Kavli Affiliate: Anna Frebel
| First 5 Authors: Yutaka Hirai, Timothy C. Beers, Young Sun Lee, Shinya Wanajo, Ian U. Roederer
| Summary:
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.
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