The cosmic origin of fluorine and sulphur: Infrared spectroscopic studies of red giants

Detta är en avhandling från Department of Astronomy and Theoretical Physics, Lund University

Sammanfattning: Disregarding the small primordial traces of the lightest elements, all metals have been formed in stellar processes, which means that the relative amount of metals in the Universe increases for every stellar generation. This build-up of elements is called chemical evolution and might be used both to constrain stellar models as well as understanding the formation and evolution of stellar populations. In this thesis I determine abundances of two of the least studied elements, fluorine and sulphur, in three different stellar populations in the Milky Way using infrared spectroscopy of giants. Regarding fluorine the chemical evolution is very unclear because the number of previous observations are small. The cosmic origin of fluorine could still be one or more of three different sources: asymptotic giant branch stars, core-collapse supernovae, or the winds of Wolf-Rayet stars. If the latter is confirmed by observations, fluorine would make a great proxy for the determining whether the initial mass function in the Bulge is different from the solar neighborhood, which has been suggested in several other types of works, but not all. If confirmed, that would tell us that the central parts of our Galaxy have evolved differently than the local Disk. In the thesis I find that all the fluorine in the solar neighborhood most likely was produced by asymptotic giant branch stars, but at the same time find possible indications of fluorine production by Wolf-Rayet stars in the Bulge, indeed suggesting an initial mass function of the Bulge that is skewed towards more massive stars as compared to the solar neighborhood. When it comes to sulphur, there have been several proposed trends for metal-poor stars. Interestingly some of these observations cannot be explained with classic models of Galactic evolution, thereby questioning some of our understanding of the formation and evolution of the Milky Way. In this thesis I find a Galactic evolution-trend of sulphur following the expected trend from established models and cannot confirm any of the more exotic trends.