98 research outputs found
Vibrational Enhancement of the Effective Donor - Acceptor Coupling
The paper deals with a simple three sites model for charge transfer phenomena
in an one-dimensional donor (D) - bridge (B) - acceptor (A) system coupled with
vibrational dynamics of the B site. It is found that in a certain range of
parameters the vibrational coupling leads to an enhancement of the effective
donor - acceptor electronic coupling as a result of the formation of the
polaron on the B site. This enhancement of the charge transfer efficiency is
maximum at the resonance, where the effective energy of the fluctuating B site
coincides with the donor (acceptor) energy.Comment: 5 pages, 3 figure
Estimating stellar mean density through seismic inversions
Determining the mass of stars is crucial both to improving stellar evolution
theory and to characterising exoplanetary systems. Asteroseismology offers a
promising way to estimate stellar mean density. When combined with accurate
radii determinations, such as is expected from GAIA, this yields accurate
stellar masses. The main difficulty is finding the best way to extract the mean
density from a set of observed frequencies.
We seek to establish a new method for estimating stellar mean density, which
combines the simplicity of a scaling law while providing the accuracy of an
inversion technique.
We provide a framework in which to construct and evaluate kernel-based linear
inversions which yield directly the mean density of a star. We then describe
three different inversion techniques (SOLA and two scaling laws) and apply them
to the sun, several test cases and three stars.
The SOLA approach and the scaling law based on the surface correcting
technique described by Kjeldsen et al. (2008) yield comparable results which
can reach an accuracy of 0.5 % and are better than scaling the large frequency
separation. The reason for this is that the averaging kernels from the two
first methods are comparable in quality and are better than what is obtained
with the large frequency separation. It is also shown that scaling the large
frequency separation is more sensitive to near-surface effects, but is much
less affected by an incorrect mode identification. As a result, one can
identify pulsation modes by looking for an l and n assignment which provides
the best agreement between the results from the large frequency separation and
those from one of the two other methods. Non-linear effects are also discussed
as is the effects of mixed modes. In particular, it is shown that mixed modes
bring little improvement as a result of their poorly adapted kernels.Comment: Accepted for publication in A&A, 20 pages, 19 figure
European Extremely Large Telescope Site Characterization II: High angular resolution parameters
This is the second article of a series devoted to European Extremely Large
Telescope (E-ELT) site characterization. In this article we present the main
properties of the parameters involved in high angular resolution observations
from the data collected in the site testing campaign of the E-ELT during the
Design Study (DS) phase. Observations were made in 2008 and 2009, in the four
sites selected to shelter the future E-ELT (characterized under the ELT-DS
contract): Aklim mountain in Morocco, Observatorio del Roque de los Muchachos
(ORM) in Spain, Mac\'on range in Argentina, and Cerro Ventarrones in Chile. The
same techniques, instruments and acquisition procedures were taken on each
site. A Multiple Aperture Scintillation Sensor (MASS) and a Differential Image
Motion Monitor (DIMM) were installed at each site. Global statistics of the
integrated seeing, the free atmosphere seeing, the boundary layer seeing and
the isoplanatic angle were studied for each site, and the results are presented
here. In order to estimate other important parameters such as the coherence
time of the wavefront and the overall parameter "coherence \'etendue"
additional information of vertical profiles of the wind speed was needed. Data
were retrieved from the National Oceanic and Atmospheric Administration (NOAA)
archive. Ground wind speed was measured by Automatic Weather Stations (AWS).
More aspects of the turbulence parameters such as their seasonal trend, their
nightly evolution and their temporal stability were also obtained and analyzed.Comment: 46 pages and 17 figures. Accepted to be published in PAS
Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results
The chromosphere is a thin layer of the solar atmosphere that bridges the
relatively cool photosphere and the intensely heated transition region and
corona. Compressible and incompressible waves propagating through the
chromosphere can supply significant amounts of energy to the interface region
and corona. In recent years an abundance of high-resolution observations from
state-of-the-art facilities have provided new and exciting ways of
disentangling the characteristics of oscillatory phenomena propagating through
the dynamic chromosphere. Coupled with rapid advancements in
magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly
investigate the role waves play in supplying energy to sustain chromospheric
and coronal heating. Here, we review the recent progress made in
characterising, categorising and interpreting oscillations manifesting in the
solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review
Pupille d’adie isolée: à propos d’un cas
La pupille d'adie correspond à une mydriase unilatérale qui ne repond pas à la stimulation lumineuse et persiste à la vision de prés. La pupille d'adie est généralement associée à de nombreuses pathologies comme la maladie cœlique, l'hépatite auto-immune, la migraine. Nous rapportons le cas d'une jeune fille présentant depuis 2 ans une pupille d'adie isolée. La pupille d'adie est une manifestation rare, l'atteinte isolée chez notre patiente ne permet pas de mettre un diagnostic précis à son atteinte
Optical seeing monitoring at the Oukaïmeden in the Moroccan high atlas mountains: first statistics
In order to complete characterization of the Oukaïmeden site
and hoping to prospect other sites in the Moroccan High Atlas, we
constructed a seeing monitor. We named the instrument DIMMAR for
Differential Image Motion Monitor of Marrakesh. After more than
one year of operation, we can conclude that the DIMMAR is quite
reliable and that it can be a prototype for other similar
low-cost instruments. In this work, the seeing measured at
Oukaïmeden site using the DIMMAR on top of a 5 meters high
tower is presented. Statistics from fifteen months seeing's are
treated (from July 19, 2003 up to September 26, 2004). The median
and mean zenith seeing for the entire period of observations are
0.84´´ and 0.92´´, respectively, at 0.5
(or 0.96´´ and 1.05´´, if corrected for exposure-time bias). The
best seeing value measured is 0.32´´.
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