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

ABSTRACT Highly r-process enhanced metal-poor stars (MP r-II; $\rm [Eu/Fe]\gt 1$ and $\rm [Fe/H]\lesssim -1.5$) 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 ($M_{\rm vir}\approx 10^7\!-\!10^8{\, {\rm M}_\odot }$ and $M_{\ast }\approx 10^3\!-\!10^4{\, {\rm M}_\odot }$ 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 $M_{\ast }\approx 10^5{\, {\rm M}_\odot }$) and europium (Eu) mass ($M_{\rm Eu}\approx 10^{-5}{\, {\rm M}_\odot }$). We identify only one simulated galaxy ($\rm M_{vir}\approx 4.6\times 10^7{\, {\rm M}_\odot }$, $M_{\ast }\approx 3.4\times 10^3{\, {\rm M}_\odot }$ at z = 0) with abundances similar to Ret II in a simulation volume that hosts ∼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 $\rm [Eu/Fe]\gt 1$.