9,036 research outputs found
Smear correction of highly-variable, frame-transfer-CCD images with application to polarimetry
Image smear, produced by the shutter-less operation of frame transfer CCD
detectors, can be detrimental for many imaging applications. Existing
algorithms used to numerically remove smear, do not contemplate cases where
intensity levels change considerably between consecutive frame exposures. In
this report we reformulate the smearing model to include specific variations of
the sensor illumination. The corresponding desmearing expression and its noise
properties are also presented and demonstrated in the context of fast imaging
polarimetry.Comment: Article accepted for publication in Applied Optics on 08 Jun 201
Size-dependent magnetization fluctuations in NiO nanoparticles
The finite size and surface roughness effects on the magnetization of NiO
nanoparticles is investigated. A large magnetic moment arises for an
antiferromagnetic nanoparticle due to these effects. The magnetic moment
without the surface roughness has a non-monotonic and oscillatory dependence on
, the size of the particles, with the amplitude of the fluctuations varying
linearly with . The geometry of the particle also matters a lot in the
calculation of the net magnetic moment. An oblate spheroid shape particle shows
an increase in net magnetic moment by increasing oblateness of the particle.
However, the magnetic moment values thus calculated are very small compared to
the experimental values for various sizes, indicating that the bulk
antiferromagnetic structure may not hold near the surface. We incorporate the
surface roughness in two different ways; an ordered surface with surface spins
inside a surface roughness shell aligned due to an internal field, and a
disordered surface with randomly oriented spins inside surface roughness shell.
Taking a variational approach we find that the core interaction strength is
modified for nontrivial values of which is a signature of
multi-sublattice ordering for nanoparticles. The surface roughness scale
is also showing size dependent fluctuations, with an envelope decay
. The net magnetic moment values calculated using
spheroidal shape and ordered surface are close to the experimental values for
different sizes.Comment: 19 pages, 8 figures, Accepted for publication in Int. J. Mod. Phys.
M dwarfs in the b201 tile of the VVV survey: Colour-based Selection, Spectral Types and Light Curves
The intrinsically faint M dwarfs are the most numerous stars in the Galaxy,
have main-sequence lifetimes longer than the Hubble time, and host some of the
most interesting planetary systems known to date. Their identification and
classification throughout the Galaxy is crucial to unravel the processes
involved in the formation of planets, stars and the Milky Way. The ESO Public
Survey VVV is a deep near-IR survey mapping the Galactic bulge and southern
plane. The VVV b201 tile, located in the border of the bulge, was specifically
selected for the characterisation of M dwarfs. We used VISTA photometry to
identify M dwarfs in the VVV b201 tile, to estimate their subtypes, and to
search for transit-like light curves from the first 26 epochs of the survey.
UKIDSS photometry from SDSS spectroscopically identified M dwarfs was used to
calculate their expected colours in the VISTA system. A colour-based
spectral subtype calibration was computed. Possible giants were identified by a
reduced proper motion diagram. The light curves of
12.8<<15.8 colour-selected M dwarfs were inspected for signals consistent
with transiting objects. We identified 23,345 objects in VVV b201 with colours
consistent with M dwarfs. We provided their spectral types and photometric
distances, up to 300 pc for M9s and 1.2 kpc for M4s, from
photometry. In the range 12<<16, we identified 753 stars as possible
giants out of 9,232 M dwarf candidates. While only the first 26 epochs of VVV
were available, and 1 epoch was excluded, we were already able to identify
transit-like signals in the light curves of 95 M dwarfs and of 12 possible
giants. Thanks to its deeper photometry (4 magnitudes deeper than 2MASS),
the VVV survey will be a major contributor to the discovery and study of M
dwarfs and possible companions towards the center of the Milky Way.Comment: 11 pages, 4 figures. Accepted for publication in Catalogs and data of
Astronomy and Astrophysic
Riemann-Langevin Particle Filtering in Track-Before-Detect
Track-before-detect (TBD) is a powerful approach that consists in providing
the tracker with sensor measurements directly without pre-detection. Due to the
measurement model non-linearities, online state estimation in TBD is most
commonly solved via particle filtering. Existing particle filters for TBD do
not incorporate measurement information in their proposal distribution. The
Langevin Monte Carlo (LMC) is a sampling method whose proposal is able to
exploit all available knowledge of the posterior (that is, both prior and
measurement information). This letter synthesizes recent advances in LMC-based
filtering to describe the Riemann-Langevin particle filter and introduces its
novel application to TBD. The benefits of our approach are illustrated in a
challenging low-noise scenario.Comment: Minor grammatical update
Income Distribution Dependence of Poverty Measure: A Theoretical Analysis
With a new deprivation (or poverty) function, in this paper, we theoretically
study the changes in poverty with respect to the `global' mean and variance of
the income distribution using Indian survey data. We show that when the income
obeys a log-normal distribution, a rising mean income generally indicates a
reduction in poverty while an increase in the variance of the income
distribution increases poverty. This altruistic view for a developing economy,
however, is not tenable anymore once the poverty index is found to follow a
pareto distribution. Here although a rising mean income indicates a reduction
in poverty, due to the presence of an inflexion point in the poverty function,
there is a critical value of the variance below which poverty decreases with
increasing variance while beyond this value, poverty undergoes a steep increase
followed by a decrease with respect to higher variance. Following these
results, we make quantitative predictions to correlate a developing with a
developed economy.Comment: 13 pages in single spaced latex, 4 figures, submitted to
'Econometrica
Effects of dust scattering albedo and 2175 A bump on ultraviolet colours of normal disc galaxies
We discuss dust properties in the interstellar medium (ISM) of nearby normal
galaxies, by comparing observations in the ultraviolet (UV) with simulations by
a radiative transfer model. The observed UV colours of nearby galaxies show a
reddening relative to their expected intrinsic colours. Some authors argued
that the Milky Way dust cannot reproduce the reddening because of the prominent
2175 \AA absorption bump. Other authors proposed a reduction mechanism of the
bump strength in an {\it attenuation law} derived from the ratio of the
observed intensity to the intrinsic one through an age-selective attenuation
(i.e., young stars are more attenuated selectively). We newly find that the
wavelength dependence of the scattering albedo also has a strong effect on the
UV colour; an albedo decreasing toward shorter wavelengths (except for the
absorption bump range) produces a significant UV reddening. After comparing the
observed UV colours of nearby normal galaxies with those expected from
radiative transfer simulations assumed several dust models, we find two sorts
of dust suitable for these galaxies: (1) dust with a bump and a smaller albedo
for a shorter wavelength (except for the bump range), and (2) dust without any
bump but with an almost constant albedo. If very small carbonaceous grains
responsible for the common unidentified infrared emission band are also the
bump carrier, the former dust is favorable. Finally, we derive mean attenuation
laws of various dust models as a function of the UV attenuation, and derive
some relations between the UV attenuation and observable/theoretical
quantities.Comment: MNRAS in pres
Magnetic structure and orbital ordering in BaCoO3 from first-principles calculations
Ab initio calculations using the APW+lo method as implemented in the WIEN2k
code have been used to describe the electronic structure of the
quasi-one-dimensional system BaCoO3. Both, GGA and LDA+U approximations were
employed to study different orbital and magnetic orderings. GGA predicts a
metallic ground state whereas LDA+U calculations yield an insulating and
ferromagnetic ground state (in a low-spin state) with an alternating orbital
ordering along the Co-Co chains, consistent with the available experimental
data.Comment: 8 pages, 9 figure
The Quest for Palladium-Catalysed Alkyl-Nitrogen Bond Formation
Our interest in the development of transition-metal catalysis for the realisation of vicinal diamination reactions of alkenes started about a decade ago. A number of successful transformations in this area have been developed using palladium catalysis. As a challenging aspect of major importance, the palladium-catalysed coupling of alkylânitrogen bonds constitutes the second step in diaminations of alkenes. We here discuss the details that led us to consider high-oxidation-state palladium catalysis as a key feature in such CâN bond-forming reactions. This work discusses both our own contributions and the ones from colleagues and combines the discussion of catalytic reactions and stoichiometric control experiments. It demonstrates that reductive alkylânitrogen bond formation from palladium(IV) proceeds with a low activation barrier and through a linear transition state of nucleophilic displacement
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