The chemical DNA of the Magellanic Clouds IV. Unveiling extreme element production: the Eu abundance in the Small Magellanic Cloud

TL;DR

This work measures Eu abundances in 209 SMC giants using high-resolution spectroscopy to quantify $r$-process enrichment relative to $α$-capture and the $s$-process. By comparing $[\mathrm{Eu/Fe}]$, $[\mathrm{Eu/Mg}]$, and $[\mathrm{Ba/Eu}]$ across a broad $[\mathrm{Fe/H}]$ range, the authors find supersolar and mostly flat $[\mathrm{Eu/Fe}]$ with metallicity, a persistent $[\mathrm{Eu/Mg}]$ offset of ~+0.5 dex, and a metallicity-driven rise in $[\mathrm{Ba/Eu}]$, indicating early, efficient Eu production and delayed s-process contribution in the SMC. The results, consistent with similar Local Group dwarfs, imply higher Eu relative to $α$-elements than the MW, suggesting different star-formation histories and r-process source efficiencies (potentially including delayed sources) in low-SF environments. Collectively, these findings enhance our understanding of chemical evolution in dwarf galaxies and inform models of MW assembly and Galactic archaeology.

Abstract

In this study we investigate the chemical enrichment of the rapid neutron-capture process in the Small Magellanic Cloud (SMC). We measure [Eu/Fe] abundance ratios in 209 giant stars that are confirmed members of the SMC, providing the first extensive dataset of Eu abundances in this galaxy across its full metallicity range, spanning more than 1.5 dex. We compare Eu abundances with those of Mg and Ba to evaluate the efficiency of the r-process relative to $α$-capture and s-process nucleosynthesis. The SMC shows enhanced [Eu/Fe] values at all metallicities (comparable with the values measured in the Milky Way), with a clear decline as [Fe/H] increases (from $\sim$ -1.75 dex to $\sim$ -0.5 dex), consistent with the onset of Type Ia supernovae. In contrast, [Eu/Mg] is enhanced by about +0.5 dex at all [Fe/H], significantly above the values observed in Milky Way stars, where [Eu/Mg] remains close to solar, reflecting comparable production of r-process and $α$-capture elements. Moreover, [Ba/Eu] increases with metallicity, beginning at [Fe/H] $\approx$ -1.5 dex, namely at a lower metallicity with respect to the Milky Way, where [Ba/Eu] starts to increase around [Fe/H] $\approx$ -1 dex. Our findings suggest the SMC has a higher production of Eu (with respect to the $α$-elements) than the Milky Way but in line with what observed in other dwarf systems within the Local Group. We confirm that galaxies with star formation efficiencies lower than the Milky Way have high [Eu/$α$], probably indicating a stronger efficiency of the delayed sources of r-process at low metallicities.

Figures (7)

• Figure 1: Difference between the RVs derived in this study and those in Paper I as a function of the Gaia DR3 G magnitude. The red circles are stars with discrepant RV differences after a 3$\sigma$-clipping iterative procedure.
• Figure 2: Abundance ratios for the analysed SMC stars: [Eu/Fe] (top panel) and [Ba/Fe] (bottom panel) as a function of [Fe/H]. The horizontal dashed lines mark the solar value.
• Figure 3: Comparison between VLT-FLAMES spectra of three stars having similar metallicity and stellar parameters but different [Eu/Fe] abundances: 4685582751830393088 ($\rm T_{eff}=3881 \ K$ and log g = 0.50, in green), 4687249169089083136 ($\rm T_{eff}=4054 \ K$ and log g = 0.45, in red) and 4684827460356570624 ($\rm T_{eff}=4087 \ K$ and log g = 0.61, in blue). The arrows mark the position of the Eu II line at 6645 Å analysed in this work and of the adjacent absorption features.
• Figure 4: [Eu/Mg] as a function of [Fe/H] for the spectroscopic sample discussed in this study.
• Figure 5: [Ba/Eu] as a function of [Fe/H] for the spectroscopic sample discussed in this study. The black dotted line in the bottom panel marks [Ba/Eu]=--0.69 dex, indicative of pure r-process content in the stars arla.