Kavli Affiliate: Anna Frebel
| First 5 Authors: Terese T. Hansen, Ian U. Roederer, Shivani P. Shah, Rana Ezzeddine, Timothy C. Beers
| Summary:
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. 1D LTE
elemental abundances are 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 are 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 (local thermodynamical equilibrium) effects.
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