Type: Article
Publication Date: 2018-02-14
Citations: 24
DOI: https://doi.org/10.3847/2041-8213/aaa9b4
Abstract A high-resolution spectroscopic analysis is presented for a new highly r -process-enhanced ([Eu/Fe] = 1.27, [Ba/Eu] = −0.65), very metal-poor ([Fe/H] = −2.09), retrograde halo star, RAVE J153830.9–180424, discovered as part of the R-Process Alliance survey. At V = 10.86, this is the brightest and most metal-rich r -II star known in the Milky Way halo. Its brightness enables high-S/N detections of a wide variety of chemical species that are mostly created by the r -process, including some infrequently detected lines from elements like Ru, Pd, Ag, Tm, Yb, Lu, Hf, and Th, with upper limits on Pb and U. This is the most complete r -process census in a very metal-poor r -II star. J1538–1804 shows no signs of s -process contamination, based on its low [Ba/Eu] and [Pb/Fe]. As with many other r -process-enhanced stars, J1538–1804's r -process pattern matches that of the Sun for elements between the first, second, and third peaks, and does not exhibit an actinide boost. Cosmo-chronometric age-dating reveals the r -process material to be quite old. This robust main r -process pattern is a necessary constraint for r -process formation scenarios (of particular interest in light of the recent neutron star merger, GW170817), and has important consequences for the origins of r -II stars. Additional r -I and r -II stars will be reported by the R-Process Alliance in the near future.