The formation of the Milky Way halo and its dwarf satellites: A NLTE-1D abundance analysis. V. The Sextans galaxy

TL;DR

This study provides a detailed NLTE abundance analysis of eleven very metal-poor stars in the Sextans dwarf galaxy, revealing alpha-enhancement, Ba deficiency, and a tight Ba-Mg relation, offering insights into early chemical enrichment.

Contribution

It presents the first homogeneous NLTE abundance analysis of Sextans dSph stars, highlighting chemical abundance patterns and their implications for galaxy formation and evolution.

Findings

Alpha-enhancement of 0.2-0.4 dex in Sextans stars.

A tight correlation between Ba and Mg abundances.

Sextans stars are deficient in Na and have near-solar Ni/Fe ratios.

Abstract

We present a homogeneous set of accurate atmospheric parameters for a sample of eleven very metal-poor (-3.32 <= [Fe/H] <= -2.61) stars in the Sextans dwarf spheroidal galaxy (dSph) and the non-local thermodynamic equilibrium (NLTE) abundances of, at least, seven chemical elements based on high-resolution UVES/VLT and HDS/Subaru spectra. For each star, its effective temperature and surface gravity were derived from the spectral energy distribution and the known distance, and the Fe abundance was obtained as the average from lines of Fe I and Fe II. Based on abundances of Mg, Ca, and Ti, we show that all the investigated stars reveal alpha-enhancements of 0.4~dex to 0.2~dex, and there is a hint of a decline in alpha/Fe for [Fe/H] > -2.8. The Sextans stars are deficient in Ba. The new result is an extremely tight relation between Ba and Mg, suggesting their common origin in massive stars and Ba synthesis in the r-process events on the timescales of standard supernovae. The exception is a C-enhanced star S 15-19 which is strongly enhanced in Ba. This star is unlikely to be a CEMP-s star because of low abundances of Sr and Y ([Y/Fe] < -1) that are produced in the s-process as efficiently as Ba and a non-detection of variation in the radial velocity. No distinctions from the Milky Way halo and the Sculptor and Ursa Minor dSphs were found in a history of early enrichment in Na and Ni, namely, the Sextans stars are deficient in Na and have close-to-solar Ni/Fe.