The R-Process Alliance: Hunting for gold in the near-UV spectrum of 2MASS J05383296–5904280

Context . Over the past few years, the R -Process Alliance (RPA) has successfully carried out a search for stars that are highly enhanced in elements produced via the rapid neutron-capture ( r -) process. In particular, the RPA has identified a number of relatively bright, highly r -process-enhanced ( r -II) stars, suitable for observations with the Hubble Space Telescope (HST), facilitating abundance derivation of elements such as gold (Au) and cadmium (Cd). Aims . This paper presents the detailed abundances derived for the metal-poor ([Fe/H] = −2.55) highly r -process-enhanced ([Eu/Fe] = +1.29) r -II star 2MASS J05383296–5904280. Methods . One-dimensional local thermodynamic equilibrium (LTE) elemental abundances were derived via equivalent width and spectral synthesis using high-resolution high signal-to-noise near-UV HST/STIS and optical Magellan/MIKE spectra. Results . Abundances were determined for 43 elements, including 26 neutron-capture elements. In particular, abundances of the rarely studied elements Nb, Mo, Cd, Lu, Os, Pt, and Au are derived from the HST spectrum. These results, combined with RPA near-UV observations of two additional r -II stars, increase the number of Cd abundances derived for r -process-enriched stars from seven to ten and Au abundances from four to seven. A large star-to-star scatter is detected for both of these elements, highlighting the need for more detections enabling further investigations, specifically into possible non-LTE effects.