Abundance Patterns of $\alpha$ and Neutron-capture Elements in the Helmi Stream

We identified 8 additional stars as members of the Helmi stream (HStr) in the combined GALAH+ DR3 and $Gaia$ EDR3 catalog. By consistently reevaluating claimed members from the literature, we consolidate a sample of 22 HStr stars with parameters determined from high-resolution spectroscopy and spanning a considerably wider (by $\sim$0.5 dex) metallicity interval ($-2.5 \lesssim \rm[Fe/H] < -1.0$)thanpreviouslyreported\mathrm{.}Ourstudyfocuseson$\alpha$ (Mg and Ca) and neutron-capture (Ba and Eu) elements. We find that the chemistry of HStr is typical of dwarf spheroidal (dSph) galaxies, in good agreement with previous $N$-body simulations of this merging event. Stars of HStr constitute a clear declining sequence in $[\alpha \mathrm{/}\mathrm{F}\mathrm{e}]$ for increasing metallicity up to $[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]\sim -1.0$. Moreover, stars of HStr show a median value of $+$0.5 dex for $[\mathrm{E}\mathrm{u}\mathrm{/}\mathrm{F}\mathrm{e}]$ with a small dispersion ($\pm$0.1 dex). Every star analyzed with $[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]<-1.2$ belong to the $r$-process enhanced ($[\mathrm{E}\mathrm{u}\mathrm{/}\mathrm{F}\mathrm{e}]>+0.3$ and $[\mathrm{B}\mathrm{a}\mathrm{/}\mathrm{E}\mathrm{u}]<0.0$) metal-poor category, providing remarkable evidence that, at such low-metallicity regime, stars of HStr experienced enrichment in neutron-capture elements predominantly via $r$-process nucleosynthesis. Finally, the extended metallicity range also suggests an increase in $[\mathrm{B}\mathrm{a}\mathrm{/}\mathrm{E}\mathrm{u}]$ for higher $[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]$, in conformity with other surviving dwarf satellite galaxies of the Milky Way.