2,066 research outputs found
A non-grey analytical model for irradiated atmospheres. II: Analytical vs. numerical solutions
The recent discovery and characterization of the diversity of the atmospheres
of exoplanets and brown dwarfs calls for the development of fast and accurate
analytical models. We quantify the accuracy of the analytical solution derived
in paper I for an irradiated, non-grey atmosphere by comparing it to a
state-of-the-art radiative transfer model. Then, using a grid of numerical
models, we calibrate the different coefficients of our analytical model for
irradiated solar-composition atmospheres of giant exoplanets and brown dwarfs.
We show that the so-called Eddington approximation used to solve the angular
dependency of the radiation field leads to relative errors of up to 5% on the
temperature profile. We show that for realistic non-grey planetary atmospheres,
the presence of a convective zone that extends to optical depths smaller than
unity can lead to changes in the radiative temperature profile on the order of
20% or more. When the convective zone is located at deeper levels (such as for
strongly irradiated hot Jupiters), its effect on the radiative atmosphere is
smaller. We show that the temperature inversion induced by a strong absorber in
the optical, such as TiO or VO is mainly due to non-grey thermal effects
reducing the ability of the upper atmosphere to cool down rather than an
enhanced absorption of the stellar light as previously thought.
Finally, we provide a functional form for the coefficients of our analytical
model for solar-composition giant exoplanets and brown dwarfs. This leads to
fully analytical pressure-temperature profiles for irradiated atmospheres with
a relative accuracy better than 10% for gravities between 2.5m/s^2 and 250
m/s^2 and effective temperatures between 100 K and 3000 K. This is a great
improvement over the commonly used Eddington boundary condition.Comment: Accepted in A&A, models are available at
http://www.oca.eu/parmentier/nongrey or in CD
Le coût du crédit aux entreprises.
Les taux moyens des concours bancaires aux entreprises (hormis les découverts) ont continué d’augmenter, dans le prolongement d’un mouvement engagé depuis le quatrième trimestre 2005.Coût du crédit, taux débiteurs, taux bancaires, taux d’intérêt, conditions de banque.
Hydrogen mean force and anharmonicity in polycrystalline and amorphous ice
The hydrogen mean force from experimental neutron Compton profiles is derived
using deep inelastic neutron scattering on amorphous and polycrystalline ice.
The formalism of mean force is extended to probe its sensitivity to
anharmonicity in the hydrogen-nucleus effective potential. The shape of the
mean force for amorphous and polycrystalline ice is primarily determined by the
anisotropy of the underlying quasi-harmonic effective potential. The data from
amorphous ice show an additional curvature reflecting the more pronounced
anharmonicity of the effective potential with respect to that of ice Ih.Comment: 12 pages, 7 figures, original researc
Root selection methods in flood analysis
International audienceIn the 1970s, de Laine developed a root-matching procedure for estimating unit hydrograph ordinates from estimates of the fast component of the total runoff from multiple storms. Later, Turner produced a root selection method which required only data from one storm event and was based on recognising a pattern typical of unit hydrograph roots. Both methods required direct runoff data, i.e. prior separation of the slow response. This paper introduces a further refinement, called root separation, which allows the estimation of both the unit hydrograph ordinates and the effective precipitation from the full discharge hydrograph. It is based on recognising and separating the quicker component of the response from the much slower components due to interflow and/or baseflow. The method analyses the z-transform roots of carefully selected segments of the full hydrograph. The root patterns of these separate segments tend to be dominated by either the fast response or the slow response. This paper shows how their respective time-scales can be distinguished with an accuracy sufficient for practical purposes. As an illustration, theoretical equations are derived for a conceptual rainfall-runoff system with the input split between fast and slow reservoirs in parallel. These are solved analytically to identify the reservoir constants and the input splitting parameter. The proposed method, called "root separation", avoids the subjective selection of rainfall roots in the Turner method as well as the subjective matching of roots in the original de Laine method. Keywords: unit hydrograph,identification methods, z-transform, polynomial roots, root separation, fast andslow response, Nash cascade</p
Triggering the Formation of Halo Globular Clusters with Galaxy Outflows
We investigate the interactions of high-redshift galaxy outflows with
low-mass virialized (Tvir < 10,000K) clouds of primordial composition. While
atomic cooling allows star formation in larger primordial objects, such
"minihalos" are generally unable to form stars by themselves. However, the
large population of high-redshift starburst galaxies may have induced
widespread star formation in these objects, via shocks that caused intense
cooling both through nonequilibrium H2 formation and metal-line emission. Using
a simple analytic model, we show that the resulting star clusters naturally
reproduce three key features of the observed population of halo globular
clusters (GCs). First, the 10,000 K maximum virial temperature corresponds to
the ~ 10^6 solar mass upper limit on the stellar mass of GCs. Secondly, the
momentum imparted in such interactions is sufficient to strip the gas from its
associated dark matter halo, explaining why GCs do not reside in dark matter
potential wells. Finally, the mixing of ejected metals into the primordial gas
is able to explain the ~ 0.1 dex homogeneity of stellar metallicities within a
given GC, while at the same time allowing for a large spread in metallicity
between different clusters. To study this possibility in detail, we use a
simple 1D numerical model of turbulence transport to simulate mixing in
cloud-outflow interactions. We find that as the shock shears across the side of
the cloud, Kelvin-Helmholtz instabilities arise, which cause mixing of enriched
material into > 20% of the cloud. Such estimates ignore the likely presence of
large-scale vortices, however, which would further enhance turbulence
generation. Thus quantitative mixing predictions must await more detailed
numerical studies.Comment: 21 pages, 11 figures, Apj in pres
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