R-process-rich stellar streams in the Milky Way

Kavli Affiliate: Anna Frebel

| First 5 Authors: Maude Gull, Anna Frebel, Karina Hinojosa, Ian U. Roederer, Alexander P. Ji

| Summary:

We present high-resolution Magellan/MIKE spectra of 22 bright ($9<V<13.5$)
metal-poor stars ($-3.18<mbox{[Fe/H]}<-1.37$) in three different stellar
streams, the Helmi debris stream, the Helmi trail stream, and the $omega$
Centauri progenitor stream. We augment our Helmi debris sample with results for
ten stars by Roederer et al. 2010 (arXiv:1001.1745), 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
$r$-process pattern, albeit with a large spread of $-0.5<mbox{[Eu/Fe]}<+1.3$.
Eight of these stars show an additional small $s$-process contribution
superposed onto their $r$-process pattern. This could be discerned because of
the relatively high $S/N$ 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 $r$-process events, such as a neutron star merger or another
prolific $r$-process source, early on that widely enriched these host systems
before their accretion by the Milky Way. The small $s$-process contribution
suggests the presence of AGB 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.

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