Master's Thesis Defense by Andrius Popovas – Niels Bohr Institute - University of Copenhagen

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Master's Thesis Defense by Andrius Popovas

Stellar Model Atmospheres of Late Spectral Type Stars

In this thesis undergoing improvements of MARCS stellar model atmospheres code (Gustafsson et al., 2008) are presented. An improved opacity sampling technique is introduced and by exploiting its speed and agility an influence of atomic line blanketing effects for the physical structure of model atmospheres is analysed and it is shown, that they can be neglected for model atmospheres with effective temperature less than 3100 K.

Furthermore, a detailed analysis of studies regarding total internal partition functions is done, involving 28 different species. We show that there are large discrepancies between them and after calculating our own partition functions for 12 atomic and 8 molecular species we show, how different partition functions affect the structure (density up to 2.2 percent, temperature up to 0.6 percent) and the synthetic spectrum (up to 12 percent in the ultraviolet wavelength range and around 2 percent in 0.7 - 2 μm wavelength range) of a model atmosphere of  effective temperature 3500 K. Lastly, we show, that if one uses total internal partition functions, which are inconsistent with the corresponding line list in terms of degeneracy factors, one can expect major errors both in structure (gas pressure and density up to 50 percent, temperature up to 4 percent) and the synthetic spectrum (up to 60 percent in the ultraviolet wavelength range) of a model atmosphere of effective temperature 3500 K.

Supervisor: Uffe Gråe Jørgensen