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

description: 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.
description: 14 pages, 5 figures, 1 table. Accepted for publication in ApJ