| Abstract [eng] |
Chromospherically active stars, such as RS CVn stars, exhibit strong magnetic activity, which has a substantial impact on stellar evolution and internal mixing processes of the stars. The standard stellar evolution models predict well-defined changes in surface chemical abundances; observations of these chromospherically active stars often reveal notable deviations. This thesis, for the first time, presents a homogeneous spectroscopic analysis of more than 50 RS CVn stars and 12 eruptive stars. High-resolution spectra of RS CVn stars were observed at the Molėtai Astronomical Observatory, and the spectra of 12 eruptive stars were observed on the Nordic Optical Telescope. The spectra were analysed to determine atmospheric parameters and abundances of elements such as Li, C, N, O, and the ¹²C/¹³C ratio using equivalent width and spectral synthesis methods, with non-local thermodynamic equilibrium corrections applied to lithium. The results reveal that the reduced ¹²C/¹³C ratios in several RS CVn giants below the red giant branch luminosity bump suggest the occurrence of early onset of extra mixing. Lithium abundances exhibit both depletion and enrichment, which points towards the presence of multiple mixing mechanisms influenced by magnetic activity. In contrast, the chemical abundances of eruptive stars remain consistent with predictions from standard stellar evolution, implying that their surface activity does not lead to significant modification of their chemical composition. These findings demonstrate that magnetic activity plays a key role in altering the timing and efficiency of internal mixing in evolved stars, providing new insights into stellar evolution and chemical enrichment in the Galaxy. |
