Enrichment in r-process Elements from Multiple Distinct Events in the Early Draco Dwarf Spheroidal Galaxy *

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 12 metal-poor stars in the Draco dwarf galaxy in the metallicity range of . 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 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.