Sulphur abundances in halo stars from Multiplet 3 at 1045 nm

TL;DR

This study investigates sulphur abundances in metal-poor halo stars using the less contaminated Multiplet 3 at 1045 nm, comparing it with the traditional Multiplet 1, and finds significant scatter in [S/Fe] ratios at low metallicity.

Contribution

It demonstrates the viability of using Multiplet 3 at 1045 nm for sulphur abundance measurements, providing an alternative to the contaminated Multiplet 1 lines.

Findings

Multiplet 3 lines are less affected by telluric contamination.

No clear [S/Fe] plateau observed at low metallicity.

Significant scatter in sulphur abundance at low metallicity.

Abstract

Sulphur is a volatile alpha-element which is not locked into dust grains in the interstellar medium (ISM). Hence, its abundance does not need to be corrected for dust depletion when comparing the ISM to the stellar atmospheres. The abundance of sulphur in the photosphere of metal-poor stars is a matter of debate: according to some authors, [S/Fe] versus [Fe/H] forms a plateau at low metallicity, while, according to other studies, there is a large scatter or perhaps a bimodal distribution. In metal-poor stars sulphur is detectable by its lines of Mult.1 at 920 nm, but this range is heavily contaminated by telluric absorptions, and one line of the multiplet is blended by the hydrogen Paschen zeta line. We study the possibility of using Mult. 3 (at 1045 nm) for deriving the sulphur abundance because this range, now observable at the VLT with the infra-red spectrograph CRIRES, is little contaminated by telluric absorption and not affected by blends at least in metal-poor stars. We compare the abundances derived from Multiplets 1 and 3, taking into account NLTE corrections and 3D effects. Here we present the results for a sample of four stars, although the scatter is less pronounced than in previous analysis, we cannot find a plateau in [S/Fe], and confirm the scatter of the sulphur abundance at low metallicity.