To calculate realistic models of objects with Ni in their atmospheres, accurate atomic data for the relevant ionization stages need to be included in model atmosphere calculations. In the context of white dwarf stars, we investigate the effect of changing the Ni IV-VI bound-bound and bound-free atomic data on model atmosphere calculations. Models including photoionization cross-section (PICS) calculated with AUTOSTRUCTURE show significant flux attenuation of up to similar to 80 per cent shortward of 180 angstrom in the extreme ultraviolet (EUV) region compared to a model using hydrogenic PICS. Comparatively, models including a larger set of Ni transitions left the EUV, UV, and optical continua unaffected. We use models calculated with permutations of these atomic data to test for potential changes to measured metal abundances of the hot DA white dwarf G191-B2B. Models including AUTOSTRUCTURE PICS were found to change the abundances of N and O by as much as similar to 22 per cent compared to models using hydrogenic PICS, but heavier species were relatively unaffected. Models including AUTOSTRUCTURE PICS caused the abundances of N/O IV and V to diverge. This is because the increased opacity in the AUTOSTRUCTURE PICS model causes these charge states to form higher in the atmosphere, more so for N/O V. Models using an extended line list caused significant changes to the Ni IV-V abundances. While both PICS and an extended line list cause changes in both synthetic spectra and measured abundances, the biggest changes are caused by using AUTOSTRUCTURE PICS for Ni.

Chemical abundances as measured from stellar spectral lines are often subject to uncertainties due to lack of accurate data for inelastic collisions, which is needed for non-local thermodynamic equilibrium (non-LTE) modelling. For cool stars, understanding of collision processes with electrons and hydrogen atoms is required to achieve high precision measurements. In this thesis, I have investigated the role of these collisions on the non-LTE formation of Li and Mg spectral lines in late-type stars. In the case of Li, electron impact excitation processes were calculated using the R-matrix with pseudo states method and the results found to agree well with recent calculations using the convergent close-coupling technique. These modern data were employed in non-LTE calculations by updating an existing model atom, which already included modern data for hydrogen collision processes. Our results were compared with calculations using older semi-empirical approximation calculations and only small differences were found: about 0.01 dex (~ 2%) or less in the abundance corrections. We therefore conclude that the influence of uncertainties in the electron collision data on non-LTE calculations is negligible. Indeed, together with the collision data for the charge transfer process Li + H ↔ Li+ + H- now available, and barring the existence of an unknown important collisional process, the collisional data in general is not a source of significant uncertainty in non-LTE Li line formation calculations. In the case of Mg, electron impact excitation processes were again calculated with the Rmatrix with pseudo states method, and used together with recent hydrogen collision calculations to build and test a model atom, without free parameters, for non-LTE modelling. Both electron and hydrogen collision processes, including charge transfer and excitation, are found to be important thermalising agents in various cases. The modelled spectra agree well with observed spectra from benchmark stars in the optical and infrared. The modelling predicts non-LTE abundance corrections ∆A(Mg)NLTE–LTE in dwarfs, both solar metallicity and metal-poor, to be very small (of order 0.01 dex), even smaller than found in previous studies. In giants, corrections vary greatly between lines, but can be as large as 0.4 dex. Results of calculations in a large grid of 1D model atmospheres are presented, and the implications for studies of Mg discussed. The propagation of uncertainties in the inelastic collision data to those in stellar abundances is investigated, and found to lead to small uncertainties, once again typically less than 0.01 dex (2%), although for few stellar models in specific lines (e.g., metal-poor suns, in the 7691 Å line) uncertainties can be as large as 0.03 dex (7%).

A complete model of helium-like line and continuum emission in astrophysical plasmas has been incorporated into the plasma simulation code CLOUDY. All elements between He and Zn are treated, any number of levels can be considered, and a full treatment of radiative and collisional processes is included. This includes photoionization from all levels, line transfer including continuum pumping and destruction by background opacities, scattering, and collisional processes. The model is calculated self-consistently with the ionization and thermal structure of the surrounding nebula. The result is a complete line and continuum spectrum of the nebula. The model helium atom is described and compared to a second standalone helium atom in the low-density case. The effects of the mixing of singlet and triplet terms, the truncation of the physical system, and the convergence of the predicted line intensities as a function of the number of quantum levels explicitly included are considered. New Case-B emissivities are calculated for the helium atom at a range of electron temperatures and densities common in planetary nebulae. Observations of the Orion Nebula are analyzed and compared with predictions of the model helium atom. Observations of low-metallicity extragalactic objects by other authors are analyzed. The methods and details of the model helium-like ions are described. The standard X-ray diagnostics of these ions are revisited and augmented with semi-analytical and numerical calculations of ultraviolet line diagnostics. Finally, a new interface between CLOUDY and the X-ray spectral analysis tool XSPEC is discussed.

This thesis contains results of electron-ion recombination processes in atomic ions relevant for plasma applications. The measurements were performed at the Stockholm Refrigerated Electron Beam Ion Trap (R-EBIT) and at the CRYRING heavy-ion storage ring. Dielectronic recombination (DR) cross sections, resonant strengths, rate coefficients and energy peak positions in H-like and He-like S are obtained for the first time from the EBIT measurements. Furthermore, the experimentally obtained DR resonant strengths are used to check the behaviour of a scaling formula for low Z, H-and He-like iso-electronic sequences and to update the fitting parameters. KLL DR peak positions for initially He- to B-like Ar ions are obtained experimentally from the EBIT measurements. Both the results from highly charged sulfur and argon are compared with the calculations performed with a distorted wave approximation. Absolute recombination rate coefficients of B-like C, B-like Ne and Be-like F ions are obtained for the first time with high energy resolution from storage ring measurements. The experimental results are compared with the intermediate coupling AUTOSTRUCTURE calculations. Plasma rate coefficients of each of these ions are obtained by convoluting the energy dependent recombination spectra with a Maxwell-Boltzmann energy distribution in the temperature range of 103-106 K. The resulting plasma rate coefficients are presented and compared with the calculated data available in literature. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Accepted. Paper 5: Accepted. Paper 6: Manuscript. Paper 7: Manuscript.</p>

Les propriétés radiatives sont d'une importance cruciale lors de l'étude des plasmas astrophysiques et de fusion, particulièrement dans des conditions hors équilibre thermodynamique local (hors-ETL). C'est pourquoi nous avons développé un modèle collisionnel-radiatif, capable de calculer les propriétés radiatives en toutes conditions de température et de densité. Nous avons intégré à ce modèle de nouveaux facteurs de Gaunt développés par nous-mêmes. Ces facteurs permettent d'obtenir des sections efficaces des processus atomiques plus précis que les formules classiques et semi-empiriques, nécessaires au calcul des populations atomiques. Ils ont été obtenus en utilisant des méthodes sophistiquées, telles que les méthodes distorted-wave et generelized-line-strength, à l'aide du code FAC. Dans le but de valider nos résultats, nous les avons comparés dans un premier temps, aux calculs de Hill et Rose d'un plasma composé d'un mélange de carbone et de 5% de silicium, dans des conditions ETL et hors-ETL. Les résultats sont très satisfaisants, aussi bien à l'ETL qu'hors-ETL. Nous avons aussi comparé nos résultats à la transmission mesurée par Xiong et al. d'un plasma de silicium à Te = 72 eV et Ne = 1.3 x 10²¹ cm⁻³. Là encore, nous obtenons un bon accord. Cependant, on constate la présence de quelques écarts au niveau du décalage spectral de certaines raies, ainsi que l'absence de certaines transitions. Cette absence peut s'expliquer par certaines configurations électroniques manquantes dans le calcul de structure atomique. Nous nous sommes aussi intéressés à l'absorption d'un mélange de carbone et de 5% de germanium avec un nombre restreint de configurations électroniques. Nous avons exploré l'effet d'interaction de configurations ainsi que l'effet d'un champ radiatif. Nous avons également déduit des lois permettant de calculer rapidement les lois de Planck et de Rosseland. Ces lois concernent pour le moment le carbone, le silicium et le germanium, pour Te entre 250 et 350 eV et Ne entre 2 et 8 x 10²³ cm⁻³. / The radiative properties are of utmost importance when studying astrophysical and ICF plasmas, particularly in non local thermodynamic equilibrium conditions. This is the reason why we have developed a collisional-radiative model that enables us to calculate the radiative properties of plasmas under any temperature and density conditions. We developed and implemented new Gaunt factors to the CR model. These factors greatly improve the precision when calculating the cross sections of the microscopic processes. These cross sections are required to calculate the populations of the atomic levels. We obtained these new factors by using the distorted-wave and generalized line strength methods of the Flexible Atomic Code (FAC).In order to validate our new-developed CR model, we compared our results to those of Hill and Rose, of a plasma composed of carbon and 5% silicon, in LTE and NLTE conditions. We obtained satisfactory results in both conditions. We also compared our results to the experimental transmission measured by Xiong et al., of a pure silicon plasma, with Te = 72 eV et Ne = 1.3 x 10²¹ cm⁻³. Again, we obtained results that are in good agreement with the measured spectrum. However, we noticed some discrepancies in the line shift of some lines, and some missing transitions. This can be explained by the absence of certain electronic configurations in the calculation of the atomic structure with the Cowan atomic code.We also explored the absorption of a plasma composed of carbon and 5% germanium with a restricted number of electronic configurations. We investigated the effect of configuration interaction and the effect of the radiative field. We also developed new laws that predict the Planck and Rosseland mean opacities very quickly. For the time being, these laws work for carbon, silicon and germanium, with Te between 250 and 350 eV, and Ne between entre 2 et 8x10²³ cm⁻³.

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)

No description available.

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)

Stellar chemical abundances provide astronomers with vital information about the production of chemical elements. Some stars preserve the composition of the environment in which they were born on their surfaces. By analysing the light from a star, the abundances of elements, its age and its path in space can be derived, and translated into the language of galactic history. The spallative history of boron in the early Galaxy was reinvestigated by observations of an ultraviolet spectral line in the old star HD 140283 with the Hubble Space Telescope. The line was barely detected and the upper limit abundance derived was lower than expected, which calls for further observations of this line in halo stars. Stars evolved into subgiants were observed with the ESO CAT, La Silla, and NOT, La Palma, to deduce their usefulness for galactic evolution studies. The high resolution spectroscopy study of the 26 objects showed that these stars are indeed useful for such studies. They are more luminous than dwarf stars and their ages can be accurately derived. They do not seem to have changed their surface abundances due to their evolution into giants. Subgiants can successfully be used to observationally reach regions further from the Earth, which can remove local biases that may appear when only observing nearby dwarf stars. A NLTE investigation of neutral Ca showed that cool metal rich dwarf stars did not deviate significantly from LTE, as had earlier been suggested. By an LTE analysis of a sample of 17 such dwarfs, using recent MARCS atmospheres, synthetic spectroscopy and modern atomic line data, cool metal rich dwarfs were shown not to deviate significantly from the expected abundance patterns in a number of elements. This increases the number of potential targets for studies of galactic chemical evolution in the metal rich regime since most stars are cool.

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)