Name/Affiliation: Cezary Galan (Nicolaus Copernicus Astronomical Center of the PAS, Warsaw, Poland)
Chemical abundance analysis of the symbiotic red giants
Symbiotic stars are the long period, interacting binary systems composed of red giant donors and white dwarf companion (occasionally replaced by a neutron star). Their study is therefore very important to understand interacting binary evolution involving RGB/AGB stars. Knowledge of the elemental abundances is of special significance since they can be used to track mass exchange history. However, the number of symbiotic giants with fairly well determined photospheric composition is insufficient for any statistical considerations, and most are G or K-type giants whereas vast majority of symbiotic stars contain M-type giants. Here we present the detailed chemical abundance analysis obtained for roughly a dozen of M-type symbiotic giants. The analysis is based on the high resolution (R~50000), high S/N~100, near-IR spectra (at H- and K-band regions) obtained with Phoenix/Gemini South spectrometer. Spectrum synthesis employing standard LTE analysis and atmosphere models was used to obtain photospheric abundances of CNO and elements around the iron peak (Sc, Ti, Fe, and Ni). Our analysis reveals a significantly sub-solar metallicity, [Fe/H]~-0.75, for the RW Hya giant confirming its membership of the Galactic halo population. We obtained slightly sub-solar metallicities ([Fe/H] ~ -0.2 - -0.45) in 60% objects and a near-solar metallicities were found in the rest cases of our sample. The very low 12C/13C isotopic ratios, ~ 6-11, in almost all these giants indicate that they have experienced the first dredge-up.