991 research outputs found
Twin CWG systems Final report
Construction, operation, and maintenance of twin control moment gyroscope system for space vehicle motion simulato
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
Alien Registration- Auclair, Germaine C. (Winthrop, Kennebec County)
https://digitalmaine.com/alien_docs/16956/thumbnail.jp
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
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
The effects of a probiotic yeast on the bacterial diversity and population structure in the rumen of cattle
It has been suggested that the ability of live yeast to improve milk yield and weight gain in cattle is because the yeast stimulates bacterial activity within the rumen. However it remains unclear if this is a general stimulation of all species or a specific stimulation of certain species. Here we characterised the change in the bacterial population within the rumen of cattle fed supplemental live yeast. Three cannulated lactating cows received a daily ration (24 kg/d) of corn silage (61% of DM), concentrates (30% of DM), dehydrated alfalfa (9% of DM) and a minerals and vitamins mix (1% of DM). The effect of yeast (BIOSAF SC 47, Lesaffre Feed Additives, France; 0.5 or 5 g/d) was compared to a control (no additive) in a 3Ă3 Latin square design. The variation in the rumen bacterial community between treatments was assessed using Serial Analysis of V1 Ribosomal Sequence Tag (SARST-V1) and 454 pyrosequencing based on analysis of the 16S rRNA gene. Compared to the control diet supplementation of probiotic yeast maintained a healthy fermentation in the rumen of lactating cattle (higher VFA concentration [high yeast dose only], higher rumen pH, and lower Eh and lactate). These improvements were accompanied with a shift in the main fibrolytic group (Fibrobacter and Ruminococcus) and lactate utilising bacteria (Megasphaera and Selenomonas). In addition we have shown that the analysis of short V1 region of 16s rRNA gene (50â60 bp) could give as much phylogenetic information as a longer read (454 pyrosequencing of 250 bp). This study also highlights the difficulty of drawing conclusions on composition and diversity of complex microbiota because of the variation caused by the use of different methods (sequencing technology and/or analysis)
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
Different prelamin A forms accumulate in human fibroblasts: a study in experimental models and progeria
Lamin A is a component of the nuclear lamina mutated in a group of human inherited disorders known as laminopathies. Among laminopathies, progeroid syndromes and lipodystrophies feature accumulation of prelamin A, the precursor protein which, in normal cells, undergoes a multi-step processing to yield mature lamin A. It is of utmost importance to characterize the prelamin A form accumulated in each laminopathy, since existing evidence shows that drugs acting on protein processing can improve some pathological aspects. We report that two antibodies raised against differently modified prelamin A peptides show a clear specificity to full-length prelamin A or carboxymethylated farnesylated prelamin A, respectively. Using these antibodies, we demonstrated that inhibition of the prelamin A endoprotease ZMPSTE24 mostly elicits accumulation of full-length prelamin A in its farnesylated form, while loss of the prelamin A cleavage site causes accumulation of carboxymethylated prelamin A in progeria cells. These results suggest a major role of ZMPSTE24 in the first prelamin A cleavage step
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