Ages and heavy element abundances from very metal-poor stars in the Sagittarius dwarf galaxy

TL;DR

This study presents the first high-resolution analysis of very metal-poor stars in the Sagittarius dwarf galaxy, revealing complex chemical enrichment patterns that challenge previous assumptions about its initial mass function and star formation history.

Contribution

It provides new detailed abundance measurements of 13 elements in very metal-poor stars in Sgr, demonstrating the influence of both low- and high-mass stars and revising previous IMF interpretations.

Findings

Sgr stars show abundance patterns similar to the metal-poor halo.

First measurements of Sr, Pb, and Th in Sgr, enabling age dating.

Evidence of early massive supernovae pollution in Sgr.

Abstract

Sagittarius (Sgr) is a massive disrupted dwarf spheroidal galaxy in the Milky Way halo that has undergone several stripping events. Previous chemical studies were restricted mainly to a few, metal- rich ([Fe/H]~ -1) stars that suggested a top-light initial mass function (IMF). Here we present the first high-resolution, very metal-poor ([Fe/H]=-1 to -3) sample of 13 giant stars in the main body of Sgr. We derive abundances of 13 elements namely C, Ca, Co, Fe, Sr, Ba, La, Ce, Nd, Eu, Dy, Pb, and Th which challenge the interpretation based on previous studies. Our abundances from Sgr mimic those of the metal-poor halo and our most metal-poor star ([Fe/H]~ -3) indicates a pure r-process pollution. Abundances of Sr, Pb, and Th are presented for the first time in Sgr, allowing for age determination using nuclear cosmochronology. We calculate ages of 9$\pm$2.5 Gyr. Most of the sample stars have been enriched by a range of asymptotic giant branch (AGB) stars with masses between 1.3 and 5 M$_{\odot}$. Sgr J190651.47-320147.23 shows a large overabundance of Pb (2.05dex) and a peculiar abundance pattern best fit by a 3 M$_{\odot}$ AGB star. Based on star-to-star scatter and observed abundance patterns a mixture of low- and high-mass AGB stars and supernovae (15-25 M$_{\odot}$) are necessary to explain these patterns. The high level (0.29$\pm$0.05 dex) of Ca indicates that massive supernovae must have existed and polluted the early ISM of Sgr before it lost its gas. This result is in contrast with a top-light IMF with no massive stars polluting Sgr.