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

• Published in: The Astrophysical journal 2023-02, Vol.944 (1), p.43
• Main Authors: 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., Rosell, A. Carnero, Kind, M. Carrasco, 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., Malagón, A. A. Plazas, 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.
• Format: Article
• Language: English
• Subjects: Astronomical models; ASTRONOMY AND ASTROPHYSICS; Astrophysics; Bursts; Chemical evolution; Delay time; Dwarf galaxies; Galaxy ages; HST photometry; Hubble Space Telescope; Ionization; Local Group; Nuclear fusion; Nucleosynthesis; R-process; Space telescopes; Star & galaxy formation; Star formation; Stars; Stars & galaxies; Stellar populations
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.3847/1538-4357/aca9d1

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.