Timing the r-Process Enrichment of the Ultra-Faint Dwarf Galaxy Reticulum II

TL;DR

This study investigates the star formation history of the ultra-faint dwarf galaxy Reticulum II and constrains the timing of r-process element enrichment, showing it occurred early within 500 million years of initial star formation.

Contribution

It provides the first detailed star formation timeline for Reticulum II and constrains the delay time for r-process enrichment, ruling out long-delay sources like neutron star mergers.

Findings

Majority of stars formed within 500 Myr of star formation onset

r-process enrichment occurred early, within 500 Myr

Star formation history includes two bursts, one short and one extended

Abstract

The ultra-faint dwarf galaxy Reticulum II (Ret II) exhibits a unique chemical evolution history, with 72 +10/-12% of its stars strongly enhanced in r-process elements. We present deep Hubble Space Telescope photometry of Ret II and analyze its star formation history. As in other ultra-faint dwarfs, the color-magnitude diagram is best fit by a model consisting of two bursts of star formation. If we assume that the bursts were instantaneous, then the older burst occurred around the epoch of reionization and formed ~80% of the stars in the galaxy, while the remainder of the stars formed ~3 Gyr later. When the bursts are allowed to have nonzero durations we obtain slightly better fits. The best-fitting model in this case consists of two bursts beginning before reionization, with approximately half the stars formed in a short (100 Myr) burst and the other half in a more extended period lasting 2.6 Gyr. Considering the full set of viable star formation history models, we find that 28% of the stars formed within 500 +/- 200 Myr of the onset of star formation. The combination of the star formation history and the prevalence of r-process-enhanced stars demonstrates that the r-process elements in Ret II must have been synthesized early in its initial star-forming phase. We therefore constrain the delay time between the formation of the first stars in Ret II and the r-process nucleosynthesis to be less than 500 Myr. This measurement rules out an r-process source with a delay time of several Gyr or more such as GW170817.