La diffusion atomique doit être prise en compte dans les modèles d'évolution stellaire car il s'agit d'une conséquence directe du fait que les étoiles sont des sphères auto-gravitantes composées d'un mélange de différents gaz (les éléments chimiques). L'équilibre des étoiles conduit à des gradients internes de pression, de densité et de température ainsi qu'à un transfert radiatif, l'ensemble produisant un effet sélectif sur les éléments (dans la plupart des cas dominé par la compétition entre le triage gravitationnel et les accélérations radiatives).Les interactions entre la diffusion atomique et les processus hydrodynamiques tels que la convection dynamique et la perte de masse sont étudiées depuis longtemps. Un processus important a cependant été oublié dans les modèles. Il s'agit de la convection double diffusive (ou thermohaline) induite par un gradient de μ instable, qui peut être produite par une accumulation locale d'éléments lourds à l'intérieur des étoiles due aux accélérations radiatives. Contrairement aux autres processus de mélange, il s'agit d'une conséquence directe de la diffusion atomique. Un effet similaire se produit à la base de la zone convective de surface en cas d'accrétion d’éléments lourds à la surface de l'étoile.Nous avons étudié la convection thermohaline induite par l'accrétion dans le cas du système 16 Cygni et les propriétés de ces deux étoiles avec le code TGEC. Nous avons inclus la prescription de Brown et al. 2013 pour la convection thermohaline. Nous avons ensuite calculé les fréquences d'oscillations de ces modèles à l'aide du code PULSE pour les comparer aux fréquences observées par Kepler. A partir de ces modèles, nous avons pu montrer qu'en accrétant 2/3 de la masse terrestre au début de la séquence principale (sur le modèle 16 Cyg B), la convection thermohaline induite par l'accrétion mélangeait l'étoile suffisamment profondément pour atteindre la zone de destruction du lithium et ainsi obtenir des abondances de lithium cohérentes avec les observations de 16 Cyg A et B.Nous avons étudié l'accumulation d’éléments lourds et l'effet de la convection thermohaline dans le cas des étoiles de type A. Dans ces étoiles, des abondances "particulières" (par rapport au soleil) ont été observées. Ceci est dû aux effets de la diffusion atomique qui sont très importants dans ces étoiles. Cependant, la diffusion atomique seule produit des variations d'abondances trop importantes et un moyen de reproduire les observations est de mélanger l'étoile assez profondément. Nous avons ensuite calculé des modèles incluant la diffusion atomique et la convection thermohaline en utilisant le code TGEC. Nous avons montré que ce processus pouvait modifier la structure interne de ces étoiles, et aussi les abondances de surface. Nous avons aussi inclus la convection thermohaline et l'accrétion dans le code de Montréal/Montpellier. Nous avons modifié plusieurs parties de ce code afin de pouvoir faire des comparaisons avec le TGEC pour comparer les résultats. Les résultats obtenus sont très similaires.Nous avons aussi déterminé les paramètres de l'étoile 94 Ceti à partir d'observations obtenues avec un instrument au sol. Cette étoile à une masse de 1.44 MΘ et est une bonne cible pour étudier l'effet des accélérations radiatives (qui ont un effet non négligeable pour des masses supérieures à 1.2 MΘ). Nous avons aussi comparé des modèles incluant des atmosphères complets afin d'en déterminer l'impact sur les fréquences.Nous avons travaillé sur les étoiles du halo pauvres en métaux pour lesquelles est observé une dispersion inexpliquée des abondances de lithium pour les métallicité très faible. Nous avons étudié la possibilité d'une accrétion sur ces étoiles qui pourrait produire de la convection thermohaline et détruire du lithium. / Atomic diffusion must be taken into account in the computations of stellar structure and evolution as it is a direct consequence of the fact that stars are self-gravitating spheres composed of a mixture of different gases (the chemical elements). The stellar equilibrium leads to internal gradients of pressure, density and temperature as well as an upward radiative transfer which produces a selective effect on the elements (in most cases dominated by the competition between gravitational settling and radiative acceleration).The interactions between atomic diffusion and well-known hydrodynamical processes like dynamical convection and mass loss have been studied for a long time. An important process was however forgotten in these computations. This is the double-diffusive (or fingering or thermohaline) convection induced by unstable μ-gradients, which can be produced by the local accumulation of heavy elements inside stars due to radiative acceleration. Contrary to the other hydrodynamical processes, fingering convection is not arbitrarily added in the computations. It is directly induced by atomic diffusion itself and cannot be avoided. It is thus very important to add this hydrodynamical process in stellar evolution modelling, which has never been done before our work. A similar effect occurs below the convective zone in case of accretion of heavy matter onto a star.We studied the accretion-induced fingering convection in the case of the stellar system 16 Cygni. We studied the properties of these two stars by computing models with the Toulouse Geneva Evolution Code (TGEC). We included the Brown et al. 2013 prescription for the computations of fingering convection in the code. We computed oscillation frequencies of these models using the PULSE code to compare it with Kepler observations. We found that if 2/3 of Earth mass is accreted at the beginning of the main sequence (on 16 Cyg B model), the accretion-induced fingering convection mixes the star deep enough to destroy the lithium and obtain the observed difference between 16 Cyg A and B.We studied the heavy element accumulation and the induced fingering convection in the case of Am stars. In these stars, peculiar surface abundances are observed (compared to the sun). This peculiarity is related to the effect of atomic diffusion, very important in these types of stars. However, atomic diffusion alone leads to abundance variations which are too large and one way to reproduce the observed abundance quantitatively is to assume mixing deep enough inside the star. We computed models including atomic diffusion (with radiative acceleration) and fingering convection with this prescription using the TGEC code. We find than this process may change the internal structure of the stars, and also the surface abundances. We also included fingering convection and the accretion process in the Montreal/Montpellier code. We modified some parts of this code (e.g. turbulence profiles) to compare the results obtained with the two codes. We computed some models and I found that the results are quite similar.We determined the stellar parameters of the star 94 Ceti (by using similar seismic computations as for 16 Cyg A and B) using ground-based observations. This star has a mass of 1.44 MΘ and is a good target to study the effect of radiative accelerations (which occur for masses larger than 1.2-1.3 MΘ). We also compared models with full atmosphere with the observations to determine the impact on oscillation frequencies.We worked on metal poor halo stars for which a dispersion of lithium surface abundance is observed for very small metallicities. We studied the possibility of an accretion of matter that can trigger fingering convection and destroy lithium.

No description available.

Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2007.

A time-dependent two-dimensional (2D) modulation model including drifts, the solar wind tennination shock (TS) with diffusive shock acceleration and a heliosheath based on the Parker (1965) transport equation is used to study the modulation of galactic cosmic rays (GCRs) and the anomalous component of cosmic rays (ACRs) in the heliosphere. In particular, the latitude dependence of the TS compression ratio and injection efficiency of the ACRs (source strength) based on the hydrodynamic modeling results of Scherer et al. (2006) is used for the first time in a modulation model. The subsequent effects on differential intensities for both GCRs and ACRs are illustrated, comparing them to the values without a latitude dependence for these parameters. It is found that the latitude dependence of these parameters is important and that it enables an improved description of the modulation of ACRs beyond the TS. With this modeling approach (without fitting observations) to the latitude dependence of the two parameters, it is possible to obtain a TS spectrum for ACRs at a polar angle of B = 55" that qualitatively approximates the main features of the Voyager 1 observations. This positive result has to be investigated further. Additionally, it is shown that the enhancement of the cosmic ray intensity just below the cut-off energy found for the ACR at the TS in an A < 0 magnetic polarity cycle in the equatorial plane with the latitude independent scenario, disappears in this region when the latitude dependence of the compression ratio and injection efficiency is assumed. Subsequent effects of these scenarios are illustrated on the global anisotropy vector of both GCRs and ACRs as the main theme of this work. For this purpose the radial and latitudinal gradients for GCRs and ACRs were accurately computed. The radial and latitudinal anisotropy components were then computed as a function of energy, radial distance and polar angle. It is also the first time that the anisotropy vector is comprehensively calculated in such a global approach to cosmic ray modeling in the heliosphere, in particular for ACRs. It is shown that the anisotropy vector inside (up-stream) and outside (down-stream) the TS behaves in a complicated way, so care must be taken in interpreting it. It is found that the latitude dependence of the two mentioned parameters can alter the direction (sign) of the anisotropy vector. Its behaviour beyond the TS is markedly different from inside the TS, mainly because of the slower solar wind velocity, with less dependence on the magnetic polarity cycles. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2007.

A time-dependent two-dimensional (2D) modulation model including drifts, the solar wind tennination shock (TS) with diffusive shock acceleration and a heliosheath based on the Parker (1965) transport equation is used to study the modulation of galactic cosmic rays (GCRs) and the anomalous component of cosmic rays (ACRs) in the heliosphere. In particular, the latitude dependence of the TS compression ratio and injection efficiency of the ACRs (source strength) based on the hydrodynamic modeling results of Scherer et al. (2006) is used for the first time in a modulation model. The subsequent effects on differential intensities for both GCRs and ACRs are illustrated, comparing them to the values without a latitude dependence for these parameters. It is found that the latitude dependence of these parameters is important and that it enables an improved description of the modulation of ACRs beyond the TS. With this modeling approach (without fitting observations) to the latitude dependence of the two parameters, it is possible to obtain a TS spectrum for ACRs at a polar angle of B = 55" that qualitatively approximates the main features of the Voyager 1 observations. This positive result has to be investigated further. Additionally, it is shown that the enhancement of the cosmic ray intensity just below the cut-off energy found for the ACR at the TS in an A < 0 magnetic polarity cycle in the equatorial plane with the latitude independent scenario, disappears in this region when the latitude dependence of the compression ratio and injection efficiency is assumed. Subsequent effects of these scenarios are illustrated on the global anisotropy vector of both GCRs and ACRs as the main theme of this work. For this purpose the radial and latitudinal gradients for GCRs and ACRs were accurately computed. The radial and latitudinal anisotropy components were then computed as a function of energy, radial distance and polar angle. It is also the first time that the anisotropy vector is comprehensively calculated in such a global approach to cosmic ray modeling in the heliosphere, in particular for ACRs. It is shown that the anisotropy vector inside (up-stream) and outside (down-stream) the TS behaves in a complicated way, so care must be taken in interpreting it. It is found that the latitude dependence of the two mentioned parameters can alter the direction (sign) of the anisotropy vector. Its behaviour beyond the TS is markedly different from inside the TS, mainly because of the slower solar wind velocity, with less dependence on the magnetic polarity cycles. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2007.

The three-dimensional (3D) steady-state electron modulation model of Ferreira (2002), based on Parker (1965) transport equation, is used to study the modulation of the 7 MeV galactic and Jovian electron anisotropies in the inner heliosphere. The Jovian electrons are produced in Jupiter's magnetosphere which is situated at ~ 5 AU in the ecliptic plane. The propagation of these particles is mainly described by the diffusion tensor applicable for the inner heliosphere. Some of the elements of the diffusion tensor are revisited in order to establish what contribution they make to the three-dimensional anisotropy vector and its components in the inner heliosphere. The 'drift' term is neglected since the focus of this study is on low-energy electrons. The effects on the electron anisotropy of different scenarios when changing the solar wind speed from minimum to maximum activity is illustrated. The effects on both the galactic and Jovian electron anisotropy of changing the polar perpendicular coefficient, in particular, are illustrated. It is shown that the computed Jovian electron anisotropy dominates the galactic anisotropy close to the Jovian electron source at ~5 AU, as expected, testifying to the validity of the3D-model. For the latitudinal anisotropy, the polar perpendicular diffusion plays a dominant role for Jovian electrons close to the source, with the polar gradient becoming the dominant factor away from the electron source. Of all three anisotropy components, the azimuthal anisotropy is dominant in the equatorial plane close to the source. It is found that there is a large azimuthal gradient close to the source because the low-energy electrons tend to follow the heliospheric magnetic field more closely than higher energy particles. The transition of the solar wind speed from minimum to intermediate to maximum solar activity condition was used to illustrate the modulation of the magnitude of the 7 MeV total anisotropy vector along the Ulysses trajectory. It was found that during the two encounters with the planet a maximum anisotropy of 38% was computed but with different anisotropy-timepeaks as the approach to Jupiter was different. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2007.

The three-dimensional (3D) steady-state electron modulation model of Ferreira (2002), based on Parker (1965) transport equation, is used to study the modulation of the 7 MeV galactic and Jovian electron anisotropies in the inner heliosphere. The Jovian electrons are produced in Jupiter's magnetosphere which is situated at ~ 5 AU in the ecliptic plane. The propagation of these particles is mainly described by the diffusion tensor applicable for the inner heliosphere. Some of the elements of the diffusion tensor are revisited in order to establish what contribution they make to the three-dimensional anisotropy vector and its components in the inner heliosphere. The 'drift' term is neglected since the focus of this study is on low-energy electrons. The effects on the electron anisotropy of different scenarios when changing the solar wind speed from minimum to maximum activity is illustrated. The effects on both the galactic and Jovian electron anisotropy of changing the polar perpendicular coefficient, in particular, are illustrated. It is shown that the computed Jovian electron anisotropy dominates the galactic anisotropy close to the Jovian electron source at ~5 AU, as expected, testifying to the validity of the3D-model. For the latitudinal anisotropy, the polar perpendicular diffusion plays a dominant role for Jovian electrons close to the source, with the polar gradient becoming the dominant factor away from the electron source. Of all three anisotropy components, the azimuthal anisotropy is dominant in the equatorial plane close to the source. It is found that there is a large azimuthal gradient close to the source because the low-energy electrons tend to follow the heliospheric magnetic field more closely than higher energy particles. The transition of the solar wind speed from minimum to intermediate to maximum solar activity condition was used to illustrate the modulation of the magnitude of the 7 MeV total anisotropy vector along the Ulysses trajectory. It was found that during the two encounters with the planet a maximum anisotropy of 38% was computed but with different anisotropy-timepeaks as the approach to Jupiter was different. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2007.

Diss. (sammanfattning) Stockholm : Tekn. högsk. / Härtill 6 uppsatser.

<p align="justify"> Depuis les premiers développements de la physique statistique réalisés au 19ème siècle, nombreux ont été les travaux dédiés à la relation entre les processus macroscopiques em>irréversibles</em>(tels que les phénomènes de transport) et les propriétés de la dynamique <em>réversible</em> des atomes et des molécules. Depuis deux décennies, l'<em>hypothèse du chaos microscopique</em> nous en apporte une plus grande compréhension. Dans cette thèse, nous nous intéressons plus particulièrement aux propriétés de <em>viscosité</em>. <br /><br /> Dans ce travail, nous considérons des systèmes périodiques de particules en interaction. Nous proposons une nouvelle méthode de calcul de la viscosité valable pour tous systèmes périodiques, quel que soit le potentiel d'interaction considéré. Cette méthode est basée sur la formule dérivée par Helfand exprimant la viscosité en fonction de la variance du <em>moment de Helfand</em> croissant linéairement dans le temps.<br /><br /> Dans les années nonante, il a été démontré qu'un système composé de seulement deux particules présente déjà de la viscosité. Les deux disques <em>durs</em> interagissent en collisions élastiques dans un domaine carré ou hexagonal avec des conditions aux bords périodiques. Nous appliquons notre méthode de calcul des propriétés de viscosité dans les deux réseaux. Nous donnons également une explication qualitative des résultats obtenus. <br /><br /> L'étude de la relation entre les propriétés de viscosité et les grandeurs du chaos microscopique représente l'une des principales tâches de cette thèse. Dans ce contexte, le <em>formalisme du taux d'échappement</em> joue un rôle majeur. Ce formalisme établit une relation directe entre cette grandeur et la viscosité. Nous étudions numériquement cette relation et la comparaison avec les résultats obtenus par notre méthode sont excellents. <br /><br /> D'autre part, le formalisme du taux d'échappement suppose l'existence d'un <em>répulseur fractal</em>. Après avoir mis en évidence son existence, nous appliquons le formalisme proposant une formule exprimant la viscosité en termes de l'exposant de Lyapunov du système (mesurant le caractère chaotique de la dynamique)et de la dimension fractale du répulseur. L'étude numérique de cette relation dans le modèle à deux disques durs est réalisée avec succès et sont en excellent accord avec les relations obtenus précédemment. <br /><br /> Enfin, nous nous penchons sur les systèmes composés de <em>N</em> disques durs ou sphères dures. Après une étude de l'équation d'état et des propriétés chaotiques, nous avons exploré les propriétés de viscosité dans ces systèmes. Les données numériques obtenues sont en très bon accord avec les prévisions théoriques d'Enskog. D'autre part, nous avons utilisé notre méthode de calcul de la viscosité dans des systèmes de Lennard-Jones. De plus, nous avons proposé une méthode analogue pour le calcul numérique de la <em>conduction thermique</em>. Nos résultats sont en très bon accord avec ceux obtenus par la méthode de Green-Kubo. </p> <br /><br /> <p align="justify"> In this thesis, we first devote a section on the history of the concept of irreversibility; of the hydrodynamics, branch of physics in which the viscosity appears; of the kinetic theory of gases establishing relationships between the microscopic dynamics and macroscopic processes like viscosity; and, finally, the interest brought in statistical mechanics of irreversible processes by the theory of chaos, more precisely, the microscopic chaos. We propose a method based on the Helfand moment in order to calculate the viscosity properties in systems of particles with periodic boundary conditions. We apply this method to the simplest system in which viscosity already exists: the two-hard-disk model. The escape-rate formalism, establishing a direct relation between chaotic quantities of the microscopic dynamics (e.g. Lyapunov exponents, fractal dimensions, etc.), is applied in this system. The results are in excellent agreement with those obtained by our Helfand-moment method. We extend the calculation of the viscosity properties to systems with more than two hard balls. Finally, we compute viscosity as well as thermal conductivity thanks to our own method also based on the Helfand moment. </p>

<p>This work focuses on the physical properties and crystallization of theophyline co-crystals. Co-crystals of theophylline with oxalic acid, glutaric acid and maleic acid have been investigated.</p><p>The DSC curves of these co-crystals show that their first endothermic peaks are all lower than the melting temperature of theophylline. The decomposition temperature of theophylline – oxalic acid co-crystal is at about 230 °C, determined by DSC together with TGA. After decomposition, the remaining theophylline melts at about 279 °C, which is higher than the known melting temperature of theophylline, suggesting a structure difference, ie. a new polymorph may have been formed. The formation of hydrogen bonds in theophylline – oxalic acid co-crystal was investigated by FTIR. Changes of FTIR peaks around 3120 cm^-1 reflects the hydrogen bond of basic N of theophylline and hydroxyl H of oxalic acid. The solubility of theophylline – oxalic acid co-crystal and theophylline – glutaric acid co-crystal was determined in 4:1 chlroform – methanol and in pure chloroform respectively. At equilibrium with the solid theophylline – oxalic acid co-crystal, the theophylline concentration is only 60 % of the corresponding value for the pure solid theophylline. At equilibrium with the solid theophylline – glutaric acid co-crystal, the theophylline concentration is at least 5 times higher than the corresponding value for the pure solid theophylline. Two phases of theophylline were found during the solubility determination. In the chloroform – methanol mixture (4:1 in volume ratio) the solubility of the stable polymorph of theophylline is found to be about 14 % lower than that of the metastable phase. Various aspects of the phase diagram of theophylline – oxalic acid co-crystal was explored.</p><p>Theophylline – oxalic acid co-crystal has been successfully prepared via primary nucleation from a stoichiometric solution mixture of the two components in chloroform – methanol mixture. By slurry conversion crystallization, the co-crystal can be prepared in several solvents, and yield and productivity can be significantly increased. Theophylline – glutaric acid can be successfully prepared via both co-grinding of the two components and slow evaporation with seeding.</p> / QC20100608