The Large Binocular Telescope Interferometer uses a near-infrared camera to measure the optical path length variations between the two AO-corrected apertures and provide high-angular resolution observations for all its science channels (1.5-13 microns). There is however a wavelength dependent component to the atmospheric turbulence, which can introduce optical path length errors when observing at a wavelength different from that of the fringe sensing camera. Water vapor in particular is highly dispersive and its effect must be taken into account for high-precision infrared interferometric observations as described previously for VLTI/MIDI or the Keck Interferometer Nuller. In this paper, we describe the new sensing approach that has been developed at the LBT to measure and monitor the optical path length fluctuations due to dry air and water vapor separately. After reviewing the current performance of the system for dry air seeing compensation, we present simultaneous H-, K-, and N-band observations that illustrate the feasibility of our feedforward approach to stabilize the path length fluctuations seen by the LBTI nuller.

Until the last decade or so, our entire knowledge of planets around Sun-like stars consisted of those in our own Solar System. This is no longer the case. Over 200 planets have been discovered through radial velocity surveys and photometric studies, both of which depend on observing the planet's effects on its host star. Much of our knowledge of the planets orbiting these stars is uncertain, based on assumptions about the stars' masses and the planets' orbital inclinations. This dissertation is comprised of two main sections. The first involves measuring the angular diameters for a sample of exoplanet host stars using Georgia State University's CHARA Array in order to learn more about the nature of these stars. These direct angular measurements are not dependent on the exoplanet systems' inclinations or the masses of the stars. Improved angular diameters lead to linear diameters when combined with HIPPARCOS parallax measurements, which in turn tell us of the stars' ages and masses. Of the 82 exoplanet systems observable with the CHARA Array, 31 host stars were observed and stellar angular diameters were measured for 26 systems. In the special case of an exoplanet system with a transiting planet, this direct measurement of the star's angular diameter leads to a direct measurement of the planet's diameter, when the planet-to-star-radii ratio is known from photometric studies. This was done for HD 189733. The star's angular diameter is 0.377 +/- 0.024 mas, which produces a stellar linear radius of 0.779 +/- 0.052 R_Sun and a planetary diameter of 1.19 +/- 0.08 R_Jupiter. The second part of this project involved the inspection of the exoplanet systems for stellar companions masquerading as planets. From radial velocity studies alone, it is impossible to distinguish between a planet in a high-inclination orbit and a low-mass stellar companion in a low-inclination orbit. Using the CHARA Array, it was possible to rule out certain secondary spectral types for each exoplanet system observed by studying the errors in the diameter fit and searching for separated fringe packets. While no definitive stellar companions were found, two expolanet systems, upsilon Andromedae and rho Coronae Borealis, exhibited behavior that were not consistent with the host star being a simple limb-darkened disk.

Over the past half century, multiplicity studies have provided a foundation for the theories of stellar formation and evolution through understanding how likely it is that stars form alone or with companions. If spectroscopic orbits are combined with techniques that can determine visual orbits, we can access the most fundamental parameter of stellar evolution, stellar mass. This dissertation is composed of two main sections. The first involves the investigation of the seminal multiplicity study of Duquennoy & Mayor (1991b) which has been the ``gold standard" for solar-type stars for nearly 20 years. Improvements in technology in the intervening years have improved the measurement accuracy for radial velocities and distances on which the study was based. Using Georgia State University's CHARA Array to search the systems in Duquennoy & Mayor's multiplicity survey for overlooked companions along with a literature search covering regimes unreachable by the CHARA Array, we have found that more than 40% of the Duquennoy & Mayor's sample was further than originally believed and the uncorrected multiplicity percentages change from 57:38:4:1:0% (single:double:triple:quad:quint%) to 48:42.5:7.5:1:1% with the discoveries of multiple previously undiscovered companions. The second part of this project describes the application of separated fringe packets for resolving the astrometric position of secondaries with small angular separations on long-baseline optical interferometers. The longest baselines of the CHARA Array allow access to a previously inaccessible range of separations compared with other techniques (<40 milliarcseconds) and the ability to very accurately angularly resolve a large number of single- and double-lined spectroscopic binaries. Combining astrometric and spectroscopic orbits provides assumption-free stellar masses and using the CHARA Array allows access to many previously unreachable systems available for high-accuracy mass determinations. We report the first angular separation measurements of seven spectroscopic binary systems, five additional separated fringe packet detections, ten systems with probably overlapping fringe packets, four systems with new data on pre-existing orbits, one completely new visual orbit for a SB2 system previously unresolved, and the detection of two previously unknown companions.

Optical interferometry is a technique by which the diameters and indeed the direct pulsations of stars are routinely being measured. As a follow-on to a 7 year interferometric campaign to measure the pulsations of over 100 mira variables, our team has been using the Spitzer Space Telescope to obtain 95 mid-infrared spectra of 25 miras during their pulsations over one year while simultaneously ascertaining their near-infrared diameters using the Palomar Testbed Interferometer. These data will then be combined with modeling from NLTE and radiative transfer codes to place hard constraints on our understanding of these stars and their circumstellar environments. We present some initial results from this work and discuss the next steps toward fully characterizing the atmosphere, molecular photosphere and dust production in mira variables.

Les étoiles massives représentent un des principaux contributeurs à l'enrichissement des galaxies en éléments lourds et en poussière interstellaire. L’ultime étape de leur évolution est représentée par le stade Wolf-Rayet (WR). Les étoiles WR présentent la particularité de générer un vent stellaire radiatif dense, qui peut interagir avec celui d’un compagnon proche, donnant naissance à un environnement de poussière en forme de spirale. Les ordres de grandeur associés à ce type d’objet sont spectaculaires : avec un taux de formation de poussière équivalent à la masse de la planète Mars produite chaque année, elles rivalisent avec les producteurs historiques de la poussière que sont les étoiles de la branche asymptotique des géantes (AGB) ou les supernovæ (SN). Les étoiles WR à poussière pourraient ainsi répondre à une problématique bien connue : d’où vient la poussière observée dans les galaxies ? Le présent travail de thèse vise donc à enrichir nos connaissances sur ce problème à travers tous les aspects de la chaîne scientifique : de l’observation à l’analyse de données en employant différents niveaux de sophistication en modélisation numérique (analytique, transfert radiatif et hydrodynamique). Le premier aspect exploré par cette thèse concerne la modélisation des nébuleuses spirales de poussières. J’ai d’abord développé un modèle analytique permettant de contraindre les aspects géométriques des spirales. Ce dernier inclut différentes hypothèses physiques comme la prise en compte d’un rayon de sublimation, de différents types de structure interne, etc. J’ai ensuite inclut le transfert de rayonnement au modèle géométrique afin de relier la distribution d’intensité de l’objet (l’image) à sa distribution en densité. Ce modèle 3-D de spirale de poussière permet d’étudier les effets d’opacité et d’ombrage liés à la masse ou au type de poussière considérée. J’ai également développé un modèle 3-D axisymétrique en transfert de rayonnement afin d’assimiler la spirale à une suite d’anneaux concentriques. Il vise à reproduire la distribution d’intensité d’une spirale à un azimut donné et permet une comparaison directe aux profils radiaux d’intensité issus d’observations. Enfin, nous avons mis en place un modèle hydrodynamique 3-D de binaire à interaction de vent, afin d’avoir une idée réaliste des conditions physiques en place au niveau de la zone de nucléation des poussières. Le second aspect abordé par cette thèse se concentre sur l’étude du prototype des nébuleuses spirales de poussière, nommé WR 104. J’explore ici toutes les échelles spatiales de l’objet : des grandes échelles avec l’imageur VLT/VISIR afin de faire le lien avec milieu interstellaire, aux régions les plus internes avec l’instrument VLTI/AMBER pour sonder la zone de nucléation de poussière, en passant par l’instrument d’optique adaptative extrême, VLT/SPHERE, afin d’étudier les premiers tours de la spirale. Le troisième et dernier aspect concerne l’instrument de seconde génération à équiper l’interféromètre européen (VLTI) : MATISSE. Il est le tout premier instrument à opérer en simultané dans les bandes L, M et N en recombinant la lumière issue de quatre télescopes. MATISSE a été conçu pour étudier une variété de cas scientifiques : des disques protoplanétaires aux noyaux actifs de galaxie, en passant par les environnements circumstellaires. Afin de préparer les premiers programmes observations, j’ai développé un outil automatisé, nommé PREVIS, visant à prédire l’observabilité des objets. Dans le cadre des nébuleuses spirales, j’ai pu explorer les capacités de l’instrument en reconstruction d’image en testant différents aspects (tailles, inclinaison, couverture (u-v), etc.). Avec un pouvoir de résolution spatiale de 3 mas à 3,5 µm, MATISSE permettra d’étudier ces objets de façon unique, en résolvant pour la première fois l’épaisseur des bras spiraux, leurs structures internes ou la position exacte du bord de sublimation. / Massive stars are one of the major contributors to the enrichment of galaxies in heavy elements and interstellar dust. The last stage of their evolution is represented by the Wolf-Rayet phase (WR). WR stars generate a dense radiative stellar wind, which can interact with the wind from a close companion and cause a spiral dust environment called pinwheel nebula. The orders of magnitude associated with this kind of object are spectacular: with a dust formation rate equivalent to the mass of the planet Mars produced each year, WR stars compete with the historical dust producers, like the stars of the asymptotic giant branch (AGB) or the supernovae (SN). Dusty WR stars could thus answer a well-known problem: where does the dust observed in galaxies come from? This thesis aims at enriching our knowledge about this problem using all aspects of the scientific chain: from observation to data analysis by using different levels of sophistication in numerical modelling (analytical, radiative transfer and hydrodynamics). The first aspect explored by this thesis concerns the modelling of spiral dust nebulae. I first developed an analytical model for the spiral to constrain the geometrical aspects of the spiral, including a number of physical hypothesis like the dust sublimation radius and different types of internal structure. The next step consisted to include the radiative transfer in the geometrical model in order to link the intensity distribution of the object (the image) to its density distribution. This 3-D model of spiral allow to study the opacity and shadowing effects related to the dust mass considered. Similarly, I developed a 3-D axisymmetric radiative transfer model to mimic the spiral into a series of concentric rings. This model aims to reproduce the intensity distribution of a spiral at a given azimuth and allows a direct comparison with the radial intensity profiles derived from observations. Finally, we implemented a 3-D hydrodynamic model of a wind-wind interacting binary to get a realistic idea of the physical conditions in places around the dust nucleation zone. The second aspect addressed by this thesis focuses to the study of the prototype of the pinwheel nebula, called WR104. Such object is an ideal laboratory to study the problem of dust nucleation around massive stars. I explored all spatial scales of WR 104: From the large scale with VLT/VISIR to study the link with the interstellar medium, to the internal regions with VLTI/AMBER to probe the dust nucleation zone, including intermediate angular resolution to study the pinwheel structure with extreme adaptive optics instrument VLT/SPHERE. The third and last aspect deals with the second generation of the instrument installed at the European Very Large Telescope Interferometer (VLTI): MATISSE. It is the first instrument operating simultaneously in the L, M and N bands by recombining the light coming from four telescopes. MATISSE was developed to study different scientific cases: protoplanetary disks, the circumstellar environments and the active galactic nuclei. To prepare the first observation programs, I developed an automated tool, called PREVIS, to determine the observability of objects according to their magnitude and celestial coordinate. In the context of spiral nebulae, I explored the image reconstruction capabilities of the instrument by testing different aspects: geometric (size, inclination, opening angle, etc.) and observational (coverage (u-v), sampling). The unprecedented spatial resolution of MATISSE of 3 mas at 3.5 µm will allow to study these objects in a unique way, resolving for the first time the thickness of the spiral arm, its internal structure or the exact position of the sublimation radius.