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There has been a concerted effort in recent years to identify theastrophysical sites of the $r$-process that can operate early in the Galaxy.The discovery of many $r$-process-enhanced (RPE) stars (especially by the$R$-process Alliance collaboration) has significantly accelerated this effort.However, only limited data exist on the detailed elemental abundances coveringthe primary neutron-capture peaks. Subtle differences in the structure of the$r$-process pattern, such as the relative abundances of elements in the thirdpeak, in particular, are expected to constrain the $r$-process sites further.Here, we present a detailed elemental-abundance analysis of four bright RPEstars selected from the HESP-GOMPA survey. Observations were carried out withthe 10-m class telescope Gran Telescopio Canarias (GTC), Spain. The highspectral signal-to-noise ratios obtained allow us to derive abundances for 20neutron-capture elements, including the third $r$-process peak element osmium(Os). We detect thorium (Th) in two stars, which we use to estimate their ages.We discuss the metallicity evolution of Mg, Sr, Ba, Eu, Os, and Th in $r$-IIand $r$-I stars, based on a compilation of RPE stars from the literature. Thestrontium (Sr) abundance trend with respect to europium (Eu) suggests the needfor an additional production site for Sr (similar to several earlier studies);this requirement could be milder for yttrium (Y) and zirconium (Zr). We alsoshow that there could be some time delay between $r$-II and $r$-I starformation, based on the Mg/Th abundance ratios.

Decoding the compositions of four bright $r$-process-enhanced stars