The R-process Alliance: The Peculiar Chemical Abundance Pattern of RAVE J183013.5−455510

We report on the spectroscopic analysis of RAVE J183013.5−455510, an extremely metal-poor star, highly enhanced in CNO, and with discernible contributions from the rapid neutron-capture process. There is no evidence of binarity for this object. At = −3.57, this star has one of the lowest metallicities currently observed, with 18 measured abundances of neutron-capture elements. The presence of Ba, La, and Ce abundances above the solar system r-process predictions suggests that there must have been a non-standard source of r-process elements operating at such low metallicities. One plausible explanation is that this enhancement originates from material ejected at unusually high velocities in a neutron star merger event. We also explore the possibility that the neutron-capture elements were produced during the evolution and explosion of a rotating massive star. In addition, based on comparisons with yields from zero-metallicity faint supernova, we speculate that RAVE J1830−4555 was formed from a gas cloud pre-enriched by both progenitor types. From analysis based on Gaia DR2 measurements, we show that this star has orbital properties similar to the Galactic metal-weak thick-disk stellar population.