Chemical abundance analysis of symbiotic giants. Metallicity and CNO abundance patterns in 14 northern S-type systems

In previous works, we computed abundances for the red giant in nearly four dozen S-type symbiotic systems (SySt). The abundances provide information about metallicity, evolutionary status, and possible memberships in Galactic stellar populations. Here, we extend our studies with a northern hemisphere sample of SySt. This northern sample is dominated by Galactic disc/halo objects, whereas our previous southern sample is heavily biased toward the bulge population. Spectrum synthesis of high-resolution (R ~ 50 000), near-IR spectra using standard LTE analysis and atmospheric models have been used to measure abundances of CNO and elements around the iron peak (Fe, Ti, Ni, and Sc) in the atmospheres of the red giant component. The SySt sample shows generally slightly sub-solar metallicity, as expected for an older disc population, with a median at [Fe/H] ~ -0.2 dex. Enhanced ¹⁴N, depleted ¹²C, and decreased ¹²C/¹³C, indicate that all these giants have experienced the first dredge-up. Comparison with theoretical predictions indicates that additional mixing processes had to occur to explain the observed C and N abundances. Relative O and Fe abundances agree with those represented by Galactic disc and bulge giant populations in the APOGEE data, with a few cases that can be attributed to membership in the extended thick-disc/halo. As an interesting byproduct of this study, we observed a blue-shifted additional component on the wings of absorption lines in the spectra of AG Peg, which could be connected with accretion on to the hot component.