From the main sequence onwards, stars of intermediate masses (1-8 Solar masses) eject a large portion of their mass with rates as high as 0.0001 Solar masses per year during their transition through the Asymptotical Giant Branch (AGB) stage. The outflows are shaped by the same mechanisms that shape the ejecta, which in turn appear to depart from spherical symmetry as early as the AGB stage. The ejecta are then evolving into asymmetrical structures. Stars like that are giant factories of dust, responsible for the enrichment of their surrounding Galactic medium in metals heavier than helium. Depending on their abundances during the AGB stage, the stars are either oxygen-rich or carbon-rich, and as such, the dust produced in their atmospheres is either O-rich or C-rich. The chemical composition of the ejecta, indicates the stellar chemistry at the moment of ejection. The disruption of the spherical symmetry of the mass loss can be caused by fast rotation, stellar magnetic fields or binarity, the latter being the most efficient and favourable mechanism. Such mechanisms can lead to the creation of circumstellar, equatorial, dusty structures, like discs, torii or spirals. Due to their small relative sizes, compared to their surrounding nebulae, they can be studied at best with the use of infrared interferometric techniques. We report the discovery of three such structures in sources at three different evolutionary stages, respectively, with the use of single- and multi-aperture interferometry. In the C-rich AGB star V Hya we imaged via aperture masking in the near-infrared, a complex and possibly orbiting structure, which is embedded within the star's molecular torus. Our MIDI observations in the mid-infrared have revealed, a silicate disc within the symbiotic nebula M2-9 that is currently being shaped by the central binary system within its core, and a C-rich disc-like structure in the born-again star Sakurai's Object, that is also aligned to an asymmetry found in its surrounding planetary nebula. Finally, we compare the properties of the structures found here with those found in the literature in order to establish a relation between late stellar evolution and the existence of dusty structures.

>Magister Scientiae - MSc / This thesis reports the results of a twofold study on the recently proposed phenomenon of 'stellar pulsations' in young brown dwarfs by the seminal study of Palla and Baraffe (2005) (PB05, thereafter). The PB05 study presents results of a non-adiabatic linear stability analysis showing that young brown dwarfs should become pulsationally unstable during the deuterium burning phase of their evolution. The PB05 calculations on which this prediction is based have already been applied in a number of ground and space-based observational campaigns aimed at searching for this newly proposed putative class of potential pulsators. However, despite their significance and implications, the theoretical calculations by PB05 have not yet, to date, been subjected to independent verification in a different computational framework. To achieve this, we have generated equilibrium brown dwarf models and performed non-adiabatic linear stability calculations similar to PB05 assuming their 'frozen-in convection' approximation and the relevant input physics. The calculations performed in this thesis show, in overall, that there is a good agreement between the results from our study and those in PB05. However, there seem to be significant differences for very low mass objects as pointed out in our comparative results. We attribute this difference to our different boundary conditions. Our outer boundary condition is equivalent to the Eddington approximation in the 3-D case (e.g see Unno and Spiegel (1966)), while PB05 use a combination of different atmospheric profiles as discussed in Chabriel and Baraffe (2000). The validity of the frozen-in assumption used by PB05, which is based on the argument that the convective time scales calculated for these objects are much less than the pulsation time scales, has not been investigated. In this thesis, we have invoked a time-dependent theory of convection similar to Kuhfuss (1986) and Stellingwerf (1982) which includes turbulent pressure, turbulent diffusion and turbulent viscosity to study the pulsations. We have also investigated the effects of varying a number of free parameters in the above theoretical models. Our results show that turbulent pressure dominates in driving the pulsations in young brown dwarfs yielding growth rates much higher than in the frozen-in scenario. This is a new result that requires further analysis. The perturbation in the convective flux is found to have a damping effect on the acoustic modes. Turbulent viscosity is found to lead to damping which increases with increase in the value of the turbulent viscosity parameter and is found to have very little effect on the fundamental mode pulsation periods. Variation in the turbulent diffusion parameter has a very small effect on the fundamental mode periods and e-folding times. As a side lobe, we have determined theoretical pulsation constants for the fundamental mode and calculated the period ratios for the fundamental mode to those of the first and second harmonics. We find values of pulsation constants falling within the theoretical values calculated for variable stars shown in Cox (1980). This is explained in relation to the terms that go into the theoretical formula discussed later in this thesis. We find a correlation between the period ratios and the BDs mass and argue that such plots of the period ratios vs mass of the BDs could be useful in constraining the masses, given known periods from observations.

Le satellite spatial Kepler a détecté des oscillations de type solaire parmi plusieurs centaines d'étoiles, permettant la détermination de leurs propriétés physiques à l'aide de l’astérosismologie. Les modèles d'évolution stellaire et les lois d'échelle employés pour déterminer les paramètres tels que la masse, le rayon et l'âge nécessitent toutefois une calibration adaptée. Dans ce contexte, l'utilisation des systèmes binaires présentant des oscillations de type solaires pour les deux étoiles semble particulièrement appropriée. Au cours de cette thèse, nous avons procédé à un travail de détection de ces binaires sismiques parmi les données de Kepler ainsi qu'au développement des outils nécessaires à leur analyse. Bien que la découverte d'une nouvelle binaire sismique semblait très peu probable, nous avons pu rapporter pour la toute première fois la détection d'oscillations de type solaire associées aux deux étoiles les plus brillantes d'un système triple, à savoir HD 188753. À partir de la modélisation, nous avons déterminé des âges semblables pour les deux étoiles détectées en astérosismologie, comme attendu en raison de leur origine commune. Par ailleurs, nous avons entrepris la première analyse orbitale de ce système hiérarchique dans le but d'obtenir une estimation directe des masses et de la parallaxe. Finalement, l'exemple de HD 188753 illustre notre capacité à détecter et à modéliser chacune des étoiles d'un système binaire ou multiple tout en réalisant l'analyse orbitale de celui-ci. Les différents outils développés au cours de cette thèse seront intensivement utilisés dans le cadre des futures missions TESS et PLATO. / The Kepler space telescope detected solar-like oscillations in several hundreds of stars, providing a way to determine their physical properties using asteroseismology. However, the stellar evolutionary models and scaling relations employed to determine parameters such as the mass, the radius and the age require a proper calibration. In this context, the use of seismic binaries showing solar-like oscillations in both stars is especially suitable. During this thesis, we have worked towards the detection of such seismic binaries from the Kepler database and developed the necessary tools to study them. Although the discovery of a new seismic binary was very unlikely, we were able to report for the first time the detection of solar-like oscillations in the two brightest stars of a triple stellar system, namely HD 188753. Using stellar modelling, we found compatible ages for the two stars derived from asteroseismology, as expected from their common origin. In addition, we performed the first orbital analysis of this hierarchical system in order to derive a direct estimate of masses and parallax. Finally, the example of HD 188753 shows our capability to detect and model each of the stars of a binary or multiple system and to perform the orbital analysis of this one. The various tools developed during this thesis will be extensively used in the context of the future missions TESS and PLATO.

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An ultraviolet instrument compatible with a CubeSat form factor is currently being developed at Virginia Tech for the purpose of measuring nitric oxide in the polar night through the stellar occultation technique. This instrument will allow the investigation of how the Sun and Earth systems are related via energetic particle precipitation in the auroral regions. The work performed in this thesis supports the instrument design and requirements development by modelling the stellar occultation geometry to identify orbit parameters and target stars that could yield nitric oxide measurements during the polar winter at consistent latitudes, to best observe the build-up and fall-off of nitric oxide. The orbit study was accomplished through the development of an open-source tool in MATLAB, the Stellar Occultation Mission Planner. The results of this analysis were used to model the instrument performance and identify the required narrowband filter parameters to meet science requirements. Additional studies were performed to explore system performance for a future flight opportunity. / Master of Science / A small, light weight instrument is being designed at Virginia Tech to allow for nitric oxide in the atmosphere to be measured during the long polar nights that occur during winter in the Arctic and Antarctic regions. This instrument will allow scientists to explore how the Sun and Earth interact through space weather at high latitudes. This will be accomplished by using star light to probe the atmosphere while the instrument is onboard a small spacecraft called a CubeSat. The work performed in this thesis simulated the spacecraft orbit to determine which stars yielded the best measurements over the course of the polar night. Using these results, the instrument performance was simulated to inform the design of a filter for the instrument. Additional studies were performed to support the design of a future mission to fly the instrument in space.

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