Non-LTE line-formation and abundances of sulfur and zinc in F, G, and K stars

Extensive statistical-equilibrium calculations on SI and ZnI were carried out, in order to investigate how the non-LTE effect plays a role in the determination of their abundances in F-K stars. Having checked on the spectra of representative F-type stars (Polaris, Procyon, a Per) and the Sun that our non-LTE corrections yield a reasonable consistency between the abundances derived from different lines, we tried an extensive non-LTE reanalysis of published equivalent-width data of S I and Zn I lines for metal-poor halo/disk stars. According to our calculations, SI 9212/9228/9237 lines suffer significant negative non-LTE corrections (less than or similar to 0.2-0.3 dex), while LTE is practically valid for S I 8683/8694 lines. As far as the very metal-poor regime is concerned, a marked discordance is observed between the [S/Fe] values from these two abundance indicators, in the sense that the former attains a nearly flat plateau (or even a slight downward bending) while the latter shows an ever-increasing trend with a further lowering of metallicity. The characteristics of [Zn/Fe] reported from recent LTE studies (i.e., an evident/slight increase of [Zn/Fe] with a decrease of [Fe/H] for very metal-poor/disk stars) were almost confirmed, since the non-LTE corrections for the Zn I 4722/4810 and 6362 lines (tending to be positive and gradually increasing toward lower [Fe/H]) are quantitatively of less significance (less than or similar to 0.1 dex).