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

Abstract

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 [Fe/H]<-1.0$) than previously reported. Our study focuses on $\alpha$ (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 $[\alpha /Fe]$ for increasing metallicity up to $[Fe/H]\sim -1.0$. Moreover, stars of HStr show a median value of $+$0.5 dex for $[Eu/Fe]$ with a small dispersion ($\pm$0.1 dex). Every star analyzed with $[Fe/H]<-1.2$ belong to the $r$-process enhanced ($[Eu/Fe]>+0.3$ and $[Ba/Eu]<0.0$) 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 $[Ba/Eu]$ for higher $[Fe/H]$, in conformity with other surviving dwarf satellite galaxies of the Milky Way.

9 pages (+2 appendix), 4 figures. Accepted for publication in ApJ Letters

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