The $R$-Process Alliance: Enrichment of $r$-process Elements in a Simulated Milky Way-like Galaxy

TL;DR

This study uses cosmological simulations to explore how different $r$-process element enhancements in stars relate to their formation in dwarf galaxies or the Milky Way, revealing links between $r$-process enrichment and galaxy origin.

Contribution

It provides a detailed simulation-based analysis of $r$-process element distribution in a Milky Way-like galaxy, connecting stellar enrichment levels to their formation environments and accretion history.

Findings

$r$-II and non-$r$-process-enhanced stars mainly formed in dwarf galaxies.

$r$-I stars predominantly formed in the in-situ disk after 6 Gyr.

Highly $r$-process-enhanced stars formed in accreted components.

Abstract

We study the formation of stars with varying amounts of heavy elements synthesized by the rapid neutron-capture process ($r$-process) based on our detailed cosmological zoom-in simulation of a Milky Way-like galaxy with an $N$-body/smoothed particle hydrodynamics code, ASURA. Most stars with no overabundance in $r$-process elements, as well as the strongly $r$-process enhanced $r$-II stars ([Eu/Fe] $>+0.7$), are formed in dwarf galaxies accreted by the Milky Way within the 6 Gyr after the Big Bang. In contrast, over half of the moderately enhanced $r$-I stars ($+0.3 <$ [Eu/Fe] $\leq +0.7$) are formed in the main in-situ disk after 6 Gyr. Our results suggest that the fraction of $r$-I and $r$-II stars formed in disrupted dwarf galaxies is larger the higher their [Eu/Fe] is. Accordingly, the most strongly enhanced $r$-III stars ([Eu/Fe] $> +2.0$) are formed in accreted components. These results suggest that non-$r$-process-enhanced stars and $r$-II stars are mainly formed in low-mass dwarf galaxies that hosted either none or a single neutron star merger, while the $r$-I stars tend to form in the well-mixed in-situ disk. We compare our findings with high-resolution spectroscopic observations of $r$-process-enhanced metal-poor stars in the halo and dwarf galaxies, including those collected by the R-Process Alliance. We conclude that observed [Eu/Fe] and [Eu/Mg] ratios can be employed in chemical tagging of the Milky Way's accretion history.