R-process-rich Stellar Streams in the Milky Way*

Abstract

<jats:title>Abstract</jats:title> <jats:p>We present high-resolution Magellan/MIKE spectra of 22 bright (9 &lt; <jats:italic>V</jats:italic> &lt; 13.5) metal-poor stars (−3.18 &lt; [Fe/H] &lt; −1.37) in three different stellar streams, the Helmi debris stream, the Helmi trail stream, and the <jats:italic>ω</jats:italic> Centauri progenitor stream. We augment our Helmi debris sample with results for 10 stars by Roederer et al. for a total of 32 stars. Detailed chemical abundances of light elements as well as heavy neutron-capture elements have been determined for our 22 stars. All three streams contain carbon-enhanced stars. For 13 stars, neutron-capture element lines were detectable, and they all show signatures in agreement with the scaled solar <jats:italic>r</jats:italic>-process pattern, albeit with a large spread of −0.5 &lt; [Eu/Fe] &lt; +1.3. Eight of these stars show an additional small <jats:italic>s</jats:italic>-process contribution superposed onto their <jats:italic>r</jats:italic>-process pattern. This could be discerned because of the relatively high signal-to-noise ratio of the spectra given that the stars are close by in the halo. Our results suggest that the progenitors of these streams experienced one or more <jats:italic>r</jats:italic>-process events early on, such as a neutron star merger or another prolific <jats:italic>r</jats:italic>-process source. This widely enriched these host systems before their accretion by the Milky Way. The small <jats:italic>s</jats:italic>-process contribution suggests the presence of asymptotic giant branch stars and associated local (inhomogeneous) enrichment as part of the ongoing chemical evolution by low-mass stars. Stars in stellar streams may thus be a promising avenue for studying the detailed history of large dwarf galaxies and their role in halo assembly with easily accessible targets for high-quality spectra of many stars.</jats:p>