Timing the r-process Enrichment of the Ultra-faint Dwarf Galaxy Reticulum II

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