Enrichment in r-process elements from multiple distinct events in the early Draco dwarf spheroidal galaxy

TL;DR

This study reveals two distinct populations of stars in the Draco dwarf galaxy with different r-process element abundances, indicating multiple enrichment events rather than a single event, challenging previous assumptions about r-process origins.

Contribution

It provides the first evidence of two separate r-process enrichment channels in a dwarf galaxy, based on high-resolution spectroscopic data of twelve stars.

Findings

Two stellar populations with different r-process abundances identified

Evidence of multiple r-process enrichment events in Draco galaxy

First detection of a sporadic, low-level r-process enrichment channel

Abstract

The stellar record of elemental abundances in satellite galaxies is important to identify the origin of r-process because such a small stellar system could have hosted a single r-process event, which would distinguish member stars that are formed before and after the event through the evidence of a considerable difference in the abundances of r-process elements, as found in the ultra-faint dwarf galaxy Reticulum II (Ret II). However, the limited mass of these systems prevents us from collecting information from a sufficient number of stars in individual satellites. Hence, it remains unclear whether the discovery of a remarkable r-process enrichment event in Ret II explains the nature of r-process abundances or is an exception. We perform high-resolution spectroscopic measurements of r-process abundances for twelve metal-poor stars in the Draco dwarf galaxy in the metallicity range of -2.5<[Fe/H]<-2. We found that these stars are separated into two groups with r-process abundances differing by one order of magnitude. A group of stars with high abundances of r-process elements was formed by a single r-process event that corresponds to the event evidenced in Ret II. On the other hand, the low r-process abundance group was formed by another sporadic enrichment channel producing a far fewer r-process elements, which is clearly identified for the first time. Accordingly, we identified two populations of stars with different r-process abundances, which are built by two r-process events that enriched gases at levels that differ by more than one order of magnitude.