New model atmospheres for chemically peculiar stars

As stars evolve, their surfaces may be altered by several different mechanisms. These introduce distinctly unusual chemical peculiarities in their atmospheres. Hydrogen-deficient stars and subdwarf B stars are examples of such objects. These stars present a challenge for constructing reliable models. as their atmospheric structure is particularly sensitive to opacity sources, and may be stratified by diffusion. In order to interpret accurately the properties of these stars, theoretical models must address these issue~. Such models are presented in this work. New model atmospheres have been constructed using up-to-date opacity data. Photoionization crosssections from the Opacity Project (OP) and the IRON Project have been incorporated into the continuous opacity calculations. An opacitysampling approach has been implemented to treat the line opacity, replacing the existing opacity distribution function method which was restricted in composition. Significant increases in the carbon and nitrogen opacities of hydrogen-deficient stars were.found when using the OP data, compared to models computed with opacity data from Peach (1970. MNRAS. 73, 1), and are reflected in the flux distributions and temperature structures of these stars. SUbsequently, systematically cooler effective temperatures were measured for hot extreme helium stars and He-sdB stars compared to previous analyses. Self-consistent model atmospheres incorporating microscopic diffusion have been computed. Based on these models, abundances predictions were made for helium, carbon, nitrogen and silicon in the atmospheres of sdB stars. An examination of helium spectral lines in the models revealed a possible diagnostic for detecting elemental stratification in subdwarf B stars. For a model atmosphere stratified by diffusion, helium abundance measurements will differ from line to line based on the depths of formation, and some line asymmetries may be observed. This was confirmed observationally in two sdBs -- Feige 48 and JL36. An analysis of helium line profiles present in the observed spectra of these stars strongly indicates that the atmospheres have been stratified by diffusion.EThOS - Electronic Theses Online ServiceGBUnited Kingdo