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Abstract

Light trans-iron elements such as Sr serve as the key to understanding the
astrophysical sites of heavy elements. Spectroscopic studies of metal-poor
stars have revealed large star-to-star scatters in the ratios of [Sr/Ba], which
indicates that there are multiple sites for the production of Sr. Here we
present the enrichment history of Sr by a series of the $N$-body/smoothed
particle hydrodynamics simulations of a dwarf galaxy with a stellar mass of 3
$\times$ 10$^{6}$ $M_{\odot}$. We show that binary neutron star mergers (NSMs)
and asymptotic giant branch (AGB) stars contribute to the enrichment of Sr in
the metallicity ranges [Fe/H] $\gtrsim$ $-$3 and [Fe/H] $\gtrsim$ $-$1,
respectively. It appears insufficient, however, to explain the overall
observational trends of Sr by considering only these sites. We find that the
models including electron-capture supernovae (ECSNe) and rotating massive stars
(RMSs), in addition to NSMs and AGBs, reasonably reproduce the enrichment
histories of Sr in dwarf galaxies. The contributions of both ECSNe and NSMs
make scatters of $\approx$ 0.2 dex in [Sr/Fe], [Sr/Ba], and [Sr/Zn] as can be
seen for observed stars in the metallicity range [Fe/H] $<$ $-2$. We also find
that the mass range of ECSN progenitors should be substantially smaller than
$1\, M_\odot$ (e.g., 0.1-$0.2\, M_\odot$) to avoid over-prediction of [Sr/Ba]
and [Sr/Zn] ratios. Our results demonstrate that NSMs, AGBs, ECSNe, and RMSs
all play roles in the enrichment histories of Local Group dwarf galaxies,
although more observational data are required to disentangle the relative
contributions of these sources.