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ABSTRACT There has been a concerted effort in recent years to identify the astrophysical 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 covering the primary neutron-capture peaks. Subtle differences in the structure of the r-process pattern, such as the relative abundances of elements in the third peak, in particular, are expected to constrain the r-process sites further. Here, we present a detailed elemental-abundance analysis of four bright RPE stars selected from the HESP–GOMPA survey. Observations were carried out with the 10-m class telescope Gran Telescopio Canarias (GTC), Spain. The high spectral signal-to-noise ratios obtained allow us to derive abundances for 20 neutron-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-II and r-I stars, based on a compilation of RPE stars from the literature. The strontium (Sr) abundance trend with respect to europium (Eu) suggests the need for an additional production site for Sr (similar to several earlier studies); this requirement could be milder for yttrium (Y) and zirconium (Zr). We also show that there could be some time delay between r-II and r-I star formation, based on the Mg/Th abundance ratios. 

Abstract The S2 stream is a kinematically cold stream that is plunging downwards through the Galactic disc. It may be part of a hotter and more diffuse structure called the Helmi stream. We present a multi-instrument chemical analysis of the stars in the metal-poor S2 stream using both high- and low-resolution spectroscopy, complemented with a re-analysis of the archival data to give a total sample of 62 S2 members. Our high-resolution program provides α-elements (C, Mg, Si, Ca and Ti), iron-peak elements (V, Cr, Mn, Fe, Ni), n-capture process elements (Sr, Ba) and other elements such as Li, Na, Al, and Sc for a subsample of S2 objects. We report coherent abundance patterns over a large metallicity spread (∼1 dex) confirming that the S2 stream was produced by a disrupted dwarf galaxy. The combination of S2’s α-elements displays a mildly decreasing trend with increasing metallicity which can be tentatively interpreted as a “knee” at [Fe/H]<−2. At the low metallicity end, the n-capture elements in S2 may be dominated by r-process production however several stars are Ba-enhanced, but unusually poor in Sr. Moreover, some of the low-[Fe/H] stars appear to be carbon-enhanced. We interpret the observed abundance patterns with the help of chemical evolution models that demonstrate the need for modest star-formation efficiency and low wind efficiency confirming that the progenitor of S2 was a primitive dwarf galaxy. 

Abstract The DESI Milky Way Survey (MWS) will observe ≥8 million stars between 16 < r < 19 mag, supplemented by observations of brighter targets under poor observing conditions. The survey will permit an accurate determination of stellar kinematics and population gradients, characterize diffuse substructure in the thick disk and stellar halo, enable the discovery of extremely metal-poor stars and other rare stellar types, and improve constraints on the Galaxy’s 3D dark matter distribution from halo star kinematics. MWS will also enable a detailed characterization of the stellar populations within 100 pc of the Sun, including a complete census of white dwarfs. The target catalog from the preliminary selection described here is public (Available at https://data.desi.lbl.gov/public/ets/target/catalogs/and detailed at  https://desidatamodel.readthedocs.io).