Metal Mixing in the r-process Enhanced Ultrafaint Dwarf Galaxy Reticulum II*

Abstract

The ultrafaint dwarf galaxy Reticulum II was enriched by a single rare and prolific r-process event. The r-process content of Reticulum II thus provides a unique opportunity to study metal mixing in a relic first galaxy. Using multi-object high-resolution spectroscopy with VLT/GIRAFFE and Magellan/M2FS, we identify 32 clear spectroscopic member stars and measure abundances of Mg, Ca, Fe, and Ba where possible. We find 72 − 12 + 10 % of the stars are r-process-enhanced, with a mean [ Ba / H ] = − 1.68 ± 0.07 and unresolved intrinsic dispersion σ [Ba/H]<0.20. The homogeneous r-process abundances imply that Ret II’s metals are well mixed by the time the r-enhanced stars form, which simulations have shown requires at least 100 Myr of metal mixing in between bursts of star formation to homogenize. This is the first direct evidence of bursty star formation in an ultrafaint dwarf galaxy. The homogeneous dilution prefers a prompt and high-yield r-process site, such as collapsar disk winds or prompt neutron star mergers. We also find evidence from [Ba/H] and [Mg/Ca] that the r-enhanced stars in Ret II formed in the absence of substantial pristine gas accretion, perhaps indicating that ≈70% of Ret II stars formed after reionization.

GHOST Commissioning Science Results: Identifying a New Chemically Peculiar Star in Reticulum II

“…All of the bright red giant branch (RGB) stars in Ret II have been previously observed with high-resolution spectroscopy with one exception, Gaia DR3 4732600514724860928 (hereafter GDR3 0928). This candidate is one of the brightest stars in Ret II, with a >99% probability for membership, yet it only has a limited chemical abundance analysis in the literature (Ji et al 2023). This star has been identified as a potential Ret II member (Massari & Helmi 2018;Ji et al 2023) as it has a consistent metallicity and radial velocity with other stars in Ret II (Star 97 in Ji et al 2023), but it has a redder color than the other Ret II RGB stars, which can be seen in Figure 1.…”

“…This candidate is one of the brightest stars in Ret II, with a >99% probability for membership, yet it only has a limited chemical abundance analysis in the literature (Ji et al 2023). This star has been identified as a potential Ret II member (Massari & Helmi 2018;Ji et al 2023) as it has a consistent metallicity and radial velocity with other stars in Ret II (Star 97 in Ji et al 2023), but it has a redder color than the other Ret II RGB stars, which can be seen in Figure 1. The discrepancy between its red color and previously estimated low metallicity, [Fe/H] = −2.35, has precluded its identification as a bona fide member of Ret II.…”

“…The first examination of the chemical abundances in Ret II from high-resolution spectra showed that seven (of nine) red giants are highly enriched in r-process elements (Ji et al 2016a) a pattern reinforced by more detailed analyses which showed the r-process abundance pattern matches well-studied metal-poor halo stars (Ji et al 2016b;Roederer et al 2016). In addition, the unenriched stars are among the most metal-poor stars in Ret II, leading to the conclusion that Ret II was enriched by a single rare and prolific r-process event after the initial star formation event (Ji et al 2023;Simon et al 2023).…”

“…However, Ji et al (2023) and Simon et al (2023) have recently argued that because the [Ba/H] abundances are roughly constant in the r-process-enhanced Ret II stars, the r-processed gas should have been well mixed into the interstellar medium (ISM) of Ret II before the births of these stars. Thus, a short SFH history of Ret II would seem to require a prompt r-process source on timescales  a few 100 Myr, whereas neutron star mergers require binary evolution that can take more than a Gyr (e.g., GW170817; Blanchard et al 2017;Pian et al 2017).…”

Constraints on the Frequency and Mass Content of r-process Events Derived from Turbulent Mixing in Galactic Disks

“…Most of these systems are very metal-poor and exhibiting low r-process enhancements (Cowan et al 2021). A notable exception is Reticulum-II, which has been shown to contain several highly enriched stars (Ji et al 2016a(Ji et al , 2016b(Ji et al , 2023Roederer et al 2016). In this context, it is interesting to consider the likelihood that any one small system hosts an r-process event given the very low SFRs associated with these systems.…”