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

TL;DR

This study uses cosmological simulations to explore how ultra-faint dwarf galaxies can produce highly r-process enhanced stars, highlighting the rarity and timing of neutron star merger events in these small galaxies.

Contribution

The paper demonstrates through simulations that rare neutron star mergers can produce highly r-process enhanced stars in ultra-faint dwarf galaxies, emphasizing the importance of star formation timing.

Findings

Only one simulated galaxy matched Reticulum II's abundance patterns.

Formation of r-II stars occurs within 10-100 Myr after enrichment.

Rarity of such stars is due to the specific timing and conditions required.

Abstract

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 $\rm [Eu/Fe]>1$.