학술논문
Timing the r-process Enrichment of the Ultra-faint Dwarf Galaxy Reticulum II
Document Type
article
Author
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
Source
The Astrophysical Journal, Vol 944, Iss 1, p 43 (2023)
Subject
Language
English
ISSN
1538-4357
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.