La période de réionisation est une transition dans l'histoire de l'Univers qui change sont état d'ionisation. Les progrès en calcul intensif permettent maintenant de modéliser l'évolution du phénomène. D'habitude, les analyses se concentrent sur l'évolution de propriétés globales dans les simulations. Différemment, j'ai mis au point une nouvelle technique d'analyse de simulations qui permet d'appréhender le phémomène global de manière locale. Elle s'appuie sur l'extraction d'un arbre de fusion de régions HII pour caractériser le processus de réionisation. La technique a été appliqué dans plusieurs simulations où différents modèles de sources ont été considérés pour étudier leur impact sur la chrono-morphologie du processus. Dans un second temps, l’arbre a permis de caractériser l'histoire de réionisation passée de galaxies de types Voie Lactée. Enfin, la technique a été appliquée dans des simulations du Groupe Local pour quantifier l'histoire de réionisation de MW et M31. / The reionization period is a major step in the history of the Universe that change its ionization state. The increasing progress in high performance computing, allows us to model the phenomenon. Usually, the simulation analyses focus on global properties to constrain the impact of the physic put in the simulations. Alternately, I implemented a method of analysis that apprehend theglobal reionization in terms of multiple ‘local reionizations’. I extracted a merger tree of HII regions from the simulations to characterize the reionization process. I applied the technique in several cosmological simulations, where different ionizing source models were tested to investigate the impact of the source model on the reionization chrono-morphology. I also used the merger tree to characterize the past reionization history of MW type galaxies. I finally applied this methodology in Local Group simulations to quantify the reionization hystory of MW and M31.

A full-sky free-free template is increasingly important for the high-sensitivity Cosmic Microwave Background (CMB) experiments, such as Planck. On the Galactic plane, where free-free estimations from Halpha measurements become unreliable, Radio Recombination Lines (RRLs) can be used to determine the thermal brightness temperature unambiguously with no dust contamination. RRLs are a powerful tool for the diagnostic of the interstellar medium, tracing the ionised component, its electron temperature, velocity and radial distributions.This thesis describes the investigation of the ionised emission from HII regions and diffuse gas along the Galactic plane using RRLs, with the aim of providing a map of the free-free emission to complement the high latitude Halpha observations. Measuring the free-free emission on the Galactic plane is of great importance to understand and characterise other Galactic emission components: synchrotron, anomalous dust and thermal dust emission. The fully-sampled HI Parkes All-Sky Survey and associated deep Zone of Avoidance Survey are re-analysed to recover extended RRL emission. They include three RRLs (H166alpha, H167alpha and H168alpha) at frequencies near 1.4 GHz. A data cube covering l=20 degree to 44 degree and |b| < 4 degree is constructed of RRL spectra with velocity and spatial resolution of 20 km/s and 14.8 arcmin, respectively. Well-known HII regions are identified as well as the diffuse RRL emission on the Galactic plane.In order to convert the integrated RRL emission into a free-free brightness temperature a value of the electron temperature (Te) of the ionised gas is needed. Using the continuum and line data from the present survey, the variation of Te with Galactocentric radius was derived for the longitude range l=20 degree to 44 degree, with a mean Te on the Galactic plane of 6000 K. The derived Te variation was used to obtain the first direct measure of the free-free brightness in this region of the Galactic plane. Subtraction of this thermal emission from the total continuum at 1.4 GHz leaves the first direct measurement of the synchrotron emission. A narrow component of width 2 degree is identified in the synchrotron latitude distribution.Determining the free-free and synchrotron emission in this region of the Galactic plane identifies the anomalous microwave component, when combined with WMAP and IRIS data. The results are in agreement with models of small spinning dust grains.

Este trabalho consiste no estudo de cubos de dados das regiões centrais de quatro galáxias com mesmo tipo morfológico da Via Láctea: NGC 1566, NGC 6744, NGC 613 e NGC 134. As observações foram feitas no período de 2013 a 2015 com o Integral Field Unit do Gemini Multi-Object Spectrograph do telescópio Gemini Sul. Foram utilizadas técnicas de análise de dados como Tomografia PCA, síntese espectral e Penalized Pixel Fitting. Além disso, para todos objetos, foram calculadas razões de linhas de emissão dos espectros das regiões centrais, imagens da emissão de [SII] 6716, 6731 das nuvens de alta e baixa densidades foram feitas e, em dois casos (NGC 1566 e NGC 613), analisaram-se imagens do Hubble Space Telescope para melhor entendimento do ambiente circumnuclear. Foram encontrados 6 espectros com emissão compatível com a de AGNs na amostra, sendo que, em duas galáxias, existe a possibilidade de AGNs duplos: NGC 6744 e NGC 613. No geral, todas as galáxias apresentaram populações estelares de idades variadas em suas regiões centrais, porém, predominantemente, com metalicidades altas (0.02 e 0.05). Todas as galáxias apresentaram cinemática estelar compatível com discos de rotação em torno do núcleo e, em duas galáxias (NGC 1566 e NGC 6744), foi observado um decréscimo da dispersão de velocidades estelar em direção ao núcleo, possivelmente devido à presença de estrelas jovens massivas. A emissão do featureless continuum do núcleo da galáxia de Seyfert NGC 1566 foi, pela primeira vez, isolada e estudada, sendo que foi encontrado um índice espectral igual 1.7. Uma amostra de quatro galáxias não é estatisticamente conclusiva e será necessário ampliar a amostra para um melhor entendimento global dos núcleos de galáxias de mesmo tipo morfológico da Via Láctea. / This work involved the analysis of data cubes of four nuclear regions of galaxies that have the same morphological type of the Milky Way: NGC 1566, NGC 6744, NGC 613 and NGC 134. The observations were taken in the period of 2013 to 2015 with the Integral Field Unit of Gemini Multi-object Spectrograph on the Gemini South telescope. The data were analyzed using techniques like PCA Tomography, spectral synthesis and the Penalized Pixel Fitting process. Besides that, for all the objects, emission-line ratios of the central regions were calculated, images of the emission-lines [SII]6716, 6731 of clouds of high and low densities were made and, in two cases (NGC 1566 and NGC 6744), their Hubble Space Telescope images were analyzed for better understanding of their circumnuclear regions. 6 spectra of the sample had emission compatible with that of AGNs, in two galaxies, there is a possibility of double AGN: NGC 613 and NGC 6744. In general, all the galaxies presented stellar populations with varying ages in their central regions, however mainly with high metallicities (0.02 and 0.05). All the galaxies presented stellar kinematics compatible with rotation disks around the central source and, in two galaxies (NGC 1566 and NGC 6744), there was a stellar dispersion velocity decrease toward the nuclei, possibly due to the presence of massive young stars. The featureless continuum emission of the Seyfert galaxy NGC 1566 was, for the rst time, isolated and studied. It was found that its spectral index is equal to 1.7. A sample of four galaxies is not conclusive and it is necessary to enlarge this sample to a better global understanding of the nuclei of galaxies with the same morphological type of the Milky Way.

Este trabalho consiste no estudo de cubos de dados das regiões centrais de quatro galáxias com mesmo tipo morfológico da Via Láctea: NGC 1566, NGC 6744, NGC 613 e NGC 134. As observações foram feitas no período de 2013 a 2015 com o Integral Field Unit do Gemini Multi-Object Spectrograph do telescópio Gemini Sul. Foram utilizadas técnicas de análise de dados como Tomografia PCA, síntese espectral e Penalized Pixel Fitting. Além disso, para todos objetos, foram calculadas razões de linhas de emissão dos espectros das regiões centrais, imagens da emissão de [SII] 6716, 6731 das nuvens de alta e baixa densidades foram feitas e, em dois casos (NGC 1566 e NGC 613), analisaram-se imagens do Hubble Space Telescope para melhor entendimento do ambiente circumnuclear. Foram encontrados 6 espectros com emissão compatível com a de AGNs na amostra, sendo que, em duas galáxias, existe a possibilidade de AGNs duplos: NGC 6744 e NGC 613. No geral, todas as galáxias apresentaram populações estelares de idades variadas em suas regiões centrais, porém, predominantemente, com metalicidades altas (0.02 e 0.05). Todas as galáxias apresentaram cinemática estelar compatível com discos de rotação em torno do núcleo e, em duas galáxias (NGC 1566 e NGC 6744), foi observado um decréscimo da dispersão de velocidades estelar em direção ao núcleo, possivelmente devido à presença de estrelas jovens massivas. A emissão do featureless continuum do núcleo da galáxia de Seyfert NGC 1566 foi, pela primeira vez, isolada e estudada, sendo que foi encontrado um índice espectral igual 1.7. Uma amostra de quatro galáxias não é estatisticamente conclusiva e será necessário ampliar a amostra para um melhor entendimento global dos núcleos de galáxias de mesmo tipo morfológico da Via Láctea. / This work involved the analysis of data cubes of four nuclear regions of galaxies that have the same morphological type of the Milky Way: NGC 1566, NGC 6744, NGC 613 and NGC 134. The observations were taken in the period of 2013 to 2015 with the Integral Field Unit of Gemini Multi-object Spectrograph on the Gemini South telescope. The data were analyzed using techniques like PCA Tomography, spectral synthesis and the Penalized Pixel Fitting process. Besides that, for all the objects, emission-line ratios of the central regions were calculated, images of the emission-lines [SII]6716, 6731 of clouds of high and low densities were made and, in two cases (NGC 1566 and NGC 6744), their Hubble Space Telescope images were analyzed for better understanding of their circumnuclear regions. 6 spectra of the sample had emission compatible with that of AGNs, in two galaxies, there is a possibility of double AGN: NGC 613 and NGC 6744. In general, all the galaxies presented stellar populations with varying ages in their central regions, however mainly with high metallicities (0.02 and 0.05). All the galaxies presented stellar kinematics compatible with rotation disks around the central source and, in two galaxies (NGC 1566 and NGC 6744), there was a stellar dispersion velocity decrease toward the nuclei, possibly due to the presence of massive young stars. The featureless continuum emission of the Seyfert galaxy NGC 1566 was, for the rst time, isolated and studied. It was found that its spectral index is equal to 1.7. A sample of four galaxies is not conclusive and it is necessary to enlarge this sample to a better global understanding of the nuclei of galaxies with the same morphological type of the Milky Way.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / We present a multi-wavelength study of the stellar content of 11 optical/infrared Southern Galactic Hii regions with 10 h < α(J2000) < −17 h and −65° < δ(J2000) < −35°. Nine optical objects with no published or uncertain distances were examined in order to identify their ionising sources and to determine their distances, whereas young stellar objects (YSOs) and main sequence ionising stars were photometrically classified in the two infrared clusters. We carried out optical spectroscopy and UBV photometry of the stellar content of these Hii regions and obtained the distance of individual stars by spectroscopic parallax. To avoid using a fixed value for the total-to-selective extinction ratio RV , the reddening AV was determined directly by the colour-difference method. We classified as types O or B 24 out of the 31 stars for which optical spectra were obtained. In particular, we identified two new O stars in RCW98 and RCW99. The values for reddening obtained correspond to a mean hRV i = 3.44, which is about 10% higher of the mean value found for field stars over all directions of the Galaxy. For the Hii regions NGC3503, NGC6334, RCW55, RCW87, RCW98 e RCW99, we obtained more precise distances calculated as the median of the spectroscopic parallaxes obtained for two to six different stars in each nebulae, with an internal dispersion of less than 5%. Among the objects more obscured in the Norma region, we analysed the stellar content of the East part of the large complex of Hii regions GAL331.5-00.1, which contains eight bright extended radio sources. This study comprises the infrared clusters [DBS2003] 156 and [DBS2003] 157, respectively associated to Hii regions GAL331.11-00.51 and GAL331.31-00.34. In order to isolate the brightest 2MASS sources of their unresolved companions, we carried out JHK photometry with resolution better than 2MASS data. 47 Near-infrared (NIR) sources with Ks-band excess were identified following usual methods. Other 70 YSOs were also identified by using Mid-IR on-line data of the GLIMPSE survey. The search for radial-velocity measurements in the literature and the similarity between the stellar population explored have indicated the two regions as physically associated. With the determination of the spectroscopic parallaxes of four O-type and two B-type stars spread over the both clusters, this hypothesis was verified. The parallaxes of these 6 stars returned very compatible distances (hdhelioci = 3.30 ± 0.29 kpc). The Near- and Mid-IR counterparts of the IRAS source 16085-5138 was found close to the field of [DBS2003] 157. This source has showed typical colours of a Ultra-compact ii region (UCHii) and spectral index α = 3.6 between 2 and 25 μm, which is typical of YSOs immersed in protostellar envelopes. A lower limit to the bolometric luminosity of the protostar embedded was computed as L = 7.7×103L⊙ (M = 10M⊙), which corresponds to a BO-B1 zero-age star. The cluster [DBS2003] 157 was found to be spread over all ∼4′ × 4′ region demarcated by a intense shell-like dust emission, where a secondary massive star formation is going on, likely as a result of the interaction between this dust and stellar winds. / Apresentamos aqui um estudo multibanda do conteúdo estelar de onze regiões Hii do Hemisfério sul com 10 h < α(J2000) < −17 h e −65◦ < δ(J2000) < −35◦. Em 9 objetos ópticos, com valores de distância discrepantes ou sem nenhuma determinação anterior publicada, visamos identificar as fontes ionizantes e determinar suas distâncias, enquanto que, no estudo de dois outros objetos no infravermelho, buscamos não apenas identificar as fontes ionizantes, mas também objetos estelares jovens (YSOs) em seus campos. Nos primeiros, realizamos espectroscopia no óptico e fotometria UBV de seus conteúdos estelares e determinamos as distâncias das estrelas massivas por meio da paralaxe espectroscópica. A fim de evitar o uso de um valor fixo para a razão entre a extinção total e a seletiva à banda V , RV , a extinção AV na direção de cada estrela foi determinada diretamente através no método das diferenças de cor. De um total de 31 estrelas espectroscopicamente estudas, 24 foram classificadas como tipos O ou B, sendo duas novas estrelas tipo O encontradas em RCW98 e RCW99. As estimativas de AV implicaram um valor médio de hRV i = 3.44. Este resultado supera em 10% o valor médio encontrado para estrelas de campo sobre todas as direções da Galáxia. Para as regiões Hii NGC3503, NGC6334, RCW55, RCW87, RCW98 e RCW99, distâncias mais precisas foram estimadas como o valor mediano da paralaxe espectroscópica de 2 a 6 diferentes estrelas ionizantes, resultando em uma dispersão interna menor que 5%. Dentre os objetos mais obscurecidos pela extinção interestelar da região de Norma, analisamos o conte´udo estelar da região leste do grande complexo de regiões Hii brilhantes em radiofrequências GAL331.5-00.1. A área estudada engloba os aglomerados infravermelhos [DBS2003] 156 e [DBS2003] 157, respectivamente associados às regiões Hii GAL331.11-00.51 e GAL331.31-00.34. Observações fotométricas nas bandas J, H e Ks, com mais alta resolução que a fotometria 2MASS, foram realizadas nas direções desses objetos, o que possibilitou isolar as fontes infravermelhas mais brilhantes de estrelas companheiras não resolvidas e selecionar potenciais estrelas ionizantes para subsequente espectroscopia. 47 fontes com excesso de emissão intrínseca na banda Ks, típico em YSOs, foram identificadas seguindo os métodos usuais de análise da fotometria no IR próximo (NIR). Outros 70 YSOs foram identificados no IR médio (Mid-IR) usando dados do survey GLIMPSE. A pesquisa por medidas de velocidade radial na direção desses dois objetos e a semelhança das populações estelares indicou que as duas sub-regiões estudas deveriam estar fisicamente associadas. Esta hipótese foi reafirmada com a determinação da paralaxe espectroscópica de 4 estrelas tipo O e outras 2 tipo B nos dois aglomerados, que retornaram valores de distâncias heliocêntricas bastante compatíveis (hdhelioci = 3.30 ± 0.29 kpc). A contrapartida nos NIR e Mid-IR da fonte IRAS 16085-5138 foi encontrada junto ao aglomerado [DBS2003] 157. Esta fonte apresentou cores típicas de uma região Hii Ultracompacta (UCHii) e índice espectral entre 2 e 25 μm de α = 3.6, típico de YSO imerso em um envelope protoestelar. Um limite inferior para a luminosidade bolométrica da protoestrela embebida foi estimado em L = 7.7×103L⊙ (M = 10M⊙), o que corresponde a uma estrela de idade zero na faixa de BO-B1. O aglomerado [DBS2003] 157 mostrou-se estar espalhado sobre toda uma região de ∼4′ ×4′, demarcada por intensa emissão de poeira quente e espacialmente distribuída como uma nuvem em forma de concha.

À toutes les échelles de l'Astrophysique, l'impact de l'ionisation venant des étoiles massives est une question cruciale. A l'échelle galactique, l'ionisation peut réguler la formation des étoiles en soutenant les nuages moléculaires contre l'effondrement gravitationnel et à l'échelle stellaire, diverses indications pointent vers une naissance possible du système solaire à proximité des étoiles massives. À l'échelle du nuage moléculaire, il est clair que le gaz chaud et ionisé comprime le gaz froid qui l'entoure, conduisant à la formation des piliers, des globules, et des coquilles de gas dense dans lesquelles des coeurs pré-stellaires sont observés. Quels sont les mécanismes de formation de ces structures? La formation de ces coeurs pré-stellaires est-elle déclenchée par l'ionisation ou préexistante ? Les étoiles massives ont-elles un impact sur la distribution en densité du gaz environnant ? Ont-elles un impact sur la distribution des étoiles en fonction de leur masse (la fonction de masse initiale, IMF) ? Cette thèse vise à apporter des éléments de réponse à ces questions, en se concentrant en particulier sur la compréhension de la formation des structures entre le gaz froid et ionisé. Nous présentons l'état de l'art des travaux théoriques et des observations des régions ionisées (régions Hii) et nous introduisons les outils numériques qui ont été développés pour modéliser l'ionisation dans les simulations d'hydrodynamique turbulente effectuées avec le code HERACLES. Grâce aux simulations, nous présentons un nouveau modèle pour la formation des piliers basés sur la courbure et l'effondrement de la coquille dense sur elle-même et un nouveau modèle pour la formation de globules basé sur la turbulence du gaz froid. Plusieurs diagnostics ont été développés pour tester ces nouveaux modèles sur les observations. Si les piliers sont formés par l'effondrement de la coquille dense sur elle-même, le spectre en vitesse d'un pilier en formation présente un spectre avec une composante décalée vers le rouge et une composante décalée vers le bleu correspondant aux parties de la coquille en avant-plan et en arrière-plan qui rentrent en collision sur la ligne de visée. Si les globules émergent en raison de la turbulence du nuage moléculaire, le spectre en vitesse de ces globules est décalé à des vitesses différentes de celles de la coquille, des piliers et des coeurs denses qui suivent l'expansion globale de la région H ii. Un autre diagnostic est l'impact de la compression sur la fonction de densité de probabilité (PDF) du gaz froid. La distribution a un double pic lorsque la pression dynamique turbulente est faible par rapport à la pression du gaz ionisé. Il s'agit de la signature de la compression causée par l'expansion de la bulle ionisée. Quand la turbulence est élevée, les deux pics fusionnent et la compression peut encore être identifiée, mais la signature est moins claire. Nous avons utilisé des cartes de densité de colonne Herschel et des données de raies moléculaires pour caractériser la structure en densité et vitesse de l'interface entre le gaz ionisé et le gaz froid dans plusieurs régions : RCW 120, RCW 36, Cygnus X, la Nébuleuse de la Rosette et de l'Aigle. En plus des diagnostics issus des simulations, des prédictions analytiques des paramètres de la coquille et des piliers ont été testées et confrontées aux observations. Dans toutes ces régions, les modèles analytiques et les diagnostics issus des simulations donnent des résultats concluants. La structure en vitesse d'un pilier en formation dans la nébuleuse de la Rosette suggère qu'il a été formé par l'effondrement de la coquille sur elle-même et la dispersion des vitesses moyennes des globules dans Cygnus X et dans la Nébuleuse de la Rosette tend à confirmer leur origine turbulente. La compression due au gaz ionisé est visible sur la PDF du gaz froid dans la plupart des régions étudiées. Ce résultat est important pour le lien entre l'IMF et les propriétés globales du nuage. Si l'IMF peut être déduite de la PDF d'un nuage, l'impact des étoiles massives sur la PDF doit être pris en compte. En outre, nous présentons des simulations dédiées de RCW 36 qui suggèrent que les coeurs denses au bord du gaz ionisé ne sont pas pré-existants, leur formation a été déclenchée par la compression due à l'ionisation. En conséquence, l'ionisation des étoiles massives est un processus clé qui doit être pris en compte pour la compréhension de l'IMF. En annexe, nous présentons également des travaux réalisés en parallèle de cette thèse : l'échange de charge dans la collision entre vents planétaires et stellaires, en collaboration avec le professeur E. Chiang, à l'école d'été ISIMA 2011 à Pékin; et le test de site en sub-millimétrique sur la station Concordia en Antarctique avec l'équipe CAMISTIC (PI : G. Durand).

La période de réionisation est une transition dans l'histoire de l'Univers qui change sont état d'ionisation. Les progrès en calcul intensif permettent maintenant de modéliser l'évolution du phénomène. D'habitude, les analyses se concentrent sur l'évolution de propriétés globales dans les simulations. Différemment, j'ai mis au point une nouvelle technique d'analyse de simulations qui permet d'appréhender le phémomène global de manière locale. Elle s'appuie sur l'extraction d'un arbre de fusion de régions HII pour caractériser le processus de réionisation. La technique a été appliqué dans plusieurs simulations où différents modèles de sources ont été considérés pour étudier leur impact sur la chrono-morphologie du processus. Dans un second temps, l'arbre a permis de caractériser l'histoire de réionisation passée de galaxies de types Voie Lactée. Enfin, la technique a été appliquée dans des simulations du Groupe Local pour quantifier l'histoire de réionisation de MW et M31.

This study consisted of an optical photometric and spectroscopic analysis on a 7′ 7′ field around the Southern high mass star forming region RCW 34. A previous study on RCW 34 in the NIR discov- ered many deeply embedded young stellar objects which were suspected to be T Tauri stars and which justified further investigation. The data used in this study consisted of three sets, the first two are photometric and spectroscopic data sets which were obtained during the first two weeks of February 2002. A third data set of spectroscopic observations was obtained by the author during the second week of 2011 of selected candidates using results from the NIR study and from the photometric data sets. All of the spectroscopy was conducted with the long slit spectrograph on the 1.9-m telescope and the photometry with DANDICAM on the 1.0-m telescope at the South African Astronomical Observatory (SAAO) in Sutherland. Objectives accomplished in the course of this study were to understand, ob- tain, reduce and interpret photometric and long slit spectroscopic CCD images. From the photometric results 57 stars showed excess blue emission on a colour-colour diagram which could be generated by circumstellar matter. The spectroscopic study showed 5 stars that showed H emission and 2 with strong Li absorption lines which confirm the suspicions of the NIR study about T Tauri stars in the region. All of the stars from the spectroscopic study in 2011 were identified as low-mass K or M type stars. Using colour-magnitude diagrams it was possible to see that the majority of the stars in the cluster are low-mass pre-main sequence stars. The stars matching between the optical and NIR filters were plotted on NIR colour-colour diagrams showing that the 5 stars that had H emission lines also had NIR colours characteristic to T Tauri stars. Out of the 5 stars that showed H emission, 2 were found to be classical T Tauris and three were found to be weak line T Tauris. / Thesis (MSc (Space Physics))--North-West University, Potchefstroom Campus, 2013

This study consisted of an optical photometric and spectroscopic analysis on a 7′ 7′ field around the Southern high mass star forming region RCW 34. A previous study on RCW 34 in the NIR discov- ered many deeply embedded young stellar objects which were suspected to be T Tauri stars and which justified further investigation. The data used in this study consisted of three sets, the first two are photometric and spectroscopic data sets which were obtained during the first two weeks of February 2002. A third data set of spectroscopic observations was obtained by the author during the second week of 2011 of selected candidates using results from the NIR study and from the photometric data sets. All of the spectroscopy was conducted with the long slit spectrograph on the 1.9-m telescope and the photometry with DANDICAM on the 1.0-m telescope at the South African Astronomical Observatory (SAAO) in Sutherland. Objectives accomplished in the course of this study were to understand, ob- tain, reduce and interpret photometric and long slit spectroscopic CCD images. From the photometric results 57 stars showed excess blue emission on a colour-colour diagram which could be generated by circumstellar matter. The spectroscopic study showed 5 stars that showed H emission and 2 with strong Li absorption lines which confirm the suspicions of the NIR study about T Tauri stars in the region. All of the stars from the spectroscopic study in 2011 were identified as low-mass K or M type stars. Using colour-magnitude diagrams it was possible to see that the majority of the stars in the cluster are low-mass pre-main sequence stars. The stars matching between the optical and NIR filters were plotted on NIR colour-colour diagrams showing that the 5 stars that had H emission lines also had NIR colours characteristic to T Tauri stars. Out of the 5 stars that showed H emission, 2 were found to be classical T Tauris and three were found to be weak line T Tauris. / Thesis (MSc (Space Physics))--North-West University, Potchefstroom Campus, 2013

Detailed knowledge of the hydrogen population structure is necessary for the interpretation of hydrogen recombination line (HRL) observations. Calculations of hydrogen departure coefficients using a capture-collision-cascade type model with the angular momentum quantum levels resolved that includes the effects of external radiation fields are presented. The stimulating processes are important at radio frequencies and can influence level populations. Updated atomic rates and new numerical techniques with a solid mathematical basis have been incorporated into the model to ensure convergence of the solution. My results differ from previous results by up to 20 per cent. The effects on departure coefficients of continuum radiation from dust, the cosmic microwave background, the stellar ionising radiation, and free-free radiation are quantified. Atomic hydrogen masers occur in recombination plasmas in sufficiently dense HII regions. These HRL masers have been observed in a handful of objects to date and the analysis of the atomic physics involved has been rudimentary. A new model of HRL masers is presented which uses an nl-model to describe the atomic populations interacting with free-free radiation from the plasma, and an escape probability framework to deal with radiative transfer effects. The importance of including the collisions between angular momentum quantum states and the free-free emission in models of HRL masers is demonstrated. The model is used to describe the general behaviour of radiative transfer of HRLs and to investigate the conditions under which HRL masers form. The model results show good agreement with observations collected over a broad range of frequencies. Theoretical predictions are made regarding the ratio of recombination lines from the same upper quantum level for these objects. / Physics / Ph. D. (Astronomy)