Metal Mixing in the R-Process Enhanced Ultra-Faint Dwarf Galaxy Reticulum II

TL;DR

This study investigates metal mixing in the ultra-faint dwarf galaxy Reticulum II, revealing well-mixed r-process elements indicative of bursty star formation and suggesting a prompt, high-yield r-process origin.

Contribution

First direct evidence of bursty star formation and metal mixing in an ultra-faint dwarf galaxy, constraining the nature of r-process sites and galaxy evolution.

Findings

72% of stars are r-process-enhanced

Metals are well-mixed within 100 Myr

Supports prompt, high-yield r-process sources

Abstract

The ultra-faint dwarf galaxy Reticulum~II was enriched by a single rare and prolific r-process event. The r-process content of Reticulum~II thus provides a unique opportunity to study metal mixing in a relic first galaxy. Using multi-object high-resolution spectroscopy with VLT/GIRAFFE and Magellan/M2FS, we identify 32 clear spectroscopic member stars and measure abundances of Mg, Ca, Fe, and Ba where possible. We find $72^{+10}_{-12}$% of the stars are r-process-enhanced, with a mean $\left\langle\mbox{[Ba/H]}\right\rangle=-1.68~\pm~0.07$ and unresolved intrinsic dispersion $\sigma_{\rm [Ba/H]} < 0.20$. The homogeneous r-process abundances imply that Ret~II's metals are well-mixed by the time the r-enhanced stars form, which simulations have shown requires at least 100 Myr of metal mixing in between bursts of star formation to homogenize. This is the first direct evidence of bursty star formation in an ultra-faint dwarf galaxy. The homogeneous dilution prefers a prompt and high-yield r-process site, such as collapsar disk winds or prompt neutron star mergers. We also find evidence from [Ba/H] and [Mg/Ca] that the r-enhanced stars in Ret~II formed in the absence of substantial pristine gas accretion, perhaps indicating that ${\approx}70$% of Ret~II stars formed after reionization.