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.
Highlights
Three main neutron (n) capture processes are known to occur in environments characterized by different n-densities: the slow (s), the intermediate (i) and the rapid (r) process.The final chemical pattern created by each process depends on the n-density and the overall physical conditions at the production site (e.g., Burbidge et al 1957; Cameron 1957; Sneden et al.2008).A generally accepted nucleosynthetic site for the main s-process are low- to intermediate-mass stars (m in the asymptotic giant branch (AGB) phase (e.g., Karakas &Lattanzio 2014)
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 slow chemical enrichment of Sculptor, and other Milky Way satellite dwarf galaxies, provides a unique opportunity to investigate the cumulative contribution of metal-poor AGB stars, which otherwise can only be studied via their mass transfer onto a binary companion
Summary
Three main neutron (n) capture processes are known to occur in environments characterized by different n-densities (low to high): the slow (s), the intermediate (i) and the rapid (r) process. The nucleosynthetic site of this process, the “Lighter-Element Primary Process” (LEPP), known as “weak” or “limited” rprocess, is unconfirmed This weak r-process might for example occur in neutrino-driven winds from ccSN (Wanajo et al 2001, 2011; Qian & Wasserburg 2003; Montes et al 2007; Arcones & Montes 2011; Hansen et al 2014; Hirai et al 2019). The slow chemical enrichment of Sculptor, and other Milky Way satellite dwarf galaxies, provides a unique opportunity to investigate the cumulative contribution of metal-poor AGB stars, which otherwise can only be studied via their mass transfer onto a binary companion
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