Application des méthodes du chaos quantique aux oscillations d'étoiles en rotation rapide

L'astérosismologie a pour but de déduire les propriétés internes des étoiles à partir de l'analyse de leurs fréquences d'oscillation. Cette analyse peut-être grandement facilitée par des informations a priori sur la structure du spectre d'oscillation, telles que celles que l'on peut obtenir par une formule asymptotique. Jusqu'à maintenant, une telle formule asymptotique n'était disponible que pour les étoiles à symétrie sphérique. Or pour une étoile en rotation rapide, la force centrifruge applatit l'étoile, et la formule asymptotique n'est plus valable. Pourtant, les étoiles pulsantes en rotation rapide sont communes parmi les étoiles massives et de masse intermédiaire de la séquence principale, et un grand nombre d'entre elles sont observées par les missions spatiales dédiées à l'astérosismologie comme CoRoT et Kepler. Dans le cas des modes d'oscillation de pression, la limite asymptotique des rayons acoustiques peut-être décrite par un système dynamique Hamiltonien. Ce système passe, lorsque l'on augmente la vitesse de rotation d'un modèle d'étoile, d'un système intégrable à un système mixte, où des régions stables et chaotiques co-existent dans l'espace des phases. Dans cette thèse, nous montrons comment obtenir des formules semi-analytiques prédisant des espacements réguliers de fréquences dans le spectre des modes de pression d'étoiles en rotation rapide, en utilisant la théorie des rayons ainsi que les méthodes du chaos quantique. Ces formules relient les espacements réguliers de fréquences d'oscillations aux quantités physiques internes des étoiles, ce qui fournit un nouvel outil théorique pour l'astérosismologie des étoiles en rotation rapide.Asteroseismology aims at inferring internal properties of stars from the analysis of their oscillation mode frequencies. This analysis can be greatly facilitated by an a priori information on the basic structure of the oscillation spec-trum given by an asymptotic formula. Up to now, the only existing asymptotic formula for stellar oscillations was derived in the spherically symmetric case. For a rapidly rotating star, spherical symmetry is broken by the centrifugal force, and thus an adequate asymptotic theory was missing. Yet, rapid rotation is common among non-evolved intermediate mass and massive pulsating stars, and many of them are found in the data of asteroseismology space missions CoRoT and Kepler. The ray limit of pressure waves that causes stellar oscillations can be described by a Hamiltonian dynamical system. It was shown that this system undergoes a transition, as the rotation rate of the star increases, from an integrable to a mixed system where stable and chaotic regions coexist in phase space. In this thesis, it is shown how to obtain semi-analytical formulas for regular frequency spacings in the pressure mode spectrum of rapidly rotating stars by using ray theory and techniques from quantum chaos. These formulas relate regular frequency spacings to physical properties of the star, which provides a new theoretical tool for the asteroseismology of rapidly rotating stars

Measurement of spectral functions of ultracold atoms in disordered potentials

We report on the measurement of the spectral functions of noninteracting ultracold atoms in a three-dimensional disordered potential resulting from an optical speckle field. Varying the disorder strength by 2 orders of magnitude, we observe the crossover from the "quantum" perturbative regime of low disorder to the "classical" regime at higher disorder strength, and find an excellent agreement with numerical simulations. The method relies on the use of state-dependent disorder and the controlled transfer of atoms to create well-defined energy states. This opens new avenues for experimental investigations of three-dimensional Anderson localization

Measurement of a topological edge invariant in a microwave network

We report on the measurement of topological invariants in an electromagnetic topological insulator analog formed by a microwave network, consisting of the winding numbers of scattering matrix eigenvalues. The experiment can be regarded as a variant of a topological pump, with non-zero winding implying the existence of topological edge states. In microwave networks, unlike most other systems exhibiting topological insulator physics, the winding can be directly observed. The effects of loss on the experimental results, and on the topological edge states, is discussed.Comment: 10 pages, 10 figure

Prototype constructions in early language acquisition

In this paper we bring together several lines of cross-linguistic research to demonstrate the role of prototypicality in young children’s acquisition of the transitive construction. Much research has shown that young children are slow to form abstract constructions because they fail to see the more general applicability of syntactic markers such as word order and case marking. Here we attempt to explain this fact by investigating the nature of the language children do and do not hear, specifically, the reliability and availability of the linguistic cues they are exposed to. We suggest that constructions redundantly marked with multiple cues could have a special status as a nucleus around which the prototype forms—which makes it difficult for them to isolate the functional significance of each cue. The implications of this view for language acquisition are discussed within a usage-based framework

Intrinsic Activity and Stability of Bifunctional Human UMP Synthase and Its Two Separate Catalytic Domains, Orotate Phosphoribosyltransferase and Orotidine-5′-phosphate Decarboxylase

Human UMP synthase is a bifunctional protein containing two separate catalytic domains, orotate phosphoribosyltransferase (EC 2.4.2.10) and orotidine-5'-phosphate decarboxylase (EC 4.1.1.23). These studies address the question of why the last two reactions in pyrimidine nucleotide synthesis are catalyzed by a bifunctional enzyme in mammalian cells, but by two separate enzymes in microorganisms. From existing data on subunit associations of the respective enzymes and calculations showing the molar concentration of enzyme to be far lower in mammalian cells than in microorganisms, we hypothesize that the covalent union in UMP synthase stabilizes the domains containing the respective catalytic centers. Evidence supporting this hypothesis comes from studies of stability of enzyme activity in vitro, at physiological concentrations, of UMP synthase, the two isolated catalytic domains prepared by site-directed mutagenesis of UMP synthase, and the yeast ODCase. The two engineered domains have activities very similar to the native UMP synthase, but unlike the bifunctional protein, the domains are quite unstable under conditions promoting the dissociated monomer

The Effects of Home Computers on Educational Outcomes: Evidence from a Field Experiment with Schoolchildren

Are home computers are an important input in the educational production function? To address this question, we conduct a field experiment involving the provision of free computers to schoolchildren for home use. Low-income children attending middle and high schools in 15 schools in California were randomly selected to receive free computers and followed over the school year. The results indicate that the experiment substantially increased computer ownership and total computer use among the schoolchildren with no substitution away from use at school or other locations outside the home. We find no evidence that the home computers improved educational outcomes for the treatment group. From detailed administrative data provided by the schools and a follow-up survey, we find no evidence of positive effects on a comprehensive set of outcomes such as grades, test scores, credits, attendance, school enrollment, computer skills, and college aspirations. The estimates also do not indicate that the effects of home computers on educational outcomes are instead negative. Our estimates are precise enough to rule out even modestly-sized positive or negative impacts. The lack of a positive net effect on educational outcomes may be due to displacement from non-educational uses such as for games, social networking, and entertainment. We find evidence that total hours of computer use for games and social networking increases substantially with having a home computer, and increases more than total hours of computer use for schoolwork

The Power of Play: A Pediatric Role in Enhancing Development in Young Children

Children need to develop a variety of skill sets to optimize their development and manage toxic stress. Research demonstrates that developmentally appropriate play with parents and peers is a singular opportunity to promote the social-emotional, cognitive, language, and self-regulation skills that build executive function and a prosocial brain. Furthermore, play supports the formation of the safe, stable, and nurturing relationships with all caregivers that children need to thrive

A large-scale test of the link between intergroup contact and support for social change

Guided by the early findings of social scientists, practitioners have long advocated for greater contact between groups to reduce prejudice and increase social cohesion. Recent work, however, suggests that intergroup contact can undermine support for social change towards greater equality, especially among disadvantaged group members. Using a large and heterogeneous dataset (12,997 individuals from 69 countries), we demonstrate that intergroup contact and support for social change towards greater equality are positively associated among members of advantaged groups (ethnic majorities and cis-heterosexuals) but negatively associated among disadvantaged groups (ethnic minorities and sexual and gender minorities). Specification-curve analysis revealed important variation in the size—and at times, direction—of correlations, depending on how contact and support for social change were measured. This allowed us to identify one type of support for change—willingness to work in solidarity— that is positively associated with intergroup contact among both advantaged and disadvantaged group members