Sulfur and zinc abundances of red giant stars

TL;DR

This study establishes reliable sulfur and zinc abundance measurements in red giant stars, demonstrating their consistency with dwarf stars and confirming their usefulness in tracing galactic chemical evolution.

Contribution

It introduces a spectrum-fitting method with non-LTE corrections for S and Zn in red giants, identifying the most reliable spectral lines for abundance determination.

Findings

Non-LTE corrections are generally small (<0.2 dex) for S and Zn lines.

The S I 6757 line is more reliable than S I 8694-5 for red giants.

Red giant S and Zn abundances agree with those of dwarf stars, validating their use in chemical evolution studies.

Abstract

Sulfur and zinc are chemically volatile elements, which play significant roles as depletion-free tracers in studying galactic chemical evolution. However, regarding red giants having evolved off the main sequence, reliable abundance determinations of S and Zn seem to be difficult despite that a few studies have been reported so far. Given this situation, we tried to establish the abundances of these elements for an extensive sample of 239 field GK giants (-0.8 < [Fe/H] < +0.2), by applying the spectrum-fitting technique to S I 8694-5, S I 6757, and Zn I 6362 lines and by taking into account the non-LTE effect. Besides, similar abundance analysis was done for 160 FGK dwarfs to be used for comparison. The non-LTE corrections for the S and Zn abundances derived from these lines turned out < 0.1(-0.2) dex for most cases and not very significant. It revealed that the S I 6757 feature is more reliable as an abundance indicator than S I 8694-5 for the case of red giants, because the latter suffers blending of unidentified lines. The finally resulting [S/Fe]-[Fe/H] and [Zn/Fe]-[Fe/H] relations for GK giants were confirmed to be in good agreement with those for FGK dwarfs, indicating that S and Zn abundances of red giants are reliably determinable from the S I 6757 and Zn I 6362 lines. Accordingly, not only main-sequence stars but also evolved red giant stars are usable for tracing the chemical evolution history of S and Zn in the regime of disk metallicity by using these lines.