Timing the r-process Enrichment of the Ultra-faint Dwarf Galaxy Reticulum II Academic Article uri icon


  • 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 of Ret II and analyze its star formation history. As in other ultra-faint dwarfs, the colormagnitude 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, forming 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.

published proceedings

  • The Astrophysical Journal

author list (cited authors)

  • Simon, J. D., Brown, T. M., Mutlu-Pakdil, B., Ji, A. P., Drlica-Wagner, A., Avila, R. J., ... Wilkinson, R. D.

publication date

  • February 2023