Highly r-process enhanced stars in ultra-faint dwarf galaxies

Highly r-process enhanced metal-poor stars (MP r-II; [Eu/Fe] > 1 and [Fe/H] less than or similar to -1.5) have been observed in ultra-faint dwarf (UFD) galaxy, specifically in Reticulum II (Ret II). The fact that only a few UFDs contain such stars implies that the r-process site may reflect very rare but individually prolific events, such as neutron star mergers (NSMs). Considering the relatively short star formation history of UFDs, it is puzzling how they could experience such a rare phenomenon. In this work, we show the results of cosmological hydrodynamic zoom-in simulations of isolated UFDs (M-vir approximate to 10(7)-10(8) M-circle dot and M-* approximate to 10(3)-10(4) M-circle dot at z = 0) to explain the formation of MP r-II stars in UFDs. We employ a simple toy model for NSM events, adopting parameters consistent with observations, such as the NSM rate (1 per M-* approximate to 10(5) M-circle dot) and europium (Eu) mass (M-Eu approximate to 10(-5) M-circle dot). We identify only one simulated galaxy (M-vir approximate to 4.6 x 10(7) M-circle dot, M-* approximate to 3.4 x 10(3) M-circle dot at z = 0) with abundances similar to Ret II in a simulation volume that hosts similar to 30 UFD analogues, indicating that such abundances are possible but rare. By exploring a range of key parameters, we demonstrate that the most important factor in determining the formation of MP r-II stars in UFDs is how quickly subsequent stars can be formed out of r-process enriched gas. We find that it takes between 10 and 100 Myr to form the first and second bursts of MP r-II stars. Over this period, Eu-polluted gas maintains the required high abundance ratios of [Eu/Fe] > 1.

Automated summary

The study shows that the formation of r-process enhanced metal-poor stars in ultra-faint dwarf galaxies is possible but rare, potentially due to individually prolific neutron star merger events. The most crucial factor in determining the formation of these stars in ultra-faint dwarf galaxies is how quickly subsequent stars can be formed from r-process enriched gas. The study demonstrates that it takes between 10 and 100 million years to form the first and second bursts of these stars, maintaining the required high abundance ratios of [Eu/Fe] > 1 over this period.