Abstract
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 ($\lesssim 0.1 \,\mathrm{dex}$).
Highlights
The subject of this paper is to investigate how the non-LTE effect influences the spectroscopic determination of sulfur and zinc abundances in F, G, and K-type stars used for studying the Galactic chemical evolution history.1.1
Do some of the departures from [Zn/Fe] ≃ 0 described above have something to do with neglecting the non-LTE effect? Or alternatively, is the non-LTE correction so negligible as to guarantee the reliability of such trends derived from LTE? In search of the answers to these questions, we decided to carry out statistical equilibrium calculations on neutral zinc, in order to estimate the non-LTE abundance corrections for Zn i 4722/4810/6362 lines for a wide range of stellar parameters, and to perform an extensive non-LTE reanalysis of published equivalent-width data of these Zn i lines toward establishing the [Zn/Fe] vs. [Fe/H] relation in our manner
Regarding the equivalent width data of the Sun, those of S i 8693/8694 and 9228/9237 lines were taken from Takada-Hidai et al.’s (2005) table 3, while the others were newly measured from Kurucz et al.’s (1984) solar flux spectrum atlas
Summary
Sulfur belongs to the group of “α-capture elements” (along with O, Mg, Si, Ca, and Ti), a large fraction of which are considered to have been synthesized in short-lived massive stars (and thrown out by type II supernovae) at the early-time of the Galaxy. The situation turned into a new aspect, when the S i lines of the 9212/9228/9237 triplet (much stronger than S i 8693/8694 lines) began to be used for exploring the sulfur abundances of very metal-deficient stars based on the two recent studies of Ryde and Lambert (2004) and Nissen et al (2004) They both arrived at the same conclusion that [S/Fe] attains an approximately constant value of ∼ +0.3 at [Fe/H] ∼< −1, which is nothing but a typical α-group trend explainable with a standard chemical evolution model. This lesson taught us the necessity of carrying out an extensive non-LTE reanalysis on almost all available observational data published so far combined together, which is the first purpose of this investigation
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