Chemical Composition of Extremely Metal-Poor Stars in the Sextans Dwarf Spheroidal Galaxy

TL;DR

This study analyzes the chemical composition of six extremely metal-poor stars in the Sextans dwarf galaxy using high-resolution spectroscopy, revealing unique abundance patterns and confirming the absence of stars with [Fe/H] below -3.

Contribution

First detailed high-resolution spectroscopic analysis of extremely metal-poor stars in Sextans dwarf galaxy, highlighting unique Mg/Fe ratios and Ba abundance patterns.

Findings

Fe abundances agree with low-resolution estimates.

First detection of low Mg/Fe ratios at such low metallicities in a dwarf galaxy.

Ba enrichment suggests early r-process activity at [Fe/H] < -2.5.

Abstract

Chemical abundances of six extremely metal-poor ([Fe/H]<-2.5) stars in the Sextans dwarf spheroidal galaxy are determined based on high resolution spectroscopy (R=40,000) with the Subaru Telescope High Dispersion Spectrograph. (1) The Fe abundances derived from the high resolution spectra are in good agreement with the metallicity estimated from the Ca triplet lines in low resolution spectra. The lack of stars with [Fe/H]=<-3 in Sextans, found by previous estimates from the Ca triplet, is confirmed by our measurements, although we note that high resolution spectroscopy for a larger sample of stars will be necessary to estimate the true fraction of stars with such low metallicity. (2) While one object shows an overabundance of Mg (similar to Galactic halo stars), the Mg/Fe ratios of the remaining five stars are similar to the solar value. This is the first time that low Mg/Fe ratios at such low metallicities have been found in a dwarf spheroidal galaxy. No evidence for over-abundances of Ca and Ti are found in these five stars, though the measurements for these elements are less certain. Possible mechanisms to produce low Mg/Fe ratios, with respect to that of Galactic halo stars, are discussed. (3) Ba is under-abundant in four objects, while the remaining two stars exhibit large and moderate excesses of this element. The abundance distribution of Ba in this galaxy is similar to that in the Galactic halo, indicating that the enrichment of heavy elements, probably by the r-process, started at metallicities [Fe/H] < -2.5, as found in the Galactic halo.