Neutron-capture elements in dwarf galaxies

II. Challenges for the s- and i-processes at low metallicity^⋆

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
² Dipartimento di Fisica e Astronomia, Universitá degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
e-mail: asa.skuladottir@unifi.it
³ INAF/Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Department of Physics, Faculty of Science and Engineering, Konan University, 8-9-1 Okamoto, Kobe, Hyogo 658-8501, Japan
⁵ Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, VIC 3800, Australia

Received: 7 November 2019
Accepted: 13 December 2019

Abstract

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.