Highly r-process enhanced stars in ultra-faint dwarf galaxies
  Highly r-process enhanced metal-poor stars (MP r-II, $\rm [Eu/Fe]>1$ 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 (SFH) 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
($M_{vir}\approx10^7-10^8$ solar mass and $M_{\ast}\approx10^3-10^4$ solar mass
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 $M_{\ast}\approx10^5$ solar mass) and europium (Eu)
mass ($M_{Eu}\approx10^{-5}$ solar mass). We identify only one simulated galaxy
($ M_{vir}\approx4.6\times10^7$, $M_{\ast}\approx 3.4\times 10^3$ solar mass at
$z=0$) with abundances similar to Ret~II in a simulation volume that hosts
$\sim30$ UFD analogs, 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 to 100~Myr to form the first and second burst of MP r-II
stars. Over this period, Eu-polluted gas maintains the required high abundance
ratios of $\rm [Eu/Fe]>1$.
