A tornado is a violent rotating column of air hanging from the cloud base of a thunderstorm. They occur most frequently in the United States, about 1000 tornadoes every year, but they can also take place in other parts of the world. For a tornado to arise the atmosphere has to be in a specific state. The state includes a steep lapse rate, a significant difference in wind speed between the ground and at 1 km altitude and moisture represented by a low cloud base level. With these three parameters present there is a good chance that a tornado will form. A tornado alone is not possible to predict with the technology that is available today, but the parameters that were just mentioned are. With climate models and Representative Concentration Pathways (RCP-scenarios) the future climate can be investigated based on greenhouse gas emissions among other things. In this research occurrence of the three parameters in the area of the United States with most tornado activity during a period of 30 years in present time was compared to a period of 30 years in the future for two different RCP-scenarios. The result was calculated to see how many times and days the three parameters occurred both at the same time and individually. It was seen that the amount of times they occurred corresponded to what was expected from the theory, about 1000 every year for both time periods. Thereby no difference was seen in the frequency of tornadoes today compared to any of the future scenarios. A difference that could be determined however was the amount of days in which they occurred. The criteria for a tornado was met in 623 days for present time, compared to RCP 8.5, which is the RCP-scenario with the largest amount of greenhouse gas emissions, where the criteria was met 655 days. RCP 2.6 that is the least devastating scenario with the smallest amount of greenhouse gas emissions had the lowest amount of 604 possible tornado days. / En tromb är ett väderfenomen som består av en intensivt roterande luftpelare mellan ett åskmoln och jordytan. De förekommer över hela världen men är vanligast i USA där det bryter ut ungefär 1000 stycken varje år. För att en tromb ska uppstå krävs det att atmosfären i området har några speciella egenskaper. Den ska vara instabilt skiktad, vilket kan förknippas med ett kraftigt temperaturavtagande med höjd, ha stor skillnad i vindhastighet mellan marken och 1 km upp, samt ha hög relativ luftfuktighet vilket indikeras av en låg molnbas. Är dessa tre kriterier uppfyllda samtidigt finns en möjlighet för en tromb att bildas. Enskilda trombers existens kan inte förutspås med dagens teknik eftersom de är kortlivade väderfenomen och inte en del av ett större vädersammanhang. Med dagens klimatmodeller och Representative Concentration Pathways (RCP-scenarier) kan däremot olika väderparametrars förekomst i framtiden undersökas bl.a. med avseende på mängden utsläpp av växthusgaser.      I den här studien har de tre ovan nämnda parametrarnas förekomst analyserats för det mest trombdrabbade området i USA och jämförts under en 30-årsperiod i nutid med en 30-årsperiod i framtiden för två olika RCP-scenarier. Resultatet togs fram utifrån hur många tillfällen och dagar som de tre parametrarna uppfylldes både enskilt och samtidigt. Antalet tillfällen de uppfylldes samtidigt motsvarade statistiken över trombförekomst relativt bra med ungefär 1000 gånger per år i båda tidsperioderna. Det var därmed ingen större skillnad mellan antalet potentiella tromber idag jämfört med något av RCP-scenarierna i framtiden. Däremot var det en viss skillnad på hur många dagar det finns en chans att det bildas tromber. Kriterierna uppfylldes under 623 dagar i nutid jämfört med 655 dagar i RCP-8.5, vilket är scenariot med störst andel utsläpp av växthusgaser. Under RCP-2.6, det mildaste scenariot med minst utsläpp, uppfylldes kriterierna under 604 dagar, färre än både nutid och RCP 8.5.

Os aglomerados globulares ricos em metais do bojo são uma peça chave na compreensão da história evolutiva da nossa Galáxia e o estudo dessas populações estelares simples pode fornecer vínculos importantes aos modelos de formação do bojo galáctico. Neste trabalho apresentamos o estudo feito com 22 estrelas dos aglomerados globulares NGC 6528 e NGC 6553, aglomerados representativos da população estelar do bojo. Através do uso de dados fotométricos e espectroscópicos determinamos a velocidade radial, temperatura efetiva, gravidade superficial e metalicidade das estrelas. Encontramos parâmetros que cobrem os intervalos 3200 <= Tef <= 5000K e -0.5 <= log g <= 2.4, representando portanto uma faixa de estágios evolutivos do aglomerado. O valor médio de [Fe/H] para os dois aglomerados foi estimado em -0.5dex, tanto pelo uso de índices de metalicidade de Lick, de calibrações de larguras equivalentes de TiO W(TiO) em função de parâmetros atmosféricos e principalmente através de espectros sintéticos. Foram encontrados também indícios de sobreabundância de elementos a. Essas estrelas estudadas são estrelas de referência para o estudo de populações estelares velhas e ricas em metais, de modo que testar e calibrar os espectros sintéticos com esses espectros estelares é um importante passo para garantir a qualidade dos espectros sintéticos na síntese de populações estelares. / The metal-rich bulge globular clusters are a keystone for the understanding of the evolutionary history of our Galaxy and the study of the bulge stellar populations provides constraints to models of bulge formation. In this work we present the study of 22 stars in the globular clusters NGC 6528 and NGC 6553, templates of the metal-rich populations of the bulge. With the use of photometric and spectroscopic data, we have found the radial velocities, effective temperatures, gravities and metallicities of these stars. The parameters found are in the range 3200 <= Tef <= 5000K and -0.5 <= log g <= 2.4, covering from the Horizontal Branch to the tip of the Red Giant Branch evolutionary stages. The mean metallicity determined was [Fe/H] = -0.5 dex, from the Lick indices, calibrations of equivalent widths of TiO as a function of stellar parameters and comparisons to the synthetic spectra. A trend is found for an overabundance of a elements. These stars are templates for the study of old metal-rich stellar populations and the fitting of the synthetic spectra to the observed stellar spectra is an important check of the quality of the synthetic spectra for stellar population synthesis.

Os aglomerados globulares ricos em metais do bojo são uma peça chave na compreensão da história evolutiva da nossa Galáxia e o estudo dessas populações estelares simples pode fornecer vínculos importantes aos modelos de formação do bojo galáctico. Neste trabalho apresentamos o estudo feito com 22 estrelas dos aglomerados globulares NGC 6528 e NGC 6553, aglomerados representativos da população estelar do bojo. Através do uso de dados fotométricos e espectroscópicos determinamos a velocidade radial, temperatura efetiva, gravidade superficial e metalicidade das estrelas. Encontramos parâmetros que cobrem os intervalos 3200 <= Tef <= 5000K e -0.5 <= log g <= 2.4, representando portanto uma faixa de estágios evolutivos do aglomerado. O valor médio de [Fe/H] para os dois aglomerados foi estimado em -0.5dex, tanto pelo uso de índices de metalicidade de Lick, de calibrações de larguras equivalentes de TiO W(TiO) em função de parâmetros atmosféricos e principalmente através de espectros sintéticos. Foram encontrados também indícios de sobreabundância de elementos a. Essas estrelas estudadas são estrelas de referência para o estudo de populações estelares velhas e ricas em metais, de modo que testar e calibrar os espectros sintéticos com esses espectros estelares é um importante passo para garantir a qualidade dos espectros sintéticos na síntese de populações estelares. / The metal-rich bulge globular clusters are a keystone for the understanding of the evolutionary history of our Galaxy and the study of the bulge stellar populations provides constraints to models of bulge formation. In this work we present the study of 22 stars in the globular clusters NGC 6528 and NGC 6553, templates of the metal-rich populations of the bulge. With the use of photometric and spectroscopic data, we have found the radial velocities, effective temperatures, gravities and metallicities of these stars. The parameters found are in the range 3200 <= Tef <= 5000K and -0.5 <= log g <= 2.4, covering from the Horizontal Branch to the tip of the Red Giant Branch evolutionary stages. The mean metallicity determined was [Fe/H] = -0.5 dex, from the Lick indices, calibrations of equivalent widths of TiO as a function of stellar parameters and comparisons to the synthetic spectra. A trend is found for an overabundance of a elements. These stars are templates for the study of old metal-rich stellar populations and the fitting of the synthetic spectra to the observed stellar spectra is an important check of the quality of the synthetic spectra for stellar population synthesis.

The performance of lasercom systems operating in the atmosphere is reduced by optical turbulence, which causes irradiance fluctuations in the received signal. The result is a randomly fading signal. Fade statistics for lasercom systems are determined from the probability density function (PDF) of the irradiance fluctuations. The expected number of fades per second and their mean fade time require the joint PDF of the fluctuating irradiance and its time derivative. Theoretical integral expressions, as well as closed form, analytical approximations, were developed for the joint PDF of a gamma-gamma distributed irradiance and its time derivative, and the corresponding expression for the expected number of fades per second. The new approximation for the conditional PDF of the time derivative of a gamma-gamma irradiance is a zero mean Gaussian distribution, with a complicated irradiance depending variance. Fade statistics obtained from experimental data were compared to theoretical predictions based on the lognormal and gamma-gamma distributions. A Gaussian beam wave was propagated through the atmosphere along a horizontal path, near ground, in the moderate-to-strong optical turbulence. To characterize the propagation path, a new method that infers atmospheric propagation parameters was developed. Scintillation theory combined with a numerical scheme was used to infer the structure constant, Cn2, the inner scale and the outer scale from the optical measurements. The inferred parameters were used in calculations for the theoretical PDFs. It was found that fade predictions made by the gamma-gamma and lognormal distributions provide an upper and lower bound, respectively, for the probability of fade and the number of fades per second for irradiance data collected in the moderate-to-strong fluctuation regime. Aperture averaging effects on the PDF of the irradiance fluctuations were investigated by comparing the irradiance distributions for the three receiver apertures at two different values of the structure parameter and, hence, different values of the coherence radius. For the moderate-to-strong fluctuation regime, the gamma-gamma distribution provides a good fit to the irradiance fluctuations collected by finite-sized apertures that are significantly smaller than the coherence radius. For apertures larger than or equal to the coherence radius, the irradiance fluctuations appear to be lognormally distributed.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Techniques for estimating unknown terms - such as eddy diffusivity and counter-gradient - in atmospheric flow are presented in this study. The methos is also used to identify the source term in atmospheric pollution. The scheme adopted is based on inverse problem methodology. The inverse problem is formulated as a non-linear optimization problem, where the objective function is defined as the square difference between observational data and data from a transport mathematical model. For estimating the properties of the atmospheric flux, an implicit strategy was used, and an Eulerian model was used as forward model. The estimation of the pollutant source term was tested employing a source-repector technique. In the pollutant dispersion simulation, a Lagrangian model was applied. For some inversions, regularized solutions should be searched. The Tikhonov and entropy regularizations were considered. Three different optimization methods were used: Levenberg-Marquardt, quasi-Newton (deterministic) e simulated annealing (stochastic). The results show a good performance of the proposed methodology in many tested situations. / Neste estudo apresentam-se técnicas para estimar termos desconhecidos em fluxos atmosféricos, tais como coeficiente de difusão turbulento e termo de contra-gradiente. O método é também usado para a estimação de termos de fonte em poluição atmosférica. O esquema adotado é baseado na metodologia de problema inverso. O problema inverso é formulado como um problema de otimização não linear, onde a função objetivo é definida como a diferença quadrática entre dados experimentais e dados obtidos através de um modelo matemático de transporte. Para a estimação de propriedades de fluxo atmosféricos, uma estratégia de inversão implícita foi utilizada, onde um modelo eureliano foi empregado como modelo matemático. Para a estimação do termo de fonte de poluição, um procedimento de inversão empregando a técnica fonte-receptor foi testado em diversos cenários físicos. Para simulação da dispersão de poluentes na atmosfera foi empregado um modelo lagrangeano. Em alguns casos tornou-se necessário aplicar técnicas de regularização na obtenção da solução inversa. Regularização de Tikhonov e em entrópicas foram empregadas, quando necessário. Três diferentes métodos de otimização são utilizados: Levenberg-Marquardt e quase-Newton (determinísticos) e recozimento simulado (estocástico). Os resultados comprovam a robustez do método de inversão nas diversas situações testadas.

Ce mémoire présente, pour la première fois, des analyses détaillées d’étoiles naines blanches montrant à la fois des éléments lourds et de forts champs magnétiques. Ces analyses sont effectuées à l’aide d’un tout nouveau code d’atmosphère pouvant calculer la position et la force des raies de tous les éléments (du carbone au cuivre) en régime Paschen-Back pour une géométrie de notre choix. Dans un premier temps, nous décrivons l'effet d'un champ magnétique sur les niveaux d'énergie atomiques, par l'effet Zeeman en champ faible puis en régime Pashen-Back lorsque l'approche perturbative n'est plus valide. Nous explorons ensuite l’effet de la géométrie du champ magnétique, notamment les différences entre les modèles calculés en supposant un champ magnétique uniforme (ou constant dans l'espace) et ceux ayant une géométrie dipolaire. Certaines études ont récemment affirmé qu’en présence d’un champ magnétique intense le mouvement convectif était fortement atténué, de sorte que l’atmosphère devenait pratiquement radiative (Tremblay et al., 2015). Nous explorons brièvement l’impact que peut avoir la suppression du transport d’énergie convective sur les résultats d’analyses d’étoiles magnétiques. Dans le but d'améliorer les analyses d'étoiles magnétiques, nous avons implémentés certains changements aux modèles d'atmosphères utilisés, tel que l'ajout du traitement magnétique des raies métalliques. Nos nouveaux outils nous permermetterons d'analyser de manière rigoureuse, pour la première fois, les étoiles G165-7, J2105+0900 et LHS 2534, trois étoiles magnétiques avec présence d'éléments lourds. Nous étudions leurs paramètres atmosphériques tels que la température effective, gravité de surface et abondances de métaux à partir de modèles standards ainsi qu’à partir de modèles radiatifs ou la convection a été artificiellement inhibée par la présence d’un champ magnétique intense. Nous observons que nos modèles riches en hydrogène sans convection ne reproduisent plus les observations, contrairement à ceux riches en hélium où la convection se produit hors de la région de formation des raies (soit plus profondément). Nous observons finalement qu'un champ magnétique dipolaire centré avec l'étoile ne reproduit pas bien les profils des raies d'un même multiplet, puisque les composantes sigma (celles à gauche et à droite de la raie centrale) prédites sont trop larges. Les observations spectroscopiques sont mieux reproduites à partir de modèles ayant un champ magnétique de surface constant, ou avec un dipôle toujours fortement décalé selon l’axe -z, indiquant que la géométrie réelle du champ magnétique est probablement plus complexe qu’un simple dipôle. / We present, for the first time, detailed analyses of white dwarf stars showing heavy metal lines and large magnetic fields. These analyses are carried out with a new atmosphere code able to compute the position and strength of lines for all elements (from carbon to copper) in the Paschen-Back regime for an arbitrary geometry. Firstly, we describe the effect of a magnetic field on the atomic energy levels, first with the Zeeman effect for weak fields then in the Paschen-Back regime when the pertubative approach is no longer valid. We then explore the effect of the magnetic field geometry, especially the differences between models calculated assuming a constant field and those with a dipolar geometry. Some studies recently suggested that in the presence of an intense magnetic field, the convective movement is strongly inhibited, renderingthe atmosphere mostly radiative (Tremblay et al., 2015). We briefly explore the impact the suppression of the convective energy transport can have on the analyses' results of magnetic stars. In order to carry out a thorough analysis of some magnetic stars, we have applied changes to the atmospheric models used, for example by adding magnetic treatment of metallic lines. With these new tools, we analyse for the first time, in a rigorous manner, the stars G165-7, J2105+0900 and LHS 2534, three magnetic stars showing heavy elements. We study their atmospheric parameters such as the effective temperature, surface gravity and metal abundances, and then explore the possibility of having pure radiative atmospheres, where the convection would be inhibited by intense magnetic fields. We find that hydrogen rich models cannot reproduce observations, unlike those rich in helium where the convective movement takes place out of the line forming region. We finally note that a centered dipolar magnetic field is not able to reproduce the line profiles of a given multiplet, since the sigma components (those to the left and the right of the central line) predicted are too broad. Spectroscopic observations are better reproduced with constant magnetic field models, or with a strongly offset dipole in the -z axis, indicating that the actual field geometry probably is more complex than a simple dipole.

Cette thèse présente les avancées dans l'étude des étoiles naines blanches magnétiques. L'étude des étoiles naines blanches en général, le state final d'évolution de près de 95% des étoiles de notre galaxie, permet d'obtenir de l'information cruciale sur leurs progéniteurs. Se pencher sur les propriétés des champs magnétiques des étoiles naines blanches par opposition à celles de leur progéniteur, beaucoup plus difficiles à observer en partie à cause de la faible intensité du champ, est bien plus réalisable. Plusieurs hypothèses ont été formulées afin de décrire l'origine et l'évolution de ces objets atypiques. Que les champs soient d'origine fossile suite à l'évolution d'une étoile isolée et conservation du flux magnétique, ou créés lors de la rotation rapide de la fusion de deux étoiles, il nous faut bien comprendre leur résultat, les naines blanches magnétiques, avant de pouvoir retracer leur évolution. Toutefois, les modèles de naines blanches magnétiques d'aujourd'hui reposent sur un nombre d'approximation, qui ne sont valides jusqu'à une relativement faible intensité de champ. Ceci a été la motivation derrière ce projet de doctorat. Nous avons implémenté un traitement des raies d'absorption utilisant le calcul des énergies de transition de l'atome d'hydrogène et d'hélium sous champs magnétiques arbitraires. Ces calculs, fournis par le groupe de l'université de Tübingen en Allemagne, ont utilisé une approche numérique à la résolution de l'Hamiltonien de l'atome d'hydrogène et d'hélium en présence d'un champ magnétique d'intensité arbitraire. Cet ajout aux modèles d'atmosphères permet de modéliser la position et l'intensité des raies d'absorption dans le spectre des étoiles. Une procédure de discrétisation de la surface visible de l'étoile nous permet de modéliser des champs magnétiques avec une géométrie arbitraire, tel qu'un dipôle décentré et incliné par rapport à l'observateur. Ces nouveaux outils ont été utilisés afin d'étudier en premier lieu un ensemble des naines blanches magnétiques avec une atmosphère riche en hydrogène. De ces résultats, nous avons déterminé une distribution de masse de ces étoiles, avec une masse moyenne significativement plus élevée que leurs homologues non-magnétiques, tels qu'attendu par les théories d'évolution binaires. Les informations extraites des différentes caractéristiques des distributions obtenues suite à cette analyse ne nous permettent cependant pas de favoriser une hypothèse d'évolution plutôt qu'une autre. Suite à ceci, celles avec une atmosphère riche en hélium ont été les prochaines à être analysées. Nous avons ainsi déterminé rigoureusement, pour la première fois, les paramètres magnétiques de ces étoiles riches en hélium. / This thesis presents advances in the study of magnetic white dwarf stars. The study of white dwarf stars in general, the final state of evolution of nearly 95% of the stars in our galaxy, provides crucial information about their progenitors. It is much more feasible to study the magnetic field properties of white dwarf stars as opposed to their progenitor stars, which are much more difficult to observe in part because of the weak field strength. Several hypotheses have been formulated to describe the origin and evolution of these atypical objects. Whether the fields are of fossil origin following the evolution of an isolated star and conservation of magnetic flux, or created during the rapid rotation of the merger of two stars, we need to understand their result, the magnetic white dwarfs, before we can trace their evolution. However, today's models of magnetic white dwarfs rely on a number of approximations, which are only valid up to relatively low field strengths. This was the motivation behind this doctoral thesis. We have implemented a treatment of the absorption lines using the calculation of the transition energies of the hydrogen and helium atom under arbitrary magnetic fields. These calculations, provided by the group of the University of Tubingen in Germany, used a numerical approach to the resolution of the Hamiltonian of the hydrogen and helium atom in the presence of a magnetic field of arbitrary intensity. This addition to atmospheric models allows to model the position and intensity of absorption lines in the spectrum of stars. A discretization procedure of the visible surface of the star allows us to model magnetic fields with an arbitrary geometry, such as an offset and inclined dipole. These new tools have been used to study first a set of magnetic white dwarfs with a hydrogen-rich atmosphere. From these results, we have determined a mass distribution of these stars, with an average mass significantly higher than their non-magnetic counterparts, as expected by the binary evolution theories. The information extracted from the different characteristics of the distributions obtained from this analysis does not however allow us to favor one evolutionary hypothesis over another. Following this, those with a helium rich atmosphere were the next to be analyzed. We have thus determined rigorously, for the first time, the magnetic parameters of these helium-rich stars.

Ce mémoire présente une analyse comparative des paramètres atmosphériques obtenus à l’aide des techniques photométrique et spectroscopique. Pour y parvenir, les données photométriques et spectroscopiques de 1375 naines blanches de type DA tirées du Sloan Digital Sky Survey (SDSS) ainsi que les données spectroscopiques du Villanova White Dwarf Catalog ont été utilisées. Il a d’abord fallu s’assurer que les données photométriques et spectroscopiques étaient bien calibrées. L’analyse photométrique a démontré que la photométrie ugriz ne semblait pas avoir de problème de calibration autre que le décalage des points zéro, qui est compensé en appliquant les corrections photométriques appropriées. De plus, le fait que le filtre u laisse passer le flux à certaines longueurs d’onde dans le rouge ne semble pas affecter la détermination des paramètres atmosphériques. L’analyse spectroscopique a ensuite confirmé que l’application de fonctions de correction permettant de tenir compte des effets hydrodynamiques 3D est la solution au problème de log g élevés. La comparaison des informations tirées des données spectroscopiques des deux différentes sources suggère que la calibration des spectres du SDSS n’est toujours pas au point. Les paramètres atmosphériques déterminés à l’aide des deux techniques ont ensuite été comparés et les températures photométriques sont systématiquement plus faibles que celles obtenues à partir des données spectroscopiques. Cet effet systématique pourrait être causé par les profils de raies utilisés dans les modèles d’atmosphère. Une méthode permettant d’obtenir une estimation de la gravité de surface d’une naine blanche à partir de sa photométrie a aussi été développée. / We present a comparative analysis of atmospheric parameters obtained with the so-called photometric and spectroscopic techniques. Photometric and spectroscopic data for 1375 DA white dwarfs from the Sloan Digital Sky survey (SDSS) are used, as well as spectroscopic data from the Villanova White Dwarf Catalog. We first test the calibration of the ugriz photometric system by using model atmosphere fits to observed data. Our photometric analysis indicates that the ugriz photometry appears well calibrated when the SDSS standard photometric corrections are applied. We also show that the reported red leak of the u filter does not affect the results of the photometric analysis. The spectroscopic analysis of the same data set reveals that the so-called high log g problem can be solved by applying published correction functions that take into account 3D hydrodynamical effects. However, a comparison between the SDSS and the White Dwarf Catalog spectroscopic data also suggests that the SDSS spectra still suffer from a small calibration problem. We then compare the atmospheric parameters obtained from both fitting techniques and show that the photometric temperatures are systematically lower than those obtained from spectroscopic data. This systematic offset may be linked to the hydrogen line profiles used in the model atmospheres. We finally explore the results of a technique aimed at measuring surface gravities using photometric data only.

Ce mémoire présente une recherche détaillée et une analyse des étoiles naines blanches hybrides chimiquement stratifiées dans le Sloan Digital Sky Survey (SDSS). Une seule étoile stratifiée, PG 1305-017, était connue avant notre recherche. L'objectif principal est de confirmer l'existence de plusieurs nouvelles étoiles stratifiées. Pour ce faire, il a fallu dans un premier temps développer une nouvelle génération de modèles d'atmosphère à partir de ceux de Bergeron et al. (1991) et Tremblay & Bergeron (2009). Nous y avons ajouté l'opacité de toutes les raies d'hélium et les calculs nécessaires pour tenir compte de la stratification chimique de l'atmosphère, où une mince quantité d’hydrogène flotte en équilibre diffusif au-dessus d’une enveloppe massive d’hélium. En parallèle, nous avons aussi calculé des modèles standards, chimiquement homogènes. Ensuite, nous avons sélectionné des naines blanches chaudes (Teff > 30,000 K) de type spectral hybride (traces d'hélium et d'hydrogène) parmi les ~38,000 naines blanches répertoriées dans le SDSS. Un total de 52 spectres d'étoile a été retenu dans notre échantillon final. La technique spectroscopique, c'est-à-dire l'ajustement des raies spectrales des modèles sur un spectre observé, a été appliquée à toutes les étoiles de notre échantillon. Nous avons ainsi mesuré la température effective, la gravité de surface et la composition chimique de l'atmosphère de ces étoiles. Par l'ajustement simultané de modèles stratifiés et homogènes, nous avons aussi pu déterminer si les étoiles étaient stratifiées ou non. Nous identifions ainsi 14 naines blanches stratifiées. Nous tirons de ces résultats plusieurs conclusions sur les processus physiques expliquant la présence d'hélium dans l'atmosphère. / We present a detailed research and analysis of chemically stratified hybrid white dwarf stars in the Sloan Digital Sky Survey (SDSS). Only one stratified star, PG 1305-017, was known before our analysis. The main objective is to confirm the existence of several new stratified stars. To do so, we initially had to develop a new generation of model atmospheres from those of Bergeron et al. (1991) and Tremblay & Bergeron (2009). We added the opacity of all the helium lines and the necessary calculations to account for the chemical stratification of the atmosphere, where a thin hydrogen layer floats in diffusive equilibrium on top of a more massive helium layer. In parallel, we also calculated a grid of standard models, which are chemically homogeneous. Then, we selected hot white dwarfs (Teff > 30,000 K) with a hybrid spectral type (traces of helium and hydrogen) from the ~38,000 white dwarfs listed in the SDSS. A total of 52 spectra were retained in our final sample. The spectroscopic technique, i.e. the fit of model spectra to an observed spectrum, was applied to all stars in our sample. Thereby, we have measured the effective temperature, the surface gravity and the chemical composition of these stars. By simultaneously fitting stratified and homogeneous models, we have also been able to determine if the stars were stratified or not. We identify 14 stratified white dwarfs. From these results, we draw several conclusions on the physical processes explaining the presence of helium in the atmosphere.

Two related bodies of work are presented, both of which aim to further the rapid development of next generation concentrating photovoltaic systems using high efficiency multi junction solar cells. They are complementary since the characterization of commercial devices and the systematic application of design principles for future designs must progress in parallel in order to accelerate iterative improvements. First addressed, is the field characterization of state of the art concentrating photovoltaic systems. Performance modeling and root cause analysis of deviations from the modeling results are critical for bringing reliable high value products to the market. Two complementary tools are presented that facilitate acceleration of the development cycle. The “Dynamic real-time I V Curve Measurement System…” provides a live picture of the current-voltage characteristics of a CPV module. This provides the user with an intuitive understanding of how module performance responds under perturbation. The “Shutter technique for noninvasive individual cell characterization in sealed concentrating photovoltaic modules,” allows the user to probe individual cell characteristics within a sealed module. This facilitates non-invasive characterization of modules that are in situ. Together, these tools were used to diagnose the wide spread failure of epoxy connections between the carrier and the emitter of bypass diodes installed in sealed commercial modules. Next, the optimization principals that are used to choose energy yield maximizing bandgap combinations for multi-junction solar cells are investigated. It is well understood that, due to differences in the solar resource in different geographical locations, this is fundamentally a local optimization problem. However, until now, a robust methodology for determining the influences of geography and atmospheric content on the ideal design point has not been developed. This analysis is presented and the influence of changing environment on the representative spectra that are used to optimize bandgap combinations is demonstrated. Calculations are confirmed with ground measurements in Ottawa, Canada and the global trends are refined for this particular location. Further, as cell designers begin to take advantage of more flexible manufacturing processes, it is critical to know if and how optimization criteria must change for solar cells with more junctions. This analysis is expanded to account for the differences between cells with up to 8 subcell bandgaps. A number of software tools were also developed for the Sunlab during this work. A multi-junction solar cell model calibration tool was developed to determine the parameters that describe each subcell. The tool fits a two diode model to temperature dependent measurements of each subcell and provides the fitting parameters so that the performance of multi-junction solar cells composed of those subcells can be modeled for real world conditions before they are put on-sun. A multi-junction bandgap optimization tool was developed to more quickly and robustly determine the ideal bandgap combinations for a set of input spectra. The optimization process outputs the current results during iteration so that they may be visualized. Finally, software tools that compute annual energy yield for input multi-junction cell parameters were developed. Both a brute force tool that computes energy harvested at each time step, and an accelerated tool that first bins time steps into discrete bins were developed. These tools will continue to be used by members of the Sunlab.