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

TL;DR

This study analyzes the chemical abundances in 14 northern S-type symbiotic giants using high-resolution near-infrared spectra, revealing metallicity patterns, evidence of stellar mixing processes, and potential accretion signatures.

Contribution

It extends previous abundance analyses to a northern sample, providing new insights into metallicity, stellar evolution, and accretion phenomena in symbiotic systems.

Findings

Median metallicity around [Fe/H] ~ -0.2 dex

Evidence of first dredge-up and additional mixing processes

Detection of a blue-shifted absorption component possibly linked to accretion

Abstract

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 disk/halo objects, whereas our previous southern sample is heavily biased toward the bulge population. Spectrum synthesis of high-resolution (R$\sim$50000), near-$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, Sc) in the atmospheres of the red giant component. The SySt sample shows generally slightly sub-solar metallicity, as expected for an older disk population, with a median at [Fe/H]\,$\sim -0.2$ dex. Enhanced $^{14}$N, depleted $^{12}$C, and decreased $^{12}$C/$^{13}$C 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 {\sl 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 onto the hot component.