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arXiv · 2212.00810

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

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

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Joshua D. Simon, Thomas M. Brown, Burçin Mutlu-Pakdil, Alexander P. Ji, Alex Drlica-Wagner, Roberto J. Avila, Clara E. Martínez-Vázquez, Ting S. Li, Eduardo Balbinot, Keith Bechtol, Anna Frebel, Marla Geha, Terese T. Hansen, David J. James, Andrew B. Pace, M. Aguena, O. Alves, F. Andrade-Oliveira, J. Annis, D. Bacon, E. Bertin, D. Brooks, D. L. Burke, A. Carnero Rosell, M. Carrasco Kind, J. Carretero, M. Costanzi, L. N. da Costa, J. De Vicente, S. Desai, P. Doel, S. Everett, I. Ferrero, J. Frieman, J. García-Bellido, M. Gatti, D. W. Gerdes, D. Gruen, R. A. Gruendl, J. Gschwend, G. Gutierrez, S. R. Hinton, D. L. Hollowood, K. Honscheid, K. Kuehn, N. Kuropatkin, J. L. Marshall, J. Mena-Fernández, R. Miquel, A. Palmese, F. Paz-Chinchón, M. E. S. Pereira, A. Pieres, A. A. Plazas Malagón, M. Raveri, M. Rodriguez-Monroy, E. Sanchez, B. Santiago, V. Scarpine, I. Sevilla-Noarbe, M. Smith, E. Suchyta, M. E. C. Swanson, G. Tarle, C. To, M. Vincenzi, N. Weaverdyck, R. D. Wilkinson. 2022-12-01. Timing the r-Process Enrichment of the Ultra-Faint Dwarf Galaxy Reticulum II. https://doi.org/10.3847/1538-4357%2Faca9d1

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