909 research outputs found
Scaling laws to understand tidal dissipation in fluid planetary regions and stars I - Rotation, stratification and thermal diffusivity
Tidal dissipation in planets and stars is one of the key physical mechanisms
driving the evolution of star-planet and planet-moon systems. Several
signatures of its action are observed in planetary systems thanks to their
orbital architecture and the rotational state of their components. Tidal
dissipation inside the fluid layers of celestial bodies are intrinsically
linked to the dynamics and the physical properties of the latter. This complex
dependence must be characterized. We compute the tidal kinetic energy
dissipated by viscous friction and thermal diffusion in a rotating local fluid
Cartesian section of a star/planet/moon submitted to a periodic tidal forcing.
The properties of tidal gravito-inertial waves excited by the perturbation are
derived analytically as explicit functions of the tidal frequency and local
fluid parameters (i.e. the rotation, the buoyancy frequency characterizing the
entropy stratification, viscous and thermal diffusivities) for periodic normal
modes. The sensitivity of the resulting possibly highly resonant dissipation
frequency-spectra to a control parameter of the system is either important or
negligible depending on the position in the regime diagram relevant for
planetary and stellar interiors. For corresponding asymptotic behaviors of
tidal gravito-inertial waves dissipated by viscous friction and thermal
diffusion, scaling laws for the frequencies, number, width, height and contrast
with the non-resonant background of resonances are derived to quantify these
variations. We characterize the strong impact of the internal physics and
dynamics of fluid planetary layers and stars on the dissipation of tidal
kinetic energy in their bulk. We point out the key control parameters that
really play a role and demonstrate how it is now necessary to develop ab-initio
modeling for tidal dissipation in celestial bodies.Comment: 24 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
Impact of the frequency dependence of tidal Q on the evolution of planetary systems
Context. Tidal dissipation in planets and in stars is one of the key physical
mechanisms that drive the evolution of planetary systems.
Aims. Tidal dissipation properties are intrisically linked to the internal
structure and the rheology of studied celestial bodies. The resulting
dependence of the dissipation upon the tidal frequency is strongly different in
the cases of solids and fluids.
Methods. We compute the tidal evolution of a two-body coplanar system, using
the tidal quality factor's frequency-dependencies appropriate to rocks and to
convective fluids.
Results. The ensuing orbital dynamics comes out smooth or strongly erratic,
dependent on how the tidal dissipation depends upon frequency.
Conclusions. We demonstrate the strong impact of the internal structure and
of the rheology of the central body on the orbital evolution of the tidal
perturber. A smooth frequency-dependence of the tidal dissipation renders a
smooth orbital evolution while a peaked dissipation can furnish erratic orbital
behaviour.Comment: Accepted for publication as a letter in Astronomy And Astrophysic
Tidal interactions in rotating multiple stars and their impact on their evolution
Tidal dissipation in stars is one of the key physical mechanisms that drive
the evolution of binary and multiple stars. As in the Earth oceans, it
corresponds to the resonant excitation of their eigenmodes of oscillation and
their damping. Therefore, it strongly depends on the internal structure,
rotation, and dissipative mechanisms in each component. In this work, we
present a local analytical modeling of tidal gravito-inertial waves excited in
stellar convective and radiative regions respectively. This model allows us to
understand in details the properties of the resonant tidal dissipation as a
function of the excitation frequencies, the rotation, the stratification, and
the viscous and thermal properties of the studied fluid regions. Then, the
frequencies, height, width at half-height, and number of resonances as well as
the non-resonant equilibrium tide are derived analytically in asymptotic
regimes that are relevant in stellar interiors. Finally, we demonstrate how
viscous dissipation of tidal waves leads to a strongly erratic orbital
evolution in the case of a coplanar binary system. We characterize such a
non-regular dynamics as a function of the height and width of resonances, which
have been previously characterized thanks to our local fluid model.Comment: 2 pages, 1 figure, IAU 307 symposium proceedings, New windows on
massive stars: asteroseismology, interferometry, and spectropolarimetry, Eds.
G. Meynet, C. Georgy, J. H. Groh, and P. Ste
Comparison of decision methods to initiate fungicide applications against cercospora blight of carrot
Un modÚle prévisionnel a été comparé, en 1991 et 1992, à d'autres méthodes pour son efficacité à déterminer le début des applications de fongicides utilisés pour lutter contre la brûlure cercosporéenne de la carotte, causée par le Cercospora carotae. Le premier traitement fongicide était appliqué lorsque : 1) les plants avaient atteint 15 cm de hauteur (méthode conventionnelle); 2) les feuilles intermédiaires de 50 % des plants présentaient des symptÎmes (méthode du 50 % d'incidence); 3) lorsque la valeur cumulative d'équivalence d'infection (CE) avait atteint 14 (méthode de prévision avec la valeur CIE = 14); 4) lorsque la valeur CIE avait atteint 18 (méthode de prévision avec la valeur CIE = 18). Pour toutes les méthodes, les applications subséquentes de fongicides ont été faites aux 10 jours lorsqu'il n'y avait pas de pluie, et aux 7 jours en présence de pluie. Les valeurs CIE ont été calculées en fonction de la durée d'humectation du feuillage et de la température durant cette période, tout en tenant compte des périodes d'humidité élevée et d'interruptions des périodes d'humectation du feuillage, et étaient cumulatives depuis l'émergence des plants. Pour les méthodes avec prévision, que ce soit pour une valeur CIE de 14 ou de 18, aucune perte de rendement n'a été observée et moins de traitements ont été nécessaires comparativement à la méthode conventionnelle et à celle du 50 % d'incidence. Dans une autre étude, les valeurs CIE ont été comparées aux pourcentages de champs qui avaient atteint des niveaux d'incidence de 50, 80 et 100% afin d'établir les seuils de risque léger (11 < CIE < 15) et de risque élevé (16 < CIE < 20). Seulement 3% des champs avaient déjà atteint l'incidence de 50 % lorsqu'une valeur CIE a atteint le seuil préconisé pour un risque faible et, 19 % des champs, une incidence de 80% pour un risque élevé.In 1991 and 1992, two thresholds of a forecasting model were compared with two other decision methods for effectiveness in timing the first fungicide application against Cercospora blight of carrot induced by Cercospora carotae. The first fungicide application was made when : 1) the plants reached 15 cm in height (conventional method); 2) the intermediate (middle) leaves of 50% of the plants were diseased (50% disease incidence threshold method); 3) the cumulative infection equivalence (CE) was 14 (forecasting model CE 14); and 4) the CE was 18 (forecasting model CE 18). In all four treatments, subsequent applications of fungicide were made at 10-d intervals when there was no rain, and at 7-d intervals when there was rain. The CE was calculated based on duration of leaf wetness and temperature during the wet period, corrected for high humidity and interrupted wet periods, and was cumulative starting at crop emergence. For thresholds of CE 14 and CE 18, no yield losses were observed and the total number of fungicide applications needed was lower compared to conventional and 50% disease incidence threshold methods. In a separate study, the CE thresholds were related to the percentage of commercial fields that reached disease incidence thresholds of 50, 80 and 100% to establish low risk (CE 11-15) and high risk (CE 16-20) thresholds. The forecasting of low and high risk CE thresholds were too late for 3 and 19% of the commercial fields because those fields had more than 50 and 80% of the plants diseased, respectively
Scaling laws to understand tidal dissipation in fluid planetary layers and stars
Tidal dissipation is known as one of the main drivers of the secular
evolution of planetary systems. It directly results from dissipative mechanisms
that occur in planets and stars' interiors and strongly depends on the
structure and dynamics of the bodies. This work focuses on the mechanism of
viscous friction in stars and planetary layers. A local model is used to study
tidal dissipation. It provides general scaling laws that give a qualitative
overview of the different possible behaviors of fluid tidal waves. Furthermore,
it highlights the sensitivity of dissipation to the tidal frequency and the
roles played by the internal parameters of the fluid such as rotation,
stratification, viscosity and thermal diffusivity that will impact the
spins/orbital architecture in planetary systems.Comment: 4 pages, 3 figures, IAU 310 symposium proceedings, Complex planetary
system
PĂ©ritonite infectieuse chez une chatte
Auclair-Sémeré G., Groulade Paul. Péritonite infectieuse chez une chatte. In: Bulletin de l'Académie Vétérinaire de France tome 128 n°6-7, 1975. pp. 289-292
Introduction de l'ouvrage : "Le retour des paysans ?"
Cet article est le texte introductif de l'ouvrage « Le retour des paysans ? », lui-mĂȘme issu d'un colloque tenu Ă Marseille les 11 et 12 dĂ©cembre 2003. Cette manifestation avait permis de rĂ©unir de nombreux chercheurs et doctorants reprĂ©sentant la plupart des disciplines en sciences sociales et analysant des situations trĂšs diverses, au Nord et au Sud. Cet ouvrage collectif est en partie le fruit de cette rencontre interdisciplinaire sur un terrain chargĂ© de passions : les paysans et l'environnement
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