The VISTA Variables in the Vía Láctea survey is a multi-epoch, near-infrared (NIR) search for RR Lyrae stars, which trace the structure and evolutionary history of the Galactic Bulge. To measure accurate light-curves for these and other variable sources in the crowded fields of the Bulge, we have built a pipeline to perform difference imaging analysis (DIA) on VVV data. This is the first time DIA has been carried out with a large-scale NIR survey, which presents certain specific challenges such as the large pixel scales and high variable background. This thesis begins with an investigation into the wider problem of reference image selection for DIA. Through this study, we found that an alternative formulation of traditional DIA - Target Image Convolution - yields better results for VVV. This technique is employed in the core image subtraction engine for the VVV DIA pipeline. This work also details the pipelines for carrying out calibration, period-finding and Fourier modelling on the light-curves of DIA sources, as well as for obtaining their dereddened NIR magnitudes and colours. To validate the pipelines, the magnitude calibration is checked against another NIR survey (in the most challenging Galactic centre field) and analysis results are crosschecked against the OGLE-IV catalogue of RR Lyrae (in a low-extinction region of the Galactic bulge). Both studies are used to determine the efficacy of the existing pipeline and identify areas for improvement. DIA generally shows a significant improvement over existing aperture photometry. Using the NIR properties of VVV DIA counterparts to the OGLE-IV RR Lyrae, we demonstrate that it is possible to identify new variables with this pipeline, including RR Lyrae.

No description available.

The kinematics and white dwarf distribution have been studied for the Globular Cluster NGC 6397. The data was obtained from NASA’s Hubble Space Telescope in 2005. In particular, we used the images of a field 5’ Southeast of the core of NGC 6397 from Advanced Camera for Surveys to conduct our analyses. The first part of the study is about the kinematics of the globular cluster. Isotropy of velocity distribution and cluster rotation have been considered. As anticipated, this relaxed cluster exhibited no strong signs of anisotropy. However, there appears to be some level of rotation. The rotational motion turns out to be mu sub alpha cos(delta) = 3.88 ± 1.41 mas yr −1 and mu sub delta = −14.83 ± 0.58 mas yr −1. This result is not entirely expected and deserves further investigation in future studies. The second of the thesis is based on white dwarf populations in the globular cluster and the Galactic Bulge. As a first glance, there appears to be a lacking of white dwarfs at the age of approximately 0.6 Gyr. Further investigation reveals this to be statistically insignificant. Through this analysis, another pattern of white dwarf abundance is discovered. There appeared to be much more stars at the age between 0.9 − 2.0 Gyr. This could be a manifestation of modeling error. As the final consideration of this thesis, white dwarf candidates in the Galactic Bulge are illustrated. Approximately 10 candidates are found at the most probable location of stars in the Bulge. The analyses conducted in this thesis set stage for further development in understanding of globular clusters. In particular, the rotation analysis raises curiosity about the dynamics of NGC 6397 in the plane of the sky. Moreover, the velocity distribution analysis confirms properties and theories pertaining to globular clusters.

Os aglomerados globulares (GCs) são laboratórios essenciais no estudo da formação e evolução quimiodinâmica da Via Láctea, pois formaram-se durante os episódios iniciais de formação estelar das galáxias. Na última década, evidências observacionais fotométricas e espectroscópicas têm desafiado o paradigma clássico da formação dos GCs em um episódio único de formação estelar, dando origem ao debate sobre as múltiplas populações estelares. Com o objetivo de explorar esse fenômeno, o programa do Telescópio Espacial Hubble GO-13297 (UV Legacy Survey of Galactic Globular Clusters, PI G. Piotto) obteve fotometria para 57 GCs com os filtros UV/azul F275W, F336W e F438W (WFC3/UVIS), sensíveis às variações nas abundâncias de CNO e, portanto, capazes de distinguir múltiplas populações ao longo dos diagramas cor-magnitude. Combinados com fotometria anterior nos filtros do óptico F606W e F814W (programa GO-10775, PI A. Sarajedini), eles formam uma base de dados fotométricos sem precedentes para GCs. Este trabalho expõe a análise de sete desses aglomerados, sendo seis do Bojo Galáctico (NGC 6304, NGC 6624, NGC 6637, NGC 6652, NGC 6717 e NGC 6723) e um localizado no Halo interno para referência (NGC 6362). A amostra é representativa dos dois picos na distribuição de metalicidades dos GCs do Bojo, em [Fe/H] ~ -0.5 e -1.0. Adotou-se métodos homogêneos para a separação das múltiplas populações nos diferentes estágios evolutivos e para o ajuste de isócronas, com o intuito de analisar comparativamente os dois grupos de metalicidade e verificar se ocorre alguma diferença de idades detectável entre as múltiplas populações. Algoritmos de classificação com aprendizado de máquina, e métodos utilizando estatística Bayesiana (máxima verossimilhança e MCMC) foram implementados e uma ferramenta em Python, nomeada SIRIUS, foi desenvolvida pelo grupo. Os resultados apontam para uma tendência de idades maiores (~ 13 Gyr) para os GCs mais pobres em metais com ramo horizontal azul, comparado com 12.0-12.5 Gyr para os outros GCs. Os resultados para as múltiplas populações indicam idades ligeiramente mais altas para a primeira geração em geral, mas com diferenças menores que 400 Myr comparado com a segunda geração. O resultado de 13.14 +0.25/-0.43 Gyr para o aglomerado NGC 6717 surpreende pois este é o aglomerado menos massivo da amostra, com maior fração de estrelas da primeira geração e com um ramo horizontal azul estendido: aspectos que o colocam como um dos objetos mais velhos da Galáxia. Esses resultados são bastante relevantes, com impactos diretos nos cenários de formação das partes centrais da Galáxia. / The globular clusters (GCs) are fundamental laboratories to study the formation and chemodynamical evolution of the Milky Way, as they formed during the initial episodes of star formation in galaxies. In the last decade, photometric and spectroscopic observational results have challenged the classical paradigm of GCs formation in a single star formation burst, giving rise to the debate about multiple stellar populations. With the purpose of better explore this phenomenon, the Hubble Space Telescope GO-13297 program (UV Legacy Survey of Galactic Globular Clusters, PI: G. Piotto) obtained photometry for 57 GCs with the UV/blue filters F275W, F336W and F438W (WFC3/UVIS), sensitive to variations in CNO abundances, and consequently able to distinguish multiple populations along the color-magnitude diagrams. Combined with previous photometry in the optical filters F606W and F814W (GO-10775 program, PI A. Sarajedini), they provide an unprecedented photometric database for GCs. This work presents the analysis of seven of these clusters, six of them from the Galactic Bulge (NGC 6304, NGC 6624, NGC 6637, NGC 6652, NGC 6717 and NGC 6723) and one located in the inner Halo for reference purposes (NGC 6362). This sample is representative of the two peaks in the metallicity distribution of Bulge GCs, with [Fe/H] ~ -0.5 and -1.0. We adopted homogeneous methods to the separation of multiple populations in the different evolutionary stages and to the isochrone fitting, with the purpose of analyze comparatively the two metallicity groups, and check whether there occurs any detectable age difference between the multiple populations. Classification algorithms with machine learning, and methods using Bayesian statistics (maximum likelihood and MCMC) were implemented and a Python tool, named SIRIUS, was developed by the group. The results point to a trend of larger ages (~ 13 Gyr) for the more metal-poor clusters with a blue horizontal branch, compared with 12.0-12.5 Gyr for the other GCs. The results for the multiple populations indicate slightly larger ages for the first generation in general, but with differences lower than 400 Myr compared with the second generation. The result of 13.14 +0.25/-0.43 Gyr for the cluster NGC 6717 is surprising because this is the least massive cluster of the sample, with the highest fraction of first generation stars and with a blue horizontal branch: features that place it as one of the oldest objects in the Galaxy. These results are quite relevant, with direct impacts on the formation scenarios of the innermost regions of the Galaxy.