Exploring the [S/Fe] Behavior of Metal-Poor Stars with the S I 1.046 micron Lines

TL;DR

This study investigates the behavior of sulfur-to-iron ratios in metal-poor stars using near-infrared spectral lines, revealing complex patterns that challenge previous simplified models of element abundance trends.

Contribution

It provides the first extensive non-LTE analysis of the S I 1.046 micron lines across a wide metallicity range, uncovering nuanced [S/Fe] behavior in metal-poor stars.

Findings

[S/Fe] reaches ~+0.7-0.8 dex at [Fe/H] ~-3

[S/Fe] shows a plateau at ~+0.3 between -2.5 and -1.5

[S/Fe] exhibits a discontinuous jump between -3 and -2.5

Abstract

In an attempt of clarifying the [S/Fe] behavior with the run of [Fe/H] in the metal-poor regime which has been a matter of debate, an extensive non-LTE analysis of near-IR S I triplet lines (multiplet 3) at 1.046 micron was carried out for selected 33 halo/disk stars in a wide metallicity range of [Fe/H] ~-3.7 to ~+0.3, based on the spectral data collected with IRCS+AO188 of the Subaru Telescope. We found an evidence of considerably large [S/Fe] ratio amounting to ~+0.7-0.8 dex at very low metallicity of [Fe/H] ~-3, which makes marked contrast with other alpha-elements (Mg, Si, Ca, Ti) flatly showing moderately supersolar [alpha/Fe] of ~0.3 dex. Meanwhile, a locally-flat tendency of [S/Fe] at ~+0.3 is seen at -2.5 <[Fe/H]< -1.5. These results may suggest that the nature of [S/Fe] in metal-poor halo stars is not so simple as has been argued (i.e., neither being globally flat independent of [Fe/H] nor monotonically increasing with a decrease in [Fe/H]), but rather complicated with a local plateau around [Fe/H] ~-2 followed by a discontinuous jump between the narrow interval of -3 <[Fe/H]< -2.5.