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

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

The Astrophysical Journal

Advertised on:
2
2023
Number of authors
68
IAC number of authors
1
Citations
15
Refereed citations
12
Description
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.
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