19,425 research outputs found
Linear Differential Constraints for Photo-polarimetric Height Estimation
In this paper we present a differential approach to photo-polarimetric shape
estimation. We propose several alternative differential constraints based on
polarisation and photometric shading information and show how to express them
in a unified partial differential system. Our method uses the image ratios
technique to combine shading and polarisation information in order to directly
reconstruct surface height, without first computing surface normal vectors.
Moreover, we are able to remove the non-linearities so that the problem reduces
to solving a linear differential problem. We also introduce a new method for
estimating a polarisation image from multichannel data and, finally, we show it
is possible to estimate the illumination directions in a two source setup,
extending the method into an uncalibrated scenario. From a numerical point of
view, we use a least-squares formulation of the discrete version of the
problem. To the best of our knowledge, this is the first work to consider a
unified differential approach to solve photo-polarimetric shape estimation
directly for height. Numerical results on synthetic and real-world data confirm
the effectiveness of our proposed method.Comment: To appear at International Conference on Computer Vision (ICCV),
Venice, Italy, October 22-29, 201
Magnetic field topology of the RS CVn star II Pegasi
The dynamo processes in cool active stars generate complex magnetic fields
responsible for prominent surface stellar activity and variability at different
time scales. For a small number of cool stars magnetic field topologies were
reconstructed from the time series of spectropolarimetric observations using
the Zeeman Doppler imaging (ZDI) method. In this study we follow a long-term
evolution of the magnetic field topology of the RS CVn binary star II Peg. We
collected high-resolution circular polarisation observations of II Peg using
the SOFIN spectropolarimeter at the Nordic Optical Telescope. These data cover
12 epochs spread over 7 years. A multi-line diagnostic technique in combination
with a new ZDI code is applied to interpret these observations. Magnetic
inversions using these data reveals evolving magnetic fields with typical local
strengths of 0.5-1.0 kG and complex topologies. Despite using a self-consistent
magnetic and temperature mapping technique, we do not find a clear correlation
between magnetic and temperature features in the ZDI maps. Neither do we
confirm the presence of persistent azimuthal field rings found in other RS CVn
stars. Reconstruction of the magnetic field topology of II Peg reveals
significant evolution of both the surface magnetic field structure and the
extended magnetospheric field geometry. From 2004 to 2010 the total field
energy drastically declined and the field became less axisymmetric. This also
coincided with the transition from predominantly poloidal to mainly toroidal
field topology. A qualitative comparison of the ZDI maps of II Peg with the
prediction of dynamo theory suggests that the magnetic field in this star is
produced mainly by the turbulent alpha^2 dynamo rather than the solar
alphaOmega dynamo. Our results do not show a clear active longitude system, nor
is there an evidence of the presence of an azimuthal dynamo wave.Comment: 20 pages, 10 figures; accepted for publication in Astronomy &
Astrophysic
Surface magnetism of cool stars
Magnetic fields are essential ingredients of many physical processes in the
interiors and envelopes of cool stars. Yet their direct detection and
characterisation is notoriously difficult, requiring high-quality observations
and advanced analysis techniques. Significant progress has been recently
achieved by several types of direct magnetic field studies on the surfaces of
cool active stars. In particular, complementary techniques of the field
topology mapping with polarisation data and total magnetic flux measurements
from intensity spectra have been systematically applied to different classes of
active stars leading to interesting and occasionally controversial results. In
this paper we summarise the current status of direct magnetic field studies of
cool stars, and investigations of surface inhomogeneities caused by the field,
based on the material presented at the Cool Stars 19 splinter session.Comment: Summary of the splinter session "Surface Magnetism of Cool Stars" at
the Cool Stars 19 conference; to be published in Astronomische Nachrichte
Weak Lensing Mass Reconstruction of the Galaxy Cluster Abell 209
Weak lensing applied to deep optical images of clusters of galaxies provides
a powerful tool to reconstruct the distribution of the gravitating mass
associated to these structures. We use the shear signal extracted by an
analysis of deep exposures of a region centered around the galaxy cluster Abell
209, at redshift z=0.2, to derive both a map of the projected mass distribution
and an estimate of the total mass within a characteristic radius. We use a
series of deep archival R-band images from CFHT-12k, covering an area of 0.3
deg^2. We determine the shear of background galaxy images using a new
implementation of the modified Kaiser-Squires-Broadhurst pipeline for shear
determination, which we has been tested against the ``Shear TEsting Program 1
and 2'' simulations. We use mass aperture statistics to produce maps of the 2
dimensional density distribution, and parametric fits using both
Navarro-Frenk-White (NFW) and singular-isothermal-sphere profiles to constrain
the total mass. The projected mass distribution shows a pronounced asymmetry,
with an elongated structure extending from the SE to the NW. This is in general
agreement with the optical distribution previously found by other authors. A
similar elongation was previously detected in the X-ray emission map, and in
the distribution of galaxy colours. The circular NFW mass profile fit gives a
total mass of M_{200} = 7.7^{+4.3}_{-2.7} 10^{14} solar masses inside the
virial radius r_{200} = 1.8\pm 0.3 Mpc. The weak lensing profile reinforces the
evidence for an elongated structure of Abell 209, as previously suggested by
studies of the galaxy distribution and velocities.Comment: accepted by A&A, 15 pages, 11 figure
Deep radio observations of the radio halo of the bullet cluster 1E 0657-55.8
We present deep 1.1-3.1 GHz Australia Telescope Compact Array observations of
the radio halo of the bullet cluster, 1E 0657-55.8. In comparison to existing
images of this radio halo the detection in our images is at higher
significance. The radio halo is as extended as the X-ray emission in the
direction of cluster merger but is significantly less extended than the X-ray
emission in the perpendicular direction. At low significance we detect a faint
second peak in the radio halo close to the X-ray centroid of the smaller
sub-cluster (the bullet) suggesting that, similarly to the X-ray emission, the
radio halo may consist of two components. Finally, we find that the distinctive
shape of the western edge of the radio halo traces out the X-ray detected bow
shock. The radio halo morphology and the lack of strong point-to-point
correlations between radio, X-ray and weak-lensing properties suggests that the
radio halo is still being formed. The colocation of the X-ray shock with a
distinctive radio brightness edge illustrates that the shock is influencing the
structure of the radio halo. These observations support the theory that shocks
and turbulence influence the formation and evolution of radio halo synchrotron
emission.Comment: 15 pages, 16 figures, 3 tables. Accepted by MNRA
Marine aerobic biofilm as biocathode catalyst
Stainless steel electrodes were immersed in open seawater and polarized for some days at â 200 mV vs. Ag/AgCl. The current increase indicated the formation of biofilms that catalysed the electrochemical reduction of oxygen. These wild, electrochemically active (EA) biofilms were scraped, resuspended in seawater and used as the inoculum in closed 0.5 L electrochemical reactors. This procedure allowed marine biofilms that are able to catalyse oxygen reduction to be formed in small, closed small vessels for the first time. Potential polarisation during biofilm formation was required to obtain EA biofilms and the roughness of the surface favoured high current values. The low availability of nutrients was shown to be a main limitation. Using an open reactor continuously fed with filtered seawater multiplied the current density by a factor of around 20, up to 60 ”A/cm2, which was higher than the current density provided in open seawater by the initial wild biofilm. These high values were attributed to continuous feeding with the nutrients contained in seawater and to suppression of the indigenous microbial species that compete with EA strains in natural open environments. Pure isolates were extracted from the wild biofilms and checked for EA properties. Of more than thirty different species tested, only Winogradskyella poriferorum and Acinetobacter johsonii gave current densities of respectively 7% and 3% of the current obtained with the wild biofilm used as inoculum. Current densities obtained with pure cultures were lower than those obtained with wild biofilms. It is suspected that synergetic effects occur in whole biofilms or/and that wild strains may be more efficient than the cultured isolates
The structure of the Au(111)/methylthiolate interface : new insights from near-edge X-ray absorption spectroscopy and X-ray standing waves
The local structure of the Au(111)([square root of]3Ă[square root of]3)R30°-methylthiolate surface phase has been investigated by S K-edge near-edge s-ray absorption fine structure (NEXAFS) both experimentally and theoretically and by experimental normal-incidence x-ray standing waves (NIXSW) at both the C and S atomic sites. NEXAFS shows not only excitation into the intramolecular sigma* SâC resonance but also into a sigma* SâAu orbital perpendicular to the surface, clearly identifying the local S headgroup site as atop a Au atom. Simulations show that it is not possible, however, to distinguish between the two possible adatom reconstruction models; a single thiolate species atop a hollow-site Au adatom or a dithiolate moiety comprising two thiolate species bonded to a bridge-bonded Au adatom. Within this dithiolate moiety a second sigma* SâAu orbital that lies near parallel to the surface has a higher energy that overlaps that of the sigma* SâC resonance. The new NIXSW data show the SâC bond to be tilted by 61° relative to the surface normal, with a preferred azimuthal orientation in , corresponding to the intermolecular nearest-neighbor directions. This azimuthal orientation is consistent with the thiolate being atop a hollow-site Au adatom, but not consistent with the originally proposed Au-adatom-dithiolate moiety. However, internal conformational changes within this species could, perhaps, render this model also consistent with the experimental data
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