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Register a new userNeutron-capture elements in dwarf galaxies II: Challenges for the s- and i-processes at low metallicity
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\\'Asa Sk\\'ulad\\'ottir Dr.
Publication date
2019
fields
Physics
and research's language is
English
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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 $-2.4<text{[Fe/H]}<-0.9$, in the Sculptor dwarf spheroidal galaxy. The chemical enrichment from AGB stars becomes apparent at $text{[Fe/H]}approx-2$ in Sculptor, and causes [Y/Ba], [La/Ba], [Nd/Ba] and [Eu/Ba] to decrease with metallicity, reaching subsolar values at the highest $text{[Fe/H]}approx-1$. 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 $text{[Sr, Y, Zr]}>+0.7$ 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 $text{[Y/Ba]}_{s/i}=-0.85pm0.16$, $text{[La/Ba]}_{s/i}=-0.49pm0.17$, and $text{[Nd/Ba]}_{s/i}=-0.48pm0.12$, 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 $text{[Y/Ba]}_{s/i}$ and $text{[La/Ba]}_{s/i}$ 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.
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We present high-resolution of spectroscopy of four stars in two candidate ultra-faint dwarf galaxies (UFDs) Grus I (Gru I) and Triangulum II (Tri II). Neither object currently has a clearly determined velocity dispersion, placing them in an ambiguous region of parameter space between dwarf galaxies and globular clusters. No significant metallicity difference is found for the two Gru I stars, but both stars are deficient in neutron-capture elements. We verify previous results that Tri II displays significant spreads in metallicity and [$alpha$/Fe]. Neutron-capture elements are not detected in our Tri II data, but we place upper limits at the lower envelope of Galactic halo stars, consistent with previous very low detections. Stars with similarly low neutron-capture element abundances are common in UFDs, but rare in other environments. This signature of low neutron-capture element abundances traces chemical enrichment in the least massive star-forming dark matter halos, and further shows that the dominant sources of neutron-capture elements in metal-poor stars are rare. In contrast, all known globular clusters have similar ratios of neutron-capture elements to those of halo stars, suggesting that globular clusters form as part of relatively massive galaxies rather than in their own dark matter halos. The low neutron-capture element abundances may be the strongest evidence that Gru I and Tri II are (or once were) galaxies rather than globular clusters, and we expect future observations of these systems to robustly find non-zero velocity dispersions or signs of tidal disruption. However, the nucleosynthetic origin of this low neutron-capture element floor remains unknown.
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