  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 [Fe/H]=-3.57, this is one of the lowest
metallicity stars 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 suggest 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 fast 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.
