2023ApJ...959...60P

We report on a chemo-dynamical analysis of SPLUS J142445.34-254247.1 (SPLUS J1424-2542), an extremely metal-poor halo star enhanced in elements formed by the rapid neutron-capture process (r-process). This star was first selected as a metal-poor candidate from its narrowband S-PLUS photometry and followed up spectroscopically in medium resolution with Gemini-South/GMOS, which confirmed its low-metallicity status. High-resolution spectroscopy was gathered with GHOST at Gemini-South, allowing for the determination of the chemical abundances for 36 elements, from carbon to thorium. At [Fe/H] = -3.39, SPLUS J1424-2542 is one of the lowest-metallicity stars with measured Th and has the highest $\mathrm{log}\epsilon (\mathrm{Th}/\mathrm{Eu})$ observed to date, making it part of the “actinide-boost” category of r-process-enhanced stars. The analysis presented here suggests that the gas cloud from which SPLUS J1424-2542 formed must have been enriched by at least two progenitor populations. The light-element (Z ≤ 30) abundance pattern is consistent with the yields from a supernova explosion of metal-free stars with 11.3-13.4 M_⊙, and the heavy-element (Z ≥ 38) abundance pattern can be reproduced by the yields from a neutron star merger (1.66 M_⊙ and 1.27 M_⊙) event. A kinematical analysis also reveals that SPLUS J1424-2542 is a low-mass, old halo star with a likely in situ origin, not associated with any known early merger events in the Milky Way.