Timing the r-process Enrichment of the Ultra-faint Dwarf Galaxy Reticulum II
Brown, Thomas M.; Bacon, D.; Smith, M.; Annis, J.; Bertin, E.; Brooks, D.; Burke, D. L.; Carnero Rosell, A.; Carrasco Kind, M.; Carretero, J.; da Costa, L. N.; Desai, S.; Doel, P.; Frieman, J.; García-Bellido, J.; Gerdes, D. W.; Gruen, D.; Gruendl, R. A.; Gschwend, J.; Gutierrez, G.; Honscheid, K.; Kuehn, K.; Kuropatkin, N.; Marshall, J. L.; Miquel, R.; Sanchez, E.; Scarpine, V.; Sevilla-Noarbe, I.; Suchyta, E.; Swanson, M. E. C.; Tarle, G.; Geha, Marla; Li, Ting S.; Hollowood, D. L.; Palmese, A.; Aguena, M.; Costanzi, M.; De Vicente, J.; Hinton, S. R.; Paz-Chinchón, F.; Wilkinson, R. D.; Hansen, Terese T.; Frebel, Anna; Simon, Joshua D.; Raveri, M.; Weaverdyck, N.; Ji, Alexander P.; James, David J.; Andrade-Oliveira, F.; Everett, S.; Ferrero, I.; Gatti, M.; Pereira, M. E. S.; Pieres, A.; Plazas Malagón, A. A.; Rodriguez-Monroy, M.; Santiago, B.; To, C.; Pace, Andrew B.; Mutlu-Pakdil, Burçin; Balbinot, Eduardo; Bechtol, Keith; Avila, Roberto J.; Drlica-Wagner, Alex; Martínez-Vázquez, Clara E.; Alves, O.; Mena-Fernández, J.; Vincenzi, M.
United States, Canada, Netherlands, Sweden, Brazil, United Kingdom, France, Spain, Italy, India, Norway, Germany, Australia
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 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, 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.