The Astrophysical Journal (Jan 2023)

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

DOI
https://doi.org/10.3847/1538-4357/aca9d1
Journal volume & issue
Vol. 944, no. 1
p. 43

Abstract

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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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