The $R$-Process Alliance: First Release from the Southern Search for $r$-Process-Enhanced Stars in the Galactic Halo

TL;DR

This study reports the first southern hemisphere sample of metal-poor stars with enhanced $r$-process elements, using spectroscopic analysis to identify various levels of $r$-process enrichment, advancing understanding of heavy element origins.

Contribution

It presents the first southern sample of $r$-process-enhanced stars, demonstrating improved detection success through refined target selection focusing on red giants.

Findings

Identified 12 $r$-II stars with strong $r$-process enhancement.

Found 42 $r$-I stars with moderate $r$-process enrichment.

Detected 20 limited-$r$ stars with light $r$-process element enhancement.

Abstract

The recent detection of a binary neutron star merger and the clear evidence for the decay of radioactive material observed in this event have, after sixty years of effort, provided an astrophysical site for the rapid neutron-capture ($r$-) process which is responsible for the production of the heaviest elements in our Universe. However, observations of metal-poor stars with highly-enhanced $r$-process elements have revealed abundance patterns suggesting that multiple sites may be involved. To address this issue, and to advance our understanding of the $r$-process, we have initiated an extensive search for bright ($V < 13.5$), very metal-poor ($\mathrm{[Fe/H]} < -2$) stars in the Milky Way halo exhibiting strongly-enhanced $r$-process signatures. This paper presents the first sample collected in the Southern Hemisphere, using the echelle spectrograph on du Pont 2.5m telescope at Las Campanas Observatory. We have observed and analyzed 107 stars with $-3.13 < \mathrm{[Fe/H]} < -0.79$. Of those, 12 stars are strongly enhanced in heavy $r$-process elements ($r$-II), 42 stars show moderate enhancements of heavy $r$-process material ($r$-I), and 20 stars exhibit low abundances of the heavy $r$-process elements and higher abundances of the light $r$-process elements relative to the heavy ones (limited-$r$). This search is more successful at finding $r$-process-enhanced stars compared to previous searches, primarily due to a refined target selection procedure that focuses on red giants.