The aim of this thesis is to revisit the properties of white dwarf stars in the Solar neighbourhood (distance > 100 pc), in particular their magnetic fields, the occurrence of binarity and their space density. This thesis presents observations and analysis of a sample of white dwarfs from the southern hemisphere. Over 80 objects were observed spectroscopically, and 65 of these were also observed with a spectropolarimeter. Many of the white dwarfs observed belong to the Solar neighbourhood, and can be used to study the star formation and evolution in this region. Our spectropolarimetric measurements helped constrain the fraction of magnetic white dwarfs in the Solar neighbourhood. Combining data from different surveys, I found a higher fraction of these objects in the relatively old local population than in other younger selections such as the Palomar-Green survey which suggests magnetic field evolution in white dwarfs, or different sets of progenitors. The progenitors of magnetic white dwarfs have been assumed to be Ap and Bp stars, however I find that the properties and number of Ap and Bp stars would only explain white dwarfs with magnetic fields larger than 100 MG. The number of known white dwarfs is believed to be complete to about 13 pc, however the sample is certainly incomplete to 20 pc from the Sun. To identify new white dwarfs in the Solar neighbourhood, some possibly magnetic or in binaries, numerous candidate white dwarfs from the Revised NLTT catalogue have been observed, which resulted in the discovery of 13 new white dwarfs, with 4 of these having a distance that places them within 20 pc of the Sun. The candidates were selected using a V − J reduced-proper-motion diagram and optical-infrared diagram. A total of 417 white dwarf candidates were selected, 200 of these have already been spectroscopically confirmed as white dwarfs. Spectroscopic confirmation is required for the remaining 217 candidates, many of these are likely to belong to the Solar neighbourhood. Four close binaries consisting of a white dwarf and a cool companion were also observed, for which atmospheric and orbital parameters were obtained. The photometry for two of these binary systems, BPM 71214 and EC 13471-1258 shows that the secondary stars are filling their Roche lobes, and combined with their orbital parameters, these systems are very good candidates for hibernating novae. The time of their previous interaction or the extent of this interaction are unknown. The two other binary systems, BPM 6502 and EUVE J0720-31.7 are post-common envelope binaries. BPM 6502 is not expected to interact within a Hubble time, however EUVE J0720-31 is expected to become a cataclysmic variable within a Hubble time. The atmospheric parameters of the white dwarfs were determined using model atmosphere codes which were modified for the present study to include convective energy transfer, self-broadening and Lyman satellite features.

Stars of all ages and evolutionary stages are seen to be surrounded by discs of matieral. during the formation of a stellar system the stars are orbited by a massive protoplanetary disc composed of interstellar gas and dust, in which planet formation occurs. Betewwen 1 and 10 Myr the protoplanetary disc disperses, leaving behind the newly formed system of planets and smaller bodies. The remaining material which has not formed into planets is referred to as a debris disc. Even though the interstellar dust grains from the protoplanetary disc have long been removed from the system, debris discs can contain large quantities of dust due to collisions between larger bodies and cometary activity. such dust can be detected by its thermal emission. This thesis focuses on observational studies at far-infrared and sub-millimetre wavelengths of debris discs and the late stages of protoplanetary disc evolution. An overview of surveys for debris discs performed to date is presented, highlighting the limitations and statistical biases. the motivation, design and sample selection for two large surveys for debris discs around nearby stars, with the Hershel space observatory and the SCUBA-2 sub-millimetre camera on the James Clerk Maxwell Telescope, are described. The combination of a uniform obstevational strategy, longer wavelengths than previous surveys, and a large, clearly chosen sample - unbiased by stellar properties - will allow robust statistical conclusions of how the incidence and properties of debris discs depend on system parameters such as stellar mass, age, metallicity, binarity and the presence of planets. As a precursor to the Hershel and SCUBA-2 surveys, a volume-limited ample of 130 A type star systems was surveyed using observations at 24 and 70 μm, which were required to determine the presence of emission from dust, were predicted by fitting model flux distributions to optical and near-infrared photometry. Debris discs were detected around 46 systems, 12 of which including the system with the largest dust mass - are new discoveries. This survey adds to the results of previous studies which show that debris disc incidence is not correleated with host star metallicity despite the wll known giant planet - metallicity correlation, This is in accordance with what is predicted from the core accretion theory of planet formation. The most signigicant result from this survey is that, contrary to results reported in a previous work, debris discs are oberall less common around binary stars. Further investigation shows that systems with separations of ~3-150 AU are especially deficient of debris, while closer binaries and the primaries of wider binaries show debris detection rates consisten with those for single stars. A sample of circumstellar discs around 29 young stellar systems with ages of 5-30 Myr were observed with the LABOCA sub-millimetre instrument on the APEX telescope at 870μm, to provide disc masses or mass upper limits in support of a large Hershel programme. These targets included the η Chamaeleontis cluster and four bright Herbig Ae/Be stars which have not previously been observed at this wavelength. All but the Herbig Ae/Be stars were not detected, and 3σ dust mass upper limits of ~ 0.1-3 M are determined, with corresponding total disc masses of ~0.03-1Mjup. These mass limits indicate that there is insufficient remaining material in these discs to form gas giant planets, and add to the prevailing view that protoplanetary discs typically disperse within 10 Myr and that gas giant planet formation must be completed before this time. A search for cold dust emimission from two of the Solar System's nearest neighbours - α Centauri AB and ε Indi - was also performed with LABOCA. In both cases no debris disc emission was detected. A bright resolved feature was detected near α Centauri AB, nowever, follow-up observations at a second epoch, two years after the initial observations, showed that the feature is not co-moving with the stars. It is argued that the feature is most likely a pre-stellar core. The stars α Centauri A and B are detected, which is one of only very few detections of main sequence stellar photospheres at sub-millimetre wavelengths.

I present the development of two near infrared proper motion pipelines for high resolution near infrared data from UKIDSS and the VISTA VVV survey. The UKIDSS pipeline is capable of accuracies of order 5-10 mas yr⁻¹ for bright sources with the largest epoch baselines (~ 8 years). The VVV pipeline reaches 1-2 mas yr⁻¹ proper motion precision at the bright end and parallax measurements at ~ 1 mas precision. It will be possible to improve upon the VVV astrometric precision due to increases in data volume and further pipeline development. I have used the proper motion pipelines to generate three near infrared proper motion catalogues of the UKIDSS LAS and GPS and the VVV survey. The LAS proper motion catalogue covers 1500 deg2 at high Galactic latitudes and contains approximately 15 million sources with two J band epochs. The GPS proper motion catalogue covers 1500 deg2 of the northern Galactic plane and contains approximately 400 million sources with two K band epochs. The VVV proper motion catalogue covers 560 deg2 of the Galactic bulge and disc and contains approximately 200 million sources with between 50 and 150 Ks band epochs. I have also produced a preliminary 5σ parallax catalogue containing 3403 VVV sources. The LAS and GPS proper motion catalogues have been used by myself and other authors to identify and study many new examples of high proper motion stars, brown dwarfs, ul-tracool dwarf benchmark candidates, cool white dwarfs, substellar subdwarfs and nearby sources within < 25 pc. These catalogues remain far from fully exploited and will be a useful resource for future research by the astronomical community. Exploitation of the VVV proper motion catalogue is still in its infancy, yet it has already generated large numbers of new high proper motion sources. These include new brown dwarf candi-dates, important benchmark objects, and nearby sources which have previously avoided detection. Parallax results from the VVV pipeline will be useful to improve low mass star/ultracool dwarf luminosity functions, significantly increasing the numbers of brown dwarfs with known parallaxes and illustrates how general purpose multi-epoch wide area surveys can generate parallaxes. Finally, I discuss the long term usefulness of such catalogues in the Gaia era and how they might be exploited in the future.

One of the key goals of Milky Way science is measuring the distribution of dark matter in the Galaxy. Through the study of Galactic dynamics, inferences can be made about the structure of the Galaxy, and hence the dark matter distribution. To this end, we present a study of methods useful for modelling and understanding dynamical systems in the Galaxy. A natural choice of coordinate system when studying dynamical systems is the canonical system of angle-action coordinates. We present methods for estimating the angle-actions in both axisymmetric and triaxial potentials. These fall into two categories: non-convergent and convergent. The non-convergent methods are fast approaches, mostly based on approximations to Stäckel potentials. We investigate the accuracy of these methods for realistic Galactic potentials. The slower convergent methods operate by constructing generating functions to take us from simple analytically-tractable potentials to our target potential. Tidal streams should prove useful for constraining the large-scale dark matter distribution in the Galaxy. Armed with our new angle-action tools, we investigate the properties of known streams in a realistic Galactic potential. We present a simple algorithm for constraining the Galactic potential using a tidal stream, which exploits the expected structure of a stream in the angle-frequency space of the true potential. We expand this approach into a fully probabilistic scheme that allows for handling of large errors, missing data and outliers. We close by discussing another tool useful for modelling the dynamics of the Galaxy: extended distribution functions for the Galactic disc. We present a simple extension of an action-based distribution function from Binney (2010) that includes metallicity information, and compare the model predictions with current data. These models are essential for incorporating the selection effects of any survey, and reveal the important chemo-dynamic correlations that expose the history and evolution of the Galaxy.

Au moment d'écrire ma Thèse plus de 900 exoplanètes été annoncées et plus de 2700 planètes détectées par le télescope spatial Kepler sont en attente d'être confirmées. La haute précision des spectres et des courbes de lumière obtenue dans les relevés Doppler et transit, permet l'étude détaillée des paramètres des étoiles hôtes, et ouvre la possibilité d'enquêter sur les corrélations étoile planètes. En outre, la détermination des paramètres stellaires avec précision est un besoin critique pour déterminer les paramètres planétaires, à savoir, la masse, le rayon et la densité.Dans le cas des naines FGK, la détermination des paramètres stellaires est bien établie et peut être utilisée avec confiance pour étudier la relation planète-étoile ainsi que pour obtenir les paramètres planétaires avec une grande precision. Cependant, ce n'est pas le cas pour les naines M, les étoiles les plus communes de la Galaxie. Par rapport à leurs cousines plus chaudes, les naines M sont plus petites, plus froides, et plus faiblement lumineuses, et donc plus difficile à étudier. Le plus grand défi qui concerne les naines M est lié à la présence de milliards de lignes moléculaires qui gomme le continuum et rend l'analyse spectrale classique presque impossible. Trouver des fac ̧ons nouvelles et novatrices pour surmonter cet obstacle et obtenir une mesure des paramètres stellaires est l'objectif principal de cette Thèse .Pour l'atteindre, j'ai concentré mes recherches sur deux approches méthodologiques, photométrique et spectroscopiques. Mon premier travail avait pour objectif d'établir l'étalonnage de métallicité pho- tométrique précis. Par manque de binaires FGK+M avec de bonnes données photométriques je ne pouvais pas atteindre cet objectif. Il m'a cependant était possible, avec les données disponibles, de comparer les étalonnages photométriques déjà établies et légèrement améliorer le meilleur d'entre eux, comme décrit au Chapitre 3.Puis, je me suis concentré sur les approches spectroscopiques pour obtenir des paramètres stel- laires plus précis pour les naines M. À cette fin, j'ai utilisé des spectres HARPS de haute résolution et développé une méthode pour mesurer les lignes spectrales sans tenir compte du continuum . En utilisant cette méthode, je créé un nouvel étalonnage visible avec une précision de 0.08 dex pour [Fe/H] et 80 K pourTeff .Ce travail est détaillé dans le Chapitre 4.Finalement , j'ai également participé à l'amélioration des paramètres de l'étoile GJ3470 et de sa planète, où mon expertise dans les paramètres stellaires de naines M avait un rôle important. Les détails concernant cette enquête sont présentés dans le Chapitre 5 / At the time of writing of this Thesis more than 900 planets have been announced and about 2700 planets from the Kepler space telescope are waiting to be confirmed. The very precise spectra and light curves obtained in Doppler and transit surveys, allows the in-depth study of the parameters of the host stars, and opens the possibility to investigate the star-plant correlations. Also, determining the stellar parameters with precision is critical for more precise determinations of the planetary parameters, namely, mass, radius, and density.In the case of the FGK dwarfs, the determination of stellar parameters is well established and can be used with confidence to study the star-planet relation as well as to obtain precise planetary parameters. However, this is not the case for M dwarfs, the most common stars in the Galaxy. Compared to their hotter cousins, M dwarfs are smaller, colder, and fainter, and therefore harder to study. The biggest challenge regarding M dwarfs is related to the presence of billions of molecular lines that depress the continuum making a classical spectral analysis almost impossible. Finding new and innovative ways to overcome this obstacle in order to obtain precise stellar parameters is the goal of this Thesis.To achieve this goal I focused my research into two main avenues: photometric and spectroscopic methods. My initial work had the objective of establishing a precise photometric metallicity calibration, but I could not reach this goal, as I did not have enough FGK+M binaries with good photometric data. However, it was possible, with the available data, to compare the already established photometric calibrations and slightly improve the best one, as described in Chapter 3.Then, I focused on spectroscopic approaches with the aim of obtaining precise M dwarf parame- ters. To this end I used HARPS high-resolution spectra and developed a method to measure the spectral lines disregarding the continuum completely. Using this method I established a new visible calibration with a precision of 0.08 dex for [Fe/H] and 80 K for Te f f . This work is detailed in Chapter 4.Finally, I also participated in the refinement of the parameters of the star GJ3470 and its planet, where my expertise in stellar parameters of M dwarfs had an important role. The details regarding this investigation are shown in Chapter 5. / No momento em que escrevo esta Tese, o número de planetas anunciados já ultrapassou os 900 e os cerca de 2700 candidatos detectados pelo telescópio espacial Kepler esperam por confirmação. Os espectros e as curvas de luz obtidos nos programas de procura de planetas permitem, também, o estudo em profundidade dos parâmetros das estrelas com planetas e abrem a possibilidade de investigar a relação estrela-planeta. Neste contexto, a determinação com precisão dos parâmetros estelares é crítica na determinação precisa dos parâmetros planetários, nomeadamente, a massa, o raio e a densidade.No caso das anãs FGK, os métodos de determinação dos parâmetros estelares estão bem estabelecidos e podem ser usados com confiança no estudo da relação estrela-planeta, assim como na obtenção de parâmetros planetários precisos. No entanto, não é esse o caso para as anãs M, as estrelas mais comuns da nossa Galáxia. Ao contrário das suas primas, as estrelas M são mais pequenas, frias e ténues e, assim sendo, mais difíceis de estudar. O grande entrave no estudo das estrelas M está relacionado com a presença de biliões de linhas moleculares que deprimem o contínuo espectral, fazendo com que uma análise espectral clássica se torne quase impossível. A procura de métodos inovadores que possibilitem ultrapassar este obstáculo, tendo em vista a obtenção de parâmetros precisos, é o objectivo desta Tese.Tendo em conta esse objetivo, foquei os meus esforços em duas linhas principais de pesquisa, baseadas em métodos fotométricos e métodos espectroscópicos. O meu trabalho inicial tinha como objetivo o estabelecimento de uma calibração fotométrica para a metalicidade, mas não me foi possível atingir esse objetivo, pois não tinha sistemas binários FGK+M suficientes com bons dados fotométricos. No entanto, foi possível, com os dados disponíveis, comparar as calibrações fotométricas existentes e refinar ligeiramente a melhor delas, como descrito no Capítulo 3.Após este trabalho passei a concentrar-me em técnicas espectroscópicas de obtenção de parâmetros estelares em estrelas M. Tendo em mente esse objetivo, usei espectros HARPS de alta resolução para desenvolver um novo método de medição de linhas espectrais independente do contínuo espectral. Seguidamente, usei este método no desenvolvimento de uma nova calibração de metalicidade e temperatura efectiva em estrelas M na região do visível, através da qual consegui atingir uma precisão de 0.08 dex para a [Fe/H] e de 80 K para a temperatura. Este trabalho está descrito no Capítulo 4.Ao mesmo tempo colaborei na determinação com precisão dos parâmetros da estrela GJ3470 e do seu planeta, onde a minha proficiência na determinação de parâmetros estelares em anãs M teve um papel importante. Os detalhes relacionados com este trabalho de investigação estão descritos no Capítulo 5.