Evoliuciniai anglies ir azoto gausų pokyčiai mažos masės metalingose žvaigždėse

Thesis is devoted to the research of mixing processes in low-mass metal-rich stars. A classical stellar evolution model is unable to explain the observed abundances of specific elements that are affected by internal mixing processes. Recently developed models aim to explain extra-mixing scenarios in the low-mass stars, however, the amount of observational data is still not large enough to test these models. Using high-resolution spectra we analysed 109 stars in ten open clusters with turn-off masses between 1.1 and 5.6 solar masses. We determined carbon, nitrogen and oxygen abundances for all stars in our sample as well as sodium and ten other chemical elements for some stars. Abundances of elements that are affected by mixing processes were compared with classical 1st dredge-up and two extra-mixing models. The first one includes material mixing induced by thermohaline instability. The second model combines both thermohaline- and rotation-induced mixing. Both models of extra-mixing are quite similar in the range up to 3 solar masses for the carbon isotope ratios and they describe observational data well enough. However, for the mass range over 3 solar masses the data of carbon isotopic ratios disagree with the extra-mixing model that includes both rotational and thermohaline mixing. However, the carbon-to-nitrogen abundance ratios act vice versa. Thus, a universal model of carbon isotopic and carbon-to-nitrogen rations has yet to be developed