Neutron-capture elements in dwarf galaxies II: Challenges for the s- and i-processes at low metallicity

The slow ( s ) and intermediate ( i ) neutron ( n ) capture processes occur both in asymptotic giant branch (AGB) stars, and in massive stars. To study the build-up of the s - and i -products at low metallicity, we investigate the abundances of Y, Ba, La, Nd, and Eu in 98 stars, at − 2 . 4 < [Fe/H] < − 0 . 9 , in the Sculptor dwarf spheroidal galaxy. The chemical enrichment from AGB stars becomes apparent at [Fe/H] ≈ − 2 in Sculptor, and causes [Y/Ba], [La/Ba], [Nd/Ba] and [Eu/Ba] to decrease with metallicity, reaching subsolar values at the highest [Fe/H] ≈ − 1 . To investigate individual nucleosynthetic sites, we compared three n -rich Sculptor stars with theoretical yields. One carbon-enhanced metal-poor (CEMP-no) star with high [Sr, Y, Zr] > +0 . 7 is best fit with a model of a rapidly-rotating massive star, the second (likely CH star) with the i -process, while the third has no satisfactory fit. For a more general understanding of the build-up of the heavy elements, we calculate for the first time the cumulative contribution of the s - and i -processes to the chemical enrichment in Sculptor, and compare with theoretical predictions. By correcting for the r -process, we derive [Y/Ba] s/i = − 0 . 85 ± 0 . 16 , [La/Ba] s/i = − 0 . 49 ± 0 . 17 , and [Nd/Ba] s/i = − 0 . 48 ± 0 . 12 , in the overall s - and/or i -process in Sculptor. These abundance ratios are within the range of those of CEMP stars in the Milky Way, which have either s - or i -process signatures. The low [Y/Ba] s/i and [La/Ba] s/i that we measure in Sculptor are inconsistent with them arising from the s -process only, but are more compatible with models of the i -process. Thus we conclude that both the s - and i -processes were important for the build-up of n -capture elements in the Sculptor dwarf spheroidal galaxy. ( r ) processes. Our results show that while the r -process is consistent with having little or no time delay relative to