Na literatura, a análise e a interpretação das linhas de emissão de H2 em nebulosas planetárias são feitas, em geral, considerando que a molécula somente seja produzida em ambientes neutros, como as regiões de fotodissociação ou com choques. No entanto, existem fortes evidências observacionais de que ao menos parte da emissão seja proveniente da região ionizada desses objetos. Em trabalhos anteriores mostramos que quantidades significativas de H2 podem sobreviver dentro dessa região hostil. No presente trabalho nosso objetivo é calcular e estudar a emissão de H2 em linhas no infravermelho produzidas na região ionizada de nebulosas planetárias, utilizando o código de fotoionização unidimensional Aangaba. Para isso, desenvolvemos diversas sub-rotinas que determinam o povoamento em níveis de energia da molécula e calculam a intensidade das linhas de emissão de H2. Obtivemos a intensidade de diversas linhas produzidas pela molécula H2 em nebulosas planetárias cujos parâmetros característicos (temperatura e luminosidade da estrela central, densidade do gás, tipo e tamanho dos grãos, etc.) estão na faixa de valores conhecidos para esses objetos. Como resultado de nosso trabalho, mostramos que a contribuição da região ionizada para a emissão de H2 de nebulosas planetárias pode ser significativa em diversas situações, particularmente quando a temperatura da estrela central é alta. Esse resultado pode explicar porque a detecção de linhas de H2 é mais provável em nebulosas planetárias bipolares, que têm estrelas tipicamente mais quentes. Além disso, verificamos que na região ionizada a excitação e a desexcitação colisional são mecanismos importantes de povoamento de todos os níveis rovibracionais do estado fundamental eletrônico de H2. Os mecanismos radiativos são também importantes, particularmente para os níveis de energia excitados. Os mecanismos de formação em estados excitados podem ter alguma influência no espectro de linhas produzidas pela desexcitação de níveis rotacionais bastante elevados, principalmente em ambientes densos. Em nossos modelos incluímos o efeito da molécula H2 no equilíbrio térmico do gás, verificando que a molécula H2 somente tem influência significativa na temperatura do gás em casos de temperatura da estrela central muito alta ou grande quantidade de grãos, principalmente através da desexcitação colisional. / The analysis and the interpretation of the H2 line emission of planetary nebulae have been done in the literature assuming that the molecule survives only in neutral environments, as in photodissociation or shocked regions. However, there is strong evidence that at least part of the H2 emission is produced inside the ionized region of such objects. In previous work we showed that significant amounts of H2 can survive inside the ionized region of planetary nebulae. The aim of the present work is to calculate and study the infrared line emission of H2 produced inside the ionized region of planetary nebulae using the one-dimensional photoionization code Aangaba. For this, we developed several numerical subroutines in order to calculate the statistical population of the H2 energy levels, as well as the intensity of the H2 infrared emission lines in physical conditions typical of planetary nebulae. We show that the contribution of the ionized region for the H2 emission can be significant, particularly in the case of nebulae with high temperature central stars. This result explains why H2 emission is more frequently observed in bipolar planetary nebulae (Gatley\'s rule), since this kind of object typically has hotter stars. We show that collisional excitation plays an important role on the H2 population of the rovibrational levels of the electronic ground state. Radiative mechanisms are also important, particularly for upper levels. Formation pumping may have some effects on the line intensities produced by de-excitation from very high rotational levels, specially in dense environments. We include the effects of H2 in the thermal equilibrium of the gas, concluding that H2 only contributes to the thermal equilibrium in the case of very high temperature of the central star or high grain to gas ratio, mainly through collisional de-excitation.

High mass loss rates in evolved stars make them the major contributors to recycling processed material back into the interstellar medium. This mass loss creates large circumstellar shells, rich in molecular material. This dissertation presents millimeter and submillimeter studies of the end stages of low mass and high mass stars in order to probe their molecular content in more detail. In low mass stars, the molecular material is carried on into the planetary nebula (PN) stage. Observations of CS, HCO⁺, and CO in planetary nebulae (PNe) of various post-asymptotic giant branch ages have shown that molecular abundances in these objects do not significantly vary with age, as previously thought. More detailed observations of the slightly oxygen-rich PN NGC 6537 resulted in the detection of CN, HCN, HNC, CCH, CS, SO, H₂CO, HCO⁺ and N₂H⁺, as well as numerous ¹³C isotopologues. Observations of the middle-aged PN M2-48 showed the presence of CN, HCN, HNC, CS, SO, SO₂, SiO, HCO⁺, N₂H⁺, and several ¹³C isotopologues. These observations represent the first detections of CS, SO, SO₂, and SiO in any planetary nebula. The implications of these observations are discussed. A 1 mm spectral survey of the supergiant star NML Cygni has been carried out with the Arizona Radio Observatory Submillimeter Telescope resulting in the observation of 102 emission features arising from 17 different molecules and 4 unidentified features. The line profiles observed in this circumstellar shell are asymmetric and vary between different molecules, akin to what has been seen in another supergiant, VY Canis Majoris. The non-LTE radiative transfer code ESCAPADE has been used to model molecular abundances in the various asymmetric outflows of VY Canis Majoris, showing just how chemically and kinematically complex these supergiant circumstellar envelopes really are.

Na literatura, a análise e a interpretação das linhas de emissão de H2 em nebulosas planetárias são feitas, em geral, considerando que a molécula somente seja produzida em ambientes neutros, como as regiões de fotodissociação ou com choques. No entanto, existem fortes evidências observacionais de que ao menos parte da emissão seja proveniente da região ionizada desses objetos. Em trabalhos anteriores mostramos que quantidades significativas de H2 podem sobreviver dentro dessa região hostil. No presente trabalho nosso objetivo é calcular e estudar a emissão de H2 em linhas no infravermelho produzidas na região ionizada de nebulosas planetárias, utilizando o código de fotoionização unidimensional Aangaba. Para isso, desenvolvemos diversas sub-rotinas que determinam o povoamento em níveis de energia da molécula e calculam a intensidade das linhas de emissão de H2. Obtivemos a intensidade de diversas linhas produzidas pela molécula H2 em nebulosas planetárias cujos parâmetros característicos (temperatura e luminosidade da estrela central, densidade do gás, tipo e tamanho dos grãos, etc.) estão na faixa de valores conhecidos para esses objetos. Como resultado de nosso trabalho, mostramos que a contribuição da região ionizada para a emissão de H2 de nebulosas planetárias pode ser significativa em diversas situações, particularmente quando a temperatura da estrela central é alta. Esse resultado pode explicar porque a detecção de linhas de H2 é mais provável em nebulosas planetárias bipolares, que têm estrelas tipicamente mais quentes. Além disso, verificamos que na região ionizada a excitação e a desexcitação colisional são mecanismos importantes de povoamento de todos os níveis rovibracionais do estado fundamental eletrônico de H2. Os mecanismos radiativos são também importantes, particularmente para os níveis de energia excitados. Os mecanismos de formação em estados excitados podem ter alguma influência no espectro de linhas produzidas pela desexcitação de níveis rotacionais bastante elevados, principalmente em ambientes densos. Em nossos modelos incluímos o efeito da molécula H2 no equilíbrio térmico do gás, verificando que a molécula H2 somente tem influência significativa na temperatura do gás em casos de temperatura da estrela central muito alta ou grande quantidade de grãos, principalmente através da desexcitação colisional. / The analysis and the interpretation of the H2 line emission of planetary nebulae have been done in the literature assuming that the molecule survives only in neutral environments, as in photodissociation or shocked regions. However, there is strong evidence that at least part of the H2 emission is produced inside the ionized region of such objects. In previous work we showed that significant amounts of H2 can survive inside the ionized region of planetary nebulae. The aim of the present work is to calculate and study the infrared line emission of H2 produced inside the ionized region of planetary nebulae using the one-dimensional photoionization code Aangaba. For this, we developed several numerical subroutines in order to calculate the statistical population of the H2 energy levels, as well as the intensity of the H2 infrared emission lines in physical conditions typical of planetary nebulae. We show that the contribution of the ionized region for the H2 emission can be significant, particularly in the case of nebulae with high temperature central stars. This result explains why H2 emission is more frequently observed in bipolar planetary nebulae (Gatley\'s rule), since this kind of object typically has hotter stars. We show that collisional excitation plays an important role on the H2 population of the rovibrational levels of the electronic ground state. Radiative mechanisms are also important, particularly for upper levels. Formation pumping may have some effects on the line intensities produced by de-excitation from very high rotational levels, specially in dense environments. We include the effects of H2 in the thermal equilibrium of the gas, concluding that H2 only contributes to the thermal equilibrium in the case of very high temperature of the central star or high grain to gas ratio, mainly through collisional de-excitation.

O presente trabalho propõe uma abordagem abrangente para descrever a evolução da região central da Via Láctea, compreendendo-se aí o bojo, a barra e as interfaces dos mesmos com o limite interno do disco e com a região central do halo. Pretende-se investigar as propriedades químicas e cinemáticas destas estruturas, que são interconectadas, com o objetivo de separá-las e aplicar os resultados daí obtidos a um modelo de formação e evolução do bojo e da região interna do disco que descreva simultaneamente distintos aspectos da evolução da região central da Galáxia. Na primeira parte do trabalho, uma amostra de nebulosas planetárias (NPs) localizadas no disco interno e no bojo da Galáxia é utilizada para encontrar a distância galactocêntrica que melhor separa estas duas populações, do ponto de vista das abundâncias. Foram utilizadas escalas de distâncias estatísticas para o estudo da distribuição das abundâncias na interface bojo-disco. A aplicação do teste Kolmogorov-Smirnov mostrou que, em média, a população interna não segue o gradiente radial de abundâncias do disco na direção do centro galáctico. Baseado neste estudo, propõe-se uma distância galactocêntrica de 1.5 kpc para definir a interface bojo-disco. Na segunda parte do trabalho, foram realizadas observações espectrofotométricas de 21 NPs localizadas na direção do centro da Galáxia com o telescópio SOAR. Estes objetos estão localizados bem próximos ao plano galáctico na direção central da Via Láctea, onde não existem dados de NPs na literatura. Os resultados mostram que as NPs localizadas nesta região apresentam baixas abundâncias de oxigênio comparadas com as NPs do disco interno e de outras regiões do bojo. Os resultados indicam que o bojo apresenta uma complexa composição de populações estelares. Por um lado, a presença de nebulosas com baixas abundâncias mostra que o bojo pode ter se formado a partir de um disco galáctico antigo através de uma evolução secular. Por outro lado, existem alguns objetos do bojo para os quais as abundâncias coincidem com o limite do gradiente radial do disco nesta região. Esta é uma evidência para um bojo composto por duas ou mais populações: uma originada do disco fino, e outra originada do disco espesso. Na última parte do trabalho propõe-se a inclusão de fluxos radiais de gás em um modelo de evolução química para simular os efeitos de uma barra localizada no centro da Galáxia nas distribuições de abundâncias, densidade de gás e taxa de formação estelar (SFR). Os resultados das simulações indicam que os modelos com fluxos de gás apresentam uma SFR mais alta no bojo e que os perfis da SFR e da densidade de gás na região central são melhor reproduzidos após a inclusão dos fluxos radiais no modelo. As simulações indicam ainda que o gradiente de abundâncias do disco é mais plano para o caso da inclusão da barra. Estes resultados indicam que a barra e os fluxos de gás exercem um importante papel na formação de estrelas no centro das galáxias espirais barradas. / This project proposes a comprehensive approach to describe the evolution of the central region of the Galaxy, comprising the bulge, the bar and their interfaces with the inner disk and the central region of the halo. We intend to investigate the chemical and kinematic properties of these structures, which are interconnected, aiming to separate them and apply these results to a model for the formation and evolution of the bulge and inner disk, capable to describe simultaneously distinct aspects of the evolution of the central region of the Galaxy. First, a sample of planetary nebulae (PNe) located in the inner-disk and bulge of the Galaxy is used in order to find the galactocentric distance that better separates these two populations, from the point of view of abundances. Statistical distance scales were used to study the distribution of abundances across the disk-bulge interface. A Kolmogorov-Smirnov test was used to find the distance in which the chemical properties of these regions better separates. The results of the statistical analysis indicate that, on the average, the inner population has lower abundances than the outer. Additionally, for the $\\alpha$-elements abundances, the inner population does not follow the disk radial gradient towards the galactic centre. Based on our results, we suggest a bulge-disk interface at 1.5 kpc, marking the transition between the bulge and inner-disk of the Galaxy, as defined by the intermediate mass population. Second, we present spectrophotometric observations for a sample of 21 PNe located towards the galactic centre of the Galaxy. The abundances are derived based on observations in the optical domain made at the SOAR telescope. Their location is interesting since there are no observations of PNe in this region. The data show lower oxygen abundances compared to those from PNe located in the inner disk and other bulge regions. The results show that the bulge has a complex composition of stellar populations. The presence of PNe with low abundances indicates that the bulge might be formed from an old galactic disk through secular evolution. On the other hand, other objects from our sample have abundances compared to those from inner disk PNe. This is evidence that two or more populations might compose the bulge: one originated from the thin disk, and the other from the thick disk. Last, we propose a chemical evolution model that includes radial gas flows. This is done in order to mimic the effects of the galactic bar on the chemical abundances distributions and the gas density profiles and the star formation rate (SFR). The results of the models with radial flows point to a high SFR in the bulge and, additionally, the SFR and gas density profiles in the inner Galaxy are better reproduced after the inclusion of radial gas flows in the model. After including a specific velocity pattern for the bar, the results show a flattening of the radial abundance gradient. Our results indicate that radial gas flows may play an important role in the star formation near the centre of barred spiral galaxies.

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)

Stellar wind bow shocks are structures created when stellar winds with supersonic relative velocities interact with the local interstellar medium (ISM). They can be studied to understand the properties of stars as well as the ISM. Since bow shocks are asymmetric, light becomes polarized by scattering in the regions of enhanced density they create. We use a Monte Carlo radiative transfer code calle SLIP to simulate the polarization signatures produced by both resolved and unresolved bow shocks with analytically derived shapes and density structures. When electron scattering is the polarizing mechanism, we find that optical depth plays an important role in the polarization signatures. While results for low optical depths reproduce theoretical predictions, higher optical depths produce higher polarization and position angle rotations at specific viewing angles. This is due to the geometrical properties of the bow shock along with multiple scattering effects. For dust scattering, we find that the polarization signature is strongly affected by wavelength, dust size, dust composition, and viewing angle. Depending on the viewing angle, the polarization magnitude may increase or decrease as a function of wavelength. We will present results from these simulations and preliminary comparisons with observational data.

The atomic physics relevant to gaseous nebulae is critically examined using modelling software with particular emphasis on radio recombination lines (RRLs). The theoretical spectral line intensities can be deduced if we know the population structure of the bound electrons in the gas under non-thermal equilibrium conditions. The population structure of hydrogen is solved for various environments using a capture-collision-cascade model that incorporates an ambient radiation eld. The validity of assuming Case B (Baker & Menzel, 1938) for nebulae is investigated. It is known that Case B is appropriate for levels with small principal quantum numbers (n < 40), but this assumption is re-examined for high levels which are relevant to RRLs. E ects of an ambient radiation eld on the population structure is examined and processes that are stimulated by a radiation eld are included in the model. This is done as a preliminary investigation to extend the model to a photoionization code. / Physics / (M. Sc. (Astronomy)

Dans le domaine de l’astrophysique des hautes énergies, de nombreuses questions restent à ce jour sans réponse et, parmi elles se trouve l’origine des rayons cosmiques Galactiques. La première preuve observationnelle de ces particules accélérées a été apportée au sein d’un vestige de supernova il y a seulement vingt ans. Depuis, nous savons que les vestiges de supernova, les pulsars et leurs nébuleuses accélèrent efficacement des particules mais de nombreuses interrogations subsistent encore. Les preuves directes concernant l’accélération de protons (constituant 90% du rayonnement cosmique) sont rares et de nombreuses sources nouvellement détectées en gamma sont de nature inconnue. Les rayonnements produits au sein des accélérateurs Galactiques fournissent d’importants éléments de réponse quant à la nature des particules accélérées. En particulier, alors que les domaines de la radio et des rayons X ne tracent que les électrons accélérés, les rayons gamma peuvent inférer la présence d’électrons et également de protons (et noyaux en général) mais l’émission s’avère le plus souvent difficile à interpréter.Les mesures des rayons gamma de très hautes énergies dépendent de notre connaissance de l’atmosphère terrestre, dans lequel ils se propagent avant d’être détectés par les télescopes Tcherenkov au sol tels que le réseau H.E.S.S. La partie technique de cette thèse concerne l’étude de l’impact des profils d’atmosphère sur les données H.E.S.S. Grâce à des simulations et des analyses prenant en compte les caractéristiques propres à chaque prise de données, nous étudions l’impact des profils d’atmosphère mesurés sur les fonctions de réponse de l’instrument et sur la reconstruction spectrale.Le premier objectif scientifique de cette thèse est de comprendre la nature de l’émission gamma au sein de deux vestiges de supernova (G326.3-1.8 et RX J1713.7-3946) par le biais d’analyses spectro-morphologiques détaillées. L’analyse de G326.3-1.8, avec les données du Fermi-LAT, a mené à deux résultats importants: une nouvelle preuve d’accélération de protons et la première séparation morphologique et spectrale de deux composantes imbriquées en gamma. L’analyse de RX J1713.7-3946, avec les données H.E.S.S. et les outils d'analyse Ctools, a confirmé une extension plus importante en gamma qu’en rayons X mais dont l’origine reste encore incertaine.La deuxième partie de cette thèse entreprend de discuter la nature des sources Galactiques non-associées au TeV. Pour ce faire, nous présentons un code générique visant à rechercher des contreparties multi-longueurs d’onde sur ces sources au TeV, et permettant de poser des contraintes sur des paramètres physiques tels que le champ magnétique moyen et l’indice spectral en radio. En appliquant ce code sur cinq sources non-identifiées du relevé du plan Galactique de H.E.S.S., nous apportons des arguments quant à leur origine. En particulier, nous étudions deux sources, dont l’émission au TeV provient probablement de multiples contributions, soulevant ainsi l’importance des données multi-longueurs d’onde pour comprendre la nature de l’émission en gamma. / In high energy astrophysics, several questions are still open and amongst them is the origin of Galactic cosmic rays. The first observational evidence of accelerated particles has only been revealed twenty years ago. Since then, supernova remnants, pulsars and their nebulae are known to efficiently accelerate particles but several questions still hold. In particular, evidence of accelerated protons (which consist on 90% of the cosmic-ray spectrum) is still elusive and several gamma-ray sources have unknown origin. Non-thermal emissions produced in Galactic accelerators provide insights about the nature of the accelerated particles. In particular, while radio and X-ray observations indicate the presence of accelerated electrons, gamma rays can be produced by both electrons and protons (or nuclei in general) but it may be difficult to assess the origin of the emission.The technical part of this thesis concerns the study of the impact on the reconstructed H.E.S.S. data when using atmospheric profiles measured with a lidar instead of a standard atmospheric model currently used. Very high energy gamma rays propagate into the atmosphere before reaching Cherenkov Telescopes and thus, the accuracy of our measurements depends on our understanding of the atmospheric composition. Using run-wise simulations based on lidar data, we study the impact on the instrument response functions and we analyse data to quantify the effect on the spectral reconstruction.The first scientific goal of this thesis is to understand the gamma-ray emission from two supernova remnants (G326.3-1.8 et RX J1713-3946) through detailed spectro-morphological analyses. The analysis of the composite supernova remnant G326.3-1.8, with Fermi-LAT data, has led to two major results: a new evidence of accelerated protons and the first morphological and spectral separation in gamma rays of two nested components. The study of RX J1713-3946, with H.E.S.S. data and using the Ctools package, confirms a significant gamma-ray extension beyond the X-ray emitting shell but its origin remains unclear.The second part of this thesis aims to constrain the nature of the unidentified TeV sources revealed in the H.E.S.S. Galactic Plane Survey. We thus present a generic code, based on a multi-wavelength approach, to find counterparts and estimate physical parameters like the radio spectral index and the mean magnetic field. We apply this code on five unidentified TeV sources and we put constraints on their nature. In particular, we present two high-confusion cases, for which the TeV emission is probably due to the contribution from different components, emphasizing the importance of multi-wavelength data to understand the origin of the gamma-ray emission.

Central stars of planetary nebulae are low-mass stars on the brink of their final evolution towards white dwarfs. Because of their surface temperature of above 25,000 K their UV radiation ionizes the surrounding material, which was ejected in an earlier phase of their evolution. Such fluorescent circumstellar gas is called a "Planetary Nebula". About one-tenth of the Galactic central stars are hydrogen-deficient. Generally, the surface of these central stars is a mixture of helium, carbon, and oxygen resulting from partial helium burning. Moreover, most of them have a strong stellar wind, similar to massive Pop-I Wolf-Rayet stars, and are in analogy classified as [WC]. The brackets distinguish the special type from the massive WC stars. Qualitative spectral analyses of [WC] stars lead to the assumption of an evolutionary sequence from the cooler, so-called late-type [WCL] stars to the very hot, early-type [WCE] stars. Quantitative analyses of the winds of [WC] stars became possible by means of computer programs that solve the radiative transfer in the co-moving frame, together with the statistical equilibrium equations for the population numbers. First analyses employing models without iron-line blanketing resulted in systematically different abundances for [WCL] and [WCE] stars. While the mass ratio of He:C is roughly 40:50 for [WCL] stars, it is 60:30 in average for [WCE] stars. The postulated evolution from [WCL] to [WCE] however could only lead to an increase of carbon, since heavier elements are built up by nuclear fusion. In the present work, improved models are used to re-analyze the [WCE] stars and to confirm their He:C abundance ratio. Refined models, calculated with the Potsdam WR model atmosphere code (PoWR), account now for line-blanketing due to iron group elements, small scale wind inhomogeneities, and complex model atoms for He, C, O, H, P, N, and Ne. Referring to stellar evolutionary models for the hydrogen-deficient [WC] stars, Ne and N abundances are of particular interest. Only one out of three different evolutionary channels, the VLTP scenario, leads to a Ne and N overabundance of a few percent by mass. A VLTP, a very late thermal pulse, is a rapid increase of the energy production of the helium-burning shell, while hydrogen burning has already ceased. Subsequently, the hydrogen envelope is mixed with deeper layers and completely burnt in the presence of C, He, and O. This results in the formation of N and Ne. A sample of eleven [WCE] stars has been analyzed. For three of them, PB 6, NGC 5189, and [S71d]3, a N overabundance of 1.5% has been found, while for three other [WCE] stars such high abundances of N can be excluded. In the case of NGC 5189, strong spectral lines of Ne can be reproduced qualitatively by our models. At present, the Ne mass fraction can only be roughly estimated from the Ne emission lines and seems to be in the order of a few percent by mass. Furthermore, using a diagnostic He-C line pair, the He:C abundance ratio of 60:30 for [WCE] stars is confirmed. Within the framework of the analysis, a new class of hydrogen-deficient central stars has been discovered, with PB 8 as its first member. Its atmospheric mixture resembles rather that of the massive WNL stars than of the [WC] stars. The determined mass fractions H:He:C:N:O are 40:55:1.3:2:1.3. As the wind of PB 8 contains significant amounts of O and C, in contrast to WN stars, a classification as [WN/WC] is suggested. / Zentralsterne Planetarischer Nebel sind massearme Sterne kurz vor ihrer finalen Entwicklung zu Weißen Zwergen. Aufgrund ihrer Oberflächentemperatur von über 25 000 K sind sie in der Lage, durch Abstrahlung von UV-Licht das sie umgebende Material, welches in einer vorigen Phase ihrer Entwicklung abgestoßen wurde, zu ionisieren. Das solchermaßen zum Leuchten angeregte Gas bezeichnet man als Planetarischen Nebel. Etwa ein Zehntel der galaktischen Zentralsterne sind wasserstoffarm. Im Allgemeinen besteht die Oberfläche dieser Zentralsterne aus einer Mischung der Elemente Helium, Kohlenstoff und Sauerstoff, welche z.T. durch Heliumbrennen erzeugt wurden. Die meisten dieser Sterne haben darüberhinaus einen starken Sternwind, ähnlich den massereichen Pop-I-Wolf-Rayet-Sternen und werden in Analogie zu diesen als [WC] klassifiziert, wobei die eckigen Klammern der Unterscheidung von den massereichen WC-Sternen dienen. Qualitative Analysen der Spektren von [WC]-Sternen lassen eine Entwicklungssequenz dieser Sterne von kühleren sogenannten late-type [WC]-Sternen (kurz [WCL]) zu sehr heißen, early-type [WC]-Sternen (kurz [WCE]) vermuten. Mithilfe von Computerprogrammen, die den Strahlungstransport im mitbewegten Beobachtersystem zusammen mit den statistischen Gleichungen der Besetzungszahlen der Ionen im Sternwind rechnen können, wurden quantitative Untersuchungen der Winde von [WC]-Sternen möglich. Erste Analysen mit Modellen ohne Eisenlinien ergaben dabei systematisch unterschiedliche Häufigkeiten für [WCL]- und [WCE]-Sterne. Während sich für [WCL]-Sterne ein Verhältnis der Massenanteile von He:C von etwas 40:50 ergab, fand man für die [WCE]-Sterne ein mittleres Verhältnis von 60:30 für die He:C-Massenanteile. Dabei sollte die Entwicklung von [WCL] nach [WCE] innerhalb einer sehr kurzen Zeit durch Aufheizung infolge der Kontraktion der Hülle erfolgen und nicht mit einer wesentlichen Abnahme der Kohlenstoffhäufigkeit bei gleichzeitiger Zunahme der Heliumhäufigkeit an der Oberfläche einhergehen. Im Rahmen der vorgelegten Arbeit wird untersucht, ob sich mittels verbesserter Modelle für die Atmosphären von [WC]-Sternen das He:C-Häufigkeitsverhältnis der [WCE]-Sterne bestätigt. Elaboriertere Modelle, welche vom Potsdamer WR-Modelatmosphären-Code (PoWR) berechnet werden können, berücksichtigen Line-Blanketing aufgrund von Elementen der Eisengruppe, kleinskalige Windinhomogenitäten und die Elemente He, C, O, H, P, N und Ne. Unter Bezug auf Sternentwicklungsmodelle, die die Ursache der Wasserstoffunterhäufigkeit von [WC]-Sternen erklären, sind insbesondere die Neon- und Stickstoff-Häufigkeiten interessant. Von den drei möglichen Entwicklungskanälen für [WC]-Sterne führt lediglich das VLTP-Szenario zu einer Stickstoff-Überhäufigkeit von einigen Prozent bezogen auf die Masse. Bei einem VLTP, einem very late thermal pulse, handelt es sich um einen plötzlichen, starken Anstieg der Energieproduktion in der helium-brennenden Schale, während das Wasserstoffbrennen bereits zum Erliegen gekommen ist. Infolge eines VLTPs wird sämtlicher Wasserstoff kurz nach dem thermischen Puls in tiefere Schichten gemischt und in Anwesenheit von C, He und O verbrannt. Infolgedessen wird N und auch Ne erzeugt. Bei der Analyse von elf [WCE]-Sternen wurden für drei von ihnen, PB 6, NGC 5189 und [S71d]3, Stickststoffmassenanteile von 1,5 % bestimmt, während für drei andere Sterne solche hohen Stickstoffhäufigkeiten ausgeschlossen werden können. Für NGC 5189 gelang außerdem die qualitative Reproduktion der beobachteten, starken Ne-Spektrallinien mittels unserer Modelle. Zur Zeit lässt sich aus der Stärke der Ne-Emissionslinien der Ne-Massenanteil leider nur abschätzen, er scheint aber im Bereich einiger Prozent zu liegen. Mittels eines diagnostischen He-C-Linienpaares konnte das He:C-Massenverhältnis von 60:30 für [WCE]-Sterne bestätigt werden. Als Ergebnis der Analyse von PB 8 postulieren wir eine neue Klasse von wasserstoffarmen Zentralsternen, die in ihrer Elementzusammensetzung eher an massereiche WNL-Sterne als an [WC]-Sterne erinnern. Die ermittelten Massenanteile H:He:C:N:O betragen 40:55:1.3:2:1.3, der Wind von PB 8 enthält daher im Unterschied zu WN-Sternen signifikante Mengen von O und C. Es wird daher eine Klassifizierung als [WN/WC] vorgeschlagen.

The atomic physics relevant to gaseous nebulae is critically examined using modelling software with particular emphasis on radio recombination lines (RRLs). The theoretical spectral line intensities can be deduced if we know the population structure of the bound electrons in the gas under non-thermal equilibrium conditions. The population structure of hydrogen is solved for various environments using a capture-collision-cascade model that incorporates an ambient radiation eld. The validity of assuming Case B (Baker & Menzel, 1938) for nebulae is investigated. It is known that Case B is appropriate for levels with small principal quantum numbers (n < 40), but this assumption is re-examined for high levels which are relevant to RRLs. E ects of an ambient radiation eld on the population structure is examined and processes that are stimulated by a radiation eld are included in the model. This is done as a preliminary investigation to extend the model to a photoionization code. / Physics / (M. Sc. (Astronomy)