3,683 research outputs found
Effect of Radiative Levitation on Calculations of Accretion Rates in White Dwarfs
Elements heavier than hydrogen or helium that are present in the atmospheres
of white dwarfs with effective temperatures lower than 25,000 K, are believed
to be the result of accretion. By measuring the abundances of these elements
and by assuming a steady-state accretion, we can derive the composition of the
accreted matter and infer its source. The presence of radiative levitation,
however, may affect the determination of the accretion rate. We present
time-dependent diffusion calculations that take into account radiative
levitation and accretion. The calculations are performed on C, N, O, Ne, Na,
Mg, Al, Si, S, Ar, and Ca in hydrogen-rich white dwarf models with effective
temperatures lower than 25,000 K and a gravity of log g = 8.0. We show that in
the presence of accretion, the abundance of an element supported by the
radiative levitation is given by the equilibrium between the radiative and
gravitational accelerations, unless the abundance predicted by the steady-state
accretion is much greater than the abundance supported by the radiative
acceleration.Comment: 6 pages, to be published in the proceedings of the 17th European
White Dwarf Workshop that was held in Tubingen, Germany, on August 16-20,
201
Parameter-free Stark Broadening of Hydrogen Lines in DA White Dwarfs
We present new calculations for the Stark broadening of the hydrogen line
profiles in the dense atmospheres of white dwarf stars. Our improved model is
based on the unified theory of Stark broadening from Vidal, Cooper & Smith, but
it also includes non-ideal gas effects from the Hummer & Mihalas occupation
probability formalism directly inside the line profile calculations. This
approach improves upon previous calculations that relied on the use of an
ad-hoc free parameter to describe the dissolution of the line wing opacity in
the presence of high electric microfields in the plasma. We present here the
first grid of model spectra for hot Teff >~ 12,000 K DA white dwarfs that has
no free parameters. The atmospheric parameters obtained from optical and UV
spectroscopic observations using these improved models are shown to differ
substantially from those published in previous studies.Comment: 8 pages, 8 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
Spin Anisotropy and Slow Dynamics in Spin Glasses
We report on an extensive study of the influence of spin anisotropy on spin
glass aging dynamics. New temperature cycle experiments allow us to compare
quantitatively the memory effect in four Heisenberg spin glasses with various
degrees of random anisotropy and one Ising spin glass. The sharpness of the
memory effect appears to decrease continuously with the spin anisotropy.
Besides, the spin glass coherence length is determined by magnetic field change
experiments for the first time in the Ising sample. For three representative
samples, from Heisenberg to Ising spin glasses, we can consistently account for
both sets of experiments (temperature cycle and magnetic field change) using a
single expression for the growth of the coherence length with time.Comment: 4 pages and 4 figures - Service de Physique de l'Etat Condense CNRS
URA 2464), DSM/DRECAM, CEA Saclay, Franc
Aging phenomena in spin glass and ferromagnetic phases: domain growth and wall dynamics
We compare aging in a disordered ferromagnet and in a spin glass, by studying
the different phases of a reentrant system. We have measured the relaxation of
the low-frequency ac susceptibility, in both the ferromagnetic and spin-glass
phases of a CdCr_{1.9}In_{0.1}S_4 sample. A restart of aging processes when the
temperature is lowered (`chaos-like' effect) is observed in both phases. The
memory of previous aging at a higher temperature can be retrieved upon
re-heating, but in the ferromagnetic phase it can rapidly be erased by the
growth of ferromagnetic domains. We interpret the behaviour observed in the
ferromagnetic phase in terms of a combination of domain growth and pinned wall
reconformations, and suggest that aging in spin glasses is dominated by such
wall reconformation processes.Comment: SPEC, CEA Saclay, 91191 Gif sur Yvette Cedex, France, to appear in
Europhys. Lett. (2000
Décharge industrielle de Bonfol : histoire d'un site contaminé et de sa gestion sous quatre régimes institutionnels successifs
Les barrières à la mise en oeuvre des politiques d'adaptation au changement climatique : le cas de la Suisse
The Adaptation Policy Paradox : the Implementation Deficit of Policies Framed as Climate Change Adaptation
The implementation of adaptation policies suffers from barriers and limits; even though adaptation is now set on the political agendas of developed and developing countries, surprisingly few examples of concrete policy realizations are found in comparative assessments. We investigate how the framings of adaptation as a policy problem can relate to tractability issues in implementation. We distinguish three framings of adaptation: climate change adaptation (CCA), climate variability adaptation (CVA), and vulnerability-centered adaptation (VCA) that imply conflicting interpretations of the collective problem to be solved and the goals to be attained through policy solutions. Through the methodology of comparative case studies, we conduct an empirical analysis of three implementation processes in India and Switzerland, and examine how adaptation framings translate into formal policy design and concrete policy realizations. We find that, regardless of the adaptive capacity of the country where implementation takes place, the CCA framing meets more tractability issues than the VCA framing. Therefore, we discuss the paradox that the innovative and additional CCA types of policies, advocated by the United Nations Framework Convention on Climate Change (UNFCCC), are more likely to face a deficit in implementation according to our analysis
Generalized Entropies
We study an entropy measure for quantum systems that generalizes the von
Neumann entropy as well as its classical counterpart, the Gibbs or Shannon
entropy. The entropy measure is based on hypothesis testing and has an elegant
formulation as a semidefinite program, a type of convex optimization. After
establishing a few basic properties, we prove upper and lower bounds in terms
of the smooth entropies, a family of entropy measures that is used to
characterize a wide range of operational quantities. From the formulation as a
semidefinite program, we also prove a result on decomposition of hypothesis
tests, which leads to a chain rule for the entropy.Comment: 21 page
Focal plane wavefront sensor achromatization : The multireference self-coherent camera
High contrast imaging and spectroscopy provide unique constraints for
exoplanet formation models as well as for planetary atmosphere models. But this
can be challenging because of the planet-to-star small angular separation and
high flux ratio. Recently, optimized instruments like SPHERE and GPI were
installed on 8m-class telescopes. These will probe young gazeous exoplanets at
large separations (~1au) but, because of uncalibrated aberrations that induce
speckles in the coronagraphic images, they are not able to detect older and
fainter planets. There are always aberrations that are slowly evolving in time.
They create quasi-static speckles that cannot be calibrated a posteriori with
sufficient accuracy. An active correction of these speckles is thus needed to
reach very high contrast levels (>1e7). This requires a focal plane wavefront
sensor. Our team proposed the SCC, the performance of which was demonstrated in
the laboratory. As for all focal plane wavefront sensors, these are sensitive
to chromatism and we propose an upgrade that mitigates the chromatism effects.
First, we recall the principle of the SCC and we explain its limitations in
polychromatic light. Then, we present and numerically study two upgrades to
mitigate chromatism effects: the optical path difference method and the
multireference self-coherent camera. Finally, we present laboratory tests of
the latter solution.
We demonstrate in the laboratory that the MRSCC camera can be used as a focal
plane wavefront sensor in polychromatic light using an 80 nm bandwidth at 640
nm. We reach a performance that is close to the chromatic limitations of our
bench: contrast of 4.5e-8 between 5 and 17 lambda/D.
The performance of the MRSCC is promising for future high-contrast imaging
instruments that aim to actively minimize the speckle intensity so as to detect
and spectrally characterize faint old or light gaseous planets.Comment: 14 pages, 20 figure
Control of drop positioning using chemical patterning
We explore how chemical patterning on surfaces can be used to control drop
wetting. Both numerical and experimental results are presented to show how the
dynamic pathway and equilibrium shape of the drops are altered by a hydrophobic
grid. The grid proves a successful way of confining drops and we show that it
can be used to alleviate {\it mottle}, a degradation in image quality which
results from uneven drop coalescence due to randomness in the positions of the
drops within the jetted array.Comment: 3 pages, 4 figure
- …