ured by [Fran[ç]{}ois ]{}(1987, 1988) and Clegg et al. (1981). NLTE effects in S abundances are found to be small and practically negligible. The behavior of \[S/Fe\] vs. \[Fe/H\] exhibits a linear increasing trend without plateau with decreasing \[Fe/H\]. Combining our results with those available in the literature, we find that the slope of the increasing trend is $-0.25$ in the NLTE behavior of \[S/Fe\], which is comparable to that observed in \[O/Fe\]. The observed behavior of S may require chemical evolution models of the Galaxy, in which scenarios of hypernovae nucleosynthesis and/or time-delayed deposition into interstellar medium are incorporated.'
author:
- 'Masahide Takada-Hidai'
- Yoichi Takeda
- Shizuka Sato
- Satoshi Honda
- Kozo Sadakane
- Satoshi Kawanomoto
- 'Wallace L. W. Sargent and Limin Lu'
- 'Thomas A. Barlow'
title: 'Behavior of Sulfur Abundances in Metal-Poor Giants and Dwarfs'
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Introduction
============

The behavior of $\alpha$-elements such as oxygen, magnesium, silicon, sulfur, calcium, and titanium in metal-poor halo and disk stars provides us with very useful information and constraints for exploring stellar nucleosynthesis and chemical evolution of the Galaxy in the early phase, since these elements are believed to be produced mainly by type II supernovae (SNe II) of massive stars (e.g. Timmes, Woosely, & Weaver 1995; Chiappini et al. 1999; Carretta, Gratton, & Sneden 2000) and are fossilized in stars which have been born at various epochs of the Galaxy evolution.

Numerous abundance works have been carried out for these elements (e.g. see a review of McWilliam 1997). It is well known that the $\alpha$-elements, except for O and S, show a general trend of abundances such that \[$\alpha$-element/Fe\] increases with decreasing metallicity \[Fe/H\] down to about $-1$ dex, and forms a plateau with \[$\alpha$-element/Fe\] $\sim 0.3$ – 0.5 dex in the range of \[Fe/H\] $< -1$ dex. Here we define \[A/B