Shear mixing in stellar radiative zones I. Effect of thermal diffusion and chemical stratification

Turbulent transport of chemical elements in radiative zones of stars is considered in current stellar evolution codes thanks to phenomenologically derived diffusion coefficients. Recent local numerical simulations (Prat & Ligni\`eres 2013, A&A, 551, L3) suggest that the coefficient for radial turbulent diffusion due to radial differential rotation satisfies $D_{\rm t}\simeq0.058\kappa/Ri$, in qualitative agreement with Zahn's model. However, this model does not apply when differential rotation is strong with respect to stable thermal stratification or when chemical stratification has a significant dynamical effect, a situation encountered at the outer boundary of nuclear-burning convective cores. We extend our numerical study to consider the effects of chemical stratification and of strong shear, and compare the results with prescriptions used in stellar evolution codes. We performed local, direct numerical simulations of stably stratified, homogeneous, sheared turbulence in the Boussinesq approximation. The regime of high thermal diffusivities, typical of stellar radiative zones, is reached thanks to the so-called small-P\'eclet-number approximation, which is an asymptotic development of the Boussinesq equations in this regime. The dependence of the diffusion coefficient on chemical stratification was explored in this approximation. Maeder's extension of Zahn's model in the strong-shear regime is not supported by our results, which are better described by a model found in the geophysical literature. As regards the effect of chemical stratification, our quantitative estimate of the diffusion coefficient as a function of the mean gradient of mean molecular weight leads to the formula $D_{\rm t}\simeq 0.45\kappa(0.12-Ri_\mu)/Ri$, which is compatible in the weak-shear regime with the model of Maeder & Meynet (1996, A&A, 313, 140).Comment: 10 pages, 9 figures, accepted in A&

Seismic diagnosis from gravity modes strongly affected by rotation

Most of the information we have about the internal rotation of stars comes from modes that are weakly affected by rotation, for example by using rotational splittings. In contrast, we present here a method, based on the asymptotic theory of Prat et al. (2016), which allows us to analyse the signature of rotation where its effect is the most important, that is in low-frequency gravity modes that are strongly affected by rotation. For such modes, we predict two spectral patterns that could be confronted to observed spectra and those computed using fully two-dimensional oscillation codes.Comment: 3 pages, 1 figure, to appear in the proceedings of the Joint TASC2 & KASC9 Workshop SPACEINN & HELAS8 Conference "Seismology of the Sun and the Distant Stars 2016

L’ours des cavernes de Font-de-Gaume III

Le niveau 4-5 (fouilles Prat) de la grotte de Font-de-Gaume est riche en ossements d’Ours des cavernes. Quelques artefacts lithiques y ont également été trouvés. Parmi eux, signalons la présence de deux pointes de Châtelperron. La reprise de ce matériel a permis de découvrir des traces de découpe sur des restes d’Ours.Layer 4-5 (Prat excavation) of Font-de-Gaume cave is reach in cave bear remains. This layer also includes a few lithic artefacts, including two châtelperron points. A recent analysis of this material shows that cutmarks are present on some of the cave bear bones

Le registre d’hominines fossiles du bassin de l’Omo-Turkana

L’objectif de cette présentation est de considérer un registre exhaustif d’hominines fossiles comme objet d’étude en tant que tel et de montrer via quelques exemples l’apport de l’analyse d’un tel registre, à condition qu’il provienne d’un système sédimentaire cohérent. Nous illustrerons ceci en utilisant le registre d’hominines du bassin de l’Omo-Turkana (Kenya/Éthiopie), où les recherches paléoanthropologiques intensives ont commencé en 1967. À ce jour, le registre d’hominines fossiles est..

Scaling regimes of active turbulence with external dissipation

Active fluids exhibit complex turbulent-like flows at low Reynolds number. Recent work predicted that 2d active nematic turbulence follows universal scaling laws. However, experimentally testing these predictions is conditioned by the coupling to the 3d environment. Here, we measure the spectrum of the kinetic energy, $E(q)$, in an active nematic film in contact with a passive oil layer. At small and intermediate scales, we find the scaling regimes $E(q)\sim q^{-4}$ and $E(q)\sim q^{-1}$, respectively, in agreement with the theoretical prediction for 2d active nematics. At large scales, however, we find a new scaling $E(q)\sim q$, which emerges when the dissipation is dominated by the 3d oil layer. In addition, we derive an explicit expression for the spectrum that spans all length scales, thus explaining and connecting the different scaling regimes. This allows us to fit the data and extract the length scale that controls the crossover to the new large-scale regime, which we tune by varying the oil viscosity. Overall, our work experimentally demonstrates the emergence of universal scaling laws in active turbulence, and it establishes how the spectrum is affected by external dissipation

Le Bison des steppes, Bison priscus Bojanus, 1827, de la grotte d’Habarra à Arudy (Pyrénées-Atlantiques)

Au cours du Quaternaire, vraisemblablement à plusieurs reprises, la grotte d’Habarra a piégé de nombreux grands mammifères passés par des deux ouvertures naturelles du toit de la caverne. Le gisement paléontologique a été découvert en 1984 et des fouilles furent pratiquées la même année réunissant des membres du Groupe Spéléologique de la Vallée d’Ossau (G.S.V.O.), du Groupe Archéologique des Pyrénées-Occidentales (G.A.P.O), de la Direction des Antiquités Préhistoriques d’Aquitaine et de l’Institut du Quaternaire de l’Université de Bordeaux I. Bien que le remplissage fossilifère n’ait été que faiblement entamé, de nombreux ossements furent exhumés se rapportant à plusieurs taxons de grands mammifères pléistocènes : Renne, Cheval, Cerf, Rhinocéros… mais le taxon le mieux représenté est le Bison des steppes tant en nombre de restes qu’en nombre d’individus. Vingt bisons au moins sont représentés et quatre d’entre eux le sont par un nombre élevé d’éléments de leurs squelettes. L’ensemble constitue une documentation sur Bison priscus exceptionnelle qui a été exploitée dans des directions qui ne peuvent être abordées qu’à partir d’échantillons importants.Des éléments de morphologie et de biométrie ont été recherchés pour une distinction 1) au niveau sexuel : pour Bison priscus de nombreux caractères distinctifs mâle-femelle ont été établis ; 2) au niveau spécifique : quelques points concernant notamment les dimensions relatives de grands segments osseux ont pu être précisés ; ils aident à caractériser Bison priscus ; relativement à Bison bison et Bison bonasus ; 3) au niveau générique : l’évidence de l’appartenance des ossements de Bovinés d’Habarra à Bison priscus a permis d’aborder la question de la pertinence de certains critères de différenciation entre les deux genres Bos et Bison.Ce travail a été conçu afin d’en rendre aisée sa consultation, un peu comme un manuel d’anatomie comparée propre aux grands Bovidés qui serait fondé non seulement sur des données actuelles mais aussi sur des restes fossiles : la table des matières, détaillée, permet de localiser rapidement l’emplacement des descriptions et informations relatives soit à un os soit à un individu particulier et le tableau 42 donne pour chaque individu la liste des os lui appartenant.Tout en sachant qu’il n’est pas sans défauts, nous souhaitons que ce travail soit utile et contribue efficacement à faire avancer les connaissances sur ce grand ongulé pléistocène qu’était le Bison des steppes ; en effet, si la silhouette de Bison priscus nous est familière grâce aux représentations que nous en ont laissées les peintres et graveurs préhistoriques, son squelette était jusqu’à présent assez mal documenté, du moins dans sa variabilité morphologique et biométrique.During the Quaternary, Habarra cave acted as a natural trap: many large mammals fell through two natural openings in the roof and became entombed within it. Discovered in 1984, the site was subsequently excavated by members of the Groupe Spéléologique de la Vallée d’Ossau (G.S.V.O.), the Groupe Archéologique des Pyrénées-Occidentales (G.A.P.O), the Direction des Antiquités Préhistoriques d’Aquitaine, and the Institut du Quaternaire of the Université de Bordeaux I. Even though the excavations were not extensive, many fossil specimens from a variety of large Pleistocene mammals were retrieved, including those of reindeer, horse, red deer, and rhinoceros. Best documented in terms of both specimens and individuals is the steppe bison (Bison priscus). The remains of at least 20 of these animals were collected, of which four individuals are particularly well-represented. The collection as a whole provides remarkably thorough documentation of the Bison priscus skeleton, in turn allowing detailed osteological analyses.We have used morphological and biometric analyses to improve our understanding of 1) sexual differences within Bison priscus, establishing a number of skeletal characters that can be used to distinguish males and females of this species; 2) species-level differences, providing biometric data that allow more secure differentiation of Bison priscus from Bison bison and Bison bonasus, and, 3) genus-level differences, since we have been able to use the Habarra Cave Bison priscus material to examine the validity of a series of criteria that may allow the differentiation of Bos and Bison.We have presented our results in the form of a manual of comparative anatomy for Bos and Bison, a manual founded on both modern material and on the fossil remains of the animals involved. The detailed table of contents will allow the reader to quickly find descriptions and information relevant to a particular skeletal element or individual, while Table 42 provides a list of specimens for each Habarra Bison priscus individual. The profile of Bison priscus is familiar to us from the depictions left by prehistoric artists, but much less has been known of the skeleton of this remarkable Pleistocene ungulate, at least in terms of morphological and biometric variability. Although we are fully aware that this work is not without fault, it is our hope that it will help fill this significant gap in our knowledge

Scaling Regimes of Active Turbulence with External Dissipation

Active fluids exhibit complex turbulentlike flows at low Reynolds number. Recent work predicted that 2D active nematic turbulence follows scaling laws with universal exponents. However, experimentally testing these predictions is conditioned by the coupling to the 3D environment. Here, we measure the spectrum of the kinetic energy in an active nematic film in contact with a passive oil layer. At small and intermediate scales, we find the scaling regimes E(q)∼q^−4 and E(q)∼q^−1, respectively, in agreement with the theoretical prediction for 2D active nematics. At large scales, however, we find a new scaling E(q)∼q^1, which emerges when the dissipation is dominated by the 3D oil layer. In addition, we derive an explicit expression for the spectrum that spans all length scales, thus explaining and connecting the different scaling regimes. This allows us to fit the data and extract the length scale that controls the crossover to the new large-scale regime, which we tune by varying the oil viscosity. Overall, our work experimentally demonstrates the emergence of scaling laws with universal exponents in active turbulence, and it establishes how the spectrum is affected by external dissipation