, and is given by the local, viscous accretion rate
associated with the flow properties near the central object.Comment: 33 pages, 12 figures, accepted by MNRA
A near-infrared morphological comparison of high-redshift submm and radio galaxies: massive star-forming discs vs relaxed spheroids
We present deep, high-quality K-band images of complete subsamples of
powerful radio and sub-mm galaxies at z=2. The data were obtained in the best
available seeing at UKIRT and Gemini North, with integration times scaled to
ensure that comparable rest-frame surface brightness levels are reached for all
galaxies. We fit two-dimensional axi-symmetric galaxy models to determine
galaxy morphologies at rest-frame optical wavelengths > 4000A, varying
luminosity, axial ratio, half-light radius, and Sersic index. We find that,
while some images show evidence of galaxy interactions, >95% of the rest-frame
optical light in all galaxies is well-described by these simple models. We also
find a clear difference in morphology between these two classes of galaxy; fits
to the individual images and image stacks reveal that the radio galaxies are
moderately large (=8.4+-1.1kpc; median r{1/2}=7.8), de Vaucouleurs
spheroids ( = 4.07+-0.27; median n=3.87), while the sub-mm galaxies appear
to be moderately compact (=3.4+-0.3kpc; median r{1/2}=3.1kpc)
exponential discs (=1.44+-0.16; median n=1.08). We show that the z=2 radio
galaxies display a well-defined Kormendy relation but that, while larger than
other recently-studied high-z massive galaxy populations, they are still ~1.5
times smaller than their local counterparts. The scalelengths of the starlight
in the sub-mm galaxies are comparable to those reported for the molecular gas.
Their sizes are also similar to those of comparably massive quiescent galaxies
at z>1.5. In terms of stellar mass surface density, the majority of the radio
galaxies lie within the locus defined by local ellipticals. In contrast, while
best modelled as discs, most of the sub-mm galaxies have higher stellar mass
densities than local galaxies, and appear destined to evolve into present-day
massive ellipticals.Comment: 24 pages, 9 figure
The Current Ability to Test Theories of Gravity with Black Hole Shadows
Our Galactic Center, Sagittarius A* (Sgr A*), is believed to harbour a
supermassive black hole (BH), as suggested by observations tracking individual
orbiting stars. Upcoming sub-millimetre very-long-baseline-interferometry
(VLBI) images of Sgr A* carried out by the Event-Horizon-Telescope
Collaboration (EHTC) are expected to provide critical evidence for the
existence of this supermassive BH. We assess our present ability to use EHTC
images to determine if they correspond to a Kerr BH as predicted by Einstein's
theory of general relativity (GR) or to a BH in alternative theories of
gravity. To this end, we perform general-relativistic magnetohydrodynamical
(GRMHD) simulations and use general-relativistic radiative transfer (GRRT)
calculations to generate synthetic shadow images of a magnetised accretion flow
onto a Kerr BH. In addition, and for the first time, we perform GRMHD
simulations and GRRT calculations for a dilaton BH, which we take as a
representative solution of an alternative theory of gravity. Adopting the VLBI
configuration from the 2017 EHTC campaign, we find that it could be extremely
difficult to distinguish between BHs from different theories of gravity, thus
highlighting that great caution is needed when interpreting BH images as tests
of GR.Comment: Published in Nature Astronomy on 16.04.18 (including supplementary
information); simulations at https://blackholecam.org/telling_bhs_apart
- âŠ