Non-LTE Line-Formation and Abundances of Sulfur and Zinc in F, G, and K Stars

Abstract

Extensive statistical-equilibrium calculations on S I and Zn I 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, $\alpha$ 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, S I 9212/9228/9237 lines suffer significant negative non-LTE corrections ($\lesssim 0.2 \hbox{--} 0.3 \,\mathrm{dex}$), 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 $\mathrm{[S/Fe]}$ 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 $\mathrm{[Zn/Fe]}$ reported from recent LTE studies (i.e., an evident/slight increase of $\mathrm{[Zn/Fe]}$ with a decrease of $\mathrm{[Fe/H]}$ 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 $\mathrm{[Fe/H]}$) are quantitatively of less significance ($\lesssim 0.1 \,\mathrm{dex}$).

First stars

“…These models have been shown to provide LTE abundances very similar (within 0.05 dex) to those of the MARCS models used by Cayrel et al (2004) and Bonifacio et al (2009). It has been shown (see Takeda et al 2005;Korotin 2008) that Mult. 1 of S I is affected by a significant negative non-LTE correction.…”

“…( * ) An asterisk after the name of the star means that the star is carbon-rich. a metallicity of about −3.0 and a temperature of 6000 K, the correction is found to be close to −0.6 dex, whereas it was only about −0.2 dex following Takeda et al (2005).…”

“…1. This correction is sometimes rather different from the correction computed by Takeda et al (2005). For example, for turnoff stars with et al (2010).…”

“…Since the Zn line is very weak in turnoff stars, we could only measure Zn abundances in our giant stars (Cayrel et al 2004). Following Takeda et al (2005), non-LTE effects for Zn are not very important in these stars (≤+0.1 dex), but we have applied the small correction obtained by interpolation in their tables.…”

“…On the other hand, Takeda et al (2005) noted that, even taking into account the non-LTE effects on the formation of the S lines, the resulting abundance of sulfur depended on the multiplet used for this determination. There was, in particular, an apparent discrepancy between the abundance trends deduced from multiplets 6 and 1.…”