Este trabalho tem como objetivo principal a determinação da distância de estrelas pré-sequência principal em regiões de formação estelar próximas. A determinação precisa da distância individual das estrelas é necessária para obter os principais parâmetros físicos de cada estrela e para investigar a estrutura da Galáxia. Em particular, investigamos as regiões de formação estelar de Lupus e Ophiuchus que contém uma das associações mais ricas em estrelas T Tauri. A grande maioria das estrelas pré-sequência principal nessas regiões não foi observada pelo satélite Hipparcos devido à sua magnitude e também não têm paralaxe trigonométrica medida a partir do solo devido à distância em que se encontram. O procedimento aqui empregado para a obter a distância individual das estrelas baseia-se na estratégia de ponto de convergência e utiliza dados de movimento próprio e velocidade radial. Desenvolvemos uma nova versão do método de ponto de convergência que permite simultaneamente determinar a posição do ponto de convergência e selecionar os membros de um moving group. Partindo dos dados de movimento próprio e o novo método aqui desenvolvido investigamos as propriedades cinemáticas e realizamos uma análise de pertinência das estrelas em cada região estudada o que nos permitiu identificar um moving group com 114 estrelas em Lupus e 55 estrelas em Ophiuchus. Calculamos a distância para cada membro do grupo usando velocidades radiais publicadas, que foram complementadas com novas observações, e a velocidade espacial do moving grup para as estrelas com velocidade radial não conhecida. Calculamos as paralaxes com precisão de 1-2~mas o que implica em um erro relativo médio de 25% nas distâncias obtidas. Finalmente, investigamos as propriedades dos diversos subgrupos e a estrutura tridimensional dos complexos de nuvens em Lupus e Ophiuchus, concluindo que existem efeitos de profundidade importantes. Utilizamos os novos resultados de distância para obter os parâmetros físicos (luminosidade, massa e idade) das estrelas e o diagrama-HR de cada região de formação estelar considerada, confirmando a distribuição de idade diferente das duas subclasses de estrelas T Tauri. Os resultados aqui obtidos representam um primeiro passo no sentido de melhor entender a estrutura das regiões de formação estelar e os estágios iniciais da formação de estrelas e planetas. / The main objective of this work is to determine the distance of pre-main sequence stars in nearby star-forming regions. A precise determination of the distance to individual stars is required to accurately determine the main physical parameters of each star and the structure of the Galaxy. Here we investigate the Lupus and Ophiuchus star-forming regions that contain one of the richest associations of T Tauri stars. Most pre-main sequence stars in these regions were neither observed by the Hipparcos satellite due to their magnitude nor have any trigonometric parallax measured from the ground due to their distance. The procedure that we use here to derive the distance to individual stars is based on the convergent point strategy and makes full use of proper motion and radial velocity data. We developed a new version of the convergent point search method that simultaneously determines the convergent point position and selects the most likely members of a moving group. Based on proper motion data and our new method we investigate the kinematic properties and perform a membership analysis of the stars in each star-forming region considered that allows us to identify a moving group with 114 stars in Lupus and 55 stars in Ophiuchus. We calculate the distance of each group member using published radial velocities, which we supplemented with new measurements, and the spatial velocity of the moving group for the remaining stars with unknown radial velocity. We derived parallaxes with accuracies of 1-2 mas yielding the average relative error of 25% on the distances. Finally, we investigate the properties of the various subgroups and the three dimensional structure of the Lupus and Ophiuchus cloud complex and conclude that significant depth effects exist. We use the new distances to refine the physical parameters (luminosity, mass and age) of stars and the HR-diagram for each star-forming region considered confirming the different age distribution of the two T Tauri subclasses. These results represent a first step towards better understanding the structure of star-forming regions and the early stages of star and planet formation.

<p>Stars form in clusters amidst complex and coupled physical phenomena. Among the most important of these is radiative feedback, which heats the surrounding gas to suppress the formation of many low-mass stars. In simulations of star formation, pre-main-sequence modeling has often been neglected and stars are assumed to have the radii and luminosities of zero-age main sequence stars. We challenge this approach by developing and integrating a one-zone protostellar evolution model for FLASH and using it to regulate the radiation output of forming stars. The impact of accurate pre-main-sequence models is less ionizing radiation and less heating during the early stages of star formation. For stars modeled in isolation, the effect of protostellar modeling resulted in ultracompact HII regions that formed slower than in the ZAMS case, but also responded to transitions in the star itself. The HII region was seen to collapse and subsequently be rebuilt as the star underwent a swelling of its radius in response to changes in stellar structure and nuclear burning. This is an important effect that has been missed in previous simulations. It implies that observed variations in HII regions may signal changes in the stars themselves, if these variation can be disentangled from other environmental effects seen in the chaotic cluster environment.</p> / Master of Science (MSc)

I develop the first magnetospheric accretion model to take account of the observed complexity of T Tauri magnetic fields, and the influence of stellar coronae. It is now accepted that accretion onto classical T Tauri stars is controlled by the stellar magnetosphere, yet to date the majority of accretion models have assumed that the stellar magnetic field is dipolar. By considering a simple steady state accretion model with both dipolar and complex magnetic fields I find a correlation between mass accretion rate and stellar mass of the form M[dot above] proportional to M[asterisk subscript, alpha superscript], with my results consistent within observed scatter. For any particular stellar mass there can be several orders of magnitude difference in the mass accretion rate, with accretion filling factors of a few percent. I demonstrate that the field geometry has a significant effect in controlling the location and distribution of hot spots, formed on the stellar surface from the high velocity impact of accreting material. I find that hot spots are often at mid to low latitudes, in contrast to what is expected for accretion to dipolar fields, and that particularly for higher mass stars, accreting material is predominantly carried by open field lines. Material accreting onto stars with fields that have a realistic degree of complexity does so with a distribution of in-fall speeds. I have also modelled the rotational modulation of X-ray emission from T Tauri stars assuming that they have isothermal, magnetically confined coronae. By extrapolating from surface magnetograms I find that T Tauri coronae are compact and clumpy, such that rotational modulation arises from X-ray emitting regions being eclipsed as the star rotates. Emitting regions are close to the stellar surface and inhomogeneously distributed about the star. However some regions of the stellar surface, which contain wind bearing open field lines, are dark in X-rays. From simulated X-ray light curves, obtained using stellar parameters from the Chandra Orion Ultradeep Project, I calculate X-ray periods and make comparisons with optically determined rotation periods. I find that X-ray periods are typically equal to, or are half of, the optical periods. Further, I find that X-ray periods are dependent upon the stellar inclination, but that the ratio of X-ray to optical period is independent of stellar mass and radius. I also present some results that show that the largest flares detected on T Tauri stars may occur inside extended magnetic structures arising from the reconnection of open field lines within the disc. I am currently working to establish whether such large field line loops can remain closed for a long enough time to fill with plasma before being torn open by the differential rotation between the star and the disc. Finally I discuss the current limitations of the model and suggest future developments and new avenues of research.