The chemical DNA of the Magellanic Clouds V. R-process dominates neutron capture elements production in the oldest SMC stars

TL;DR

This study of 12 metal-poor Small Magellanic Cloud giants reveals that neutron-capture elements in the oldest SMC stars are dominated by the $r$-process, evidenced by large star-to-star scatter in Eu and Sm and the presence of $r$-II stars. High-resolution UVES/VLT and MIKE/Magellan spectra enable precise measurements of Fe, $\alpha$-elements, and multiple neutron-capture species, with NLTE corrections applied where relevant. A stochastic chemical evolution model incorporating MRD-SNe and NSM sources reproduces the observed Eu/Fe spread, requiring a relatively high NSM fraction to match the upper envelope, and predicts minimal early $s$-process signatures due to delayed AGB contributions. Taken together, the results indicate an early, inhomogeneous chemical enrichment in the SMC with $r$-process–driven production dominating neutron-capture elements at $[Fe/H]$ below about $-1.8$, consistent with the SMC’s slow star formation history and gas mixing.

Abstract

We present the chemical abundances of Fe, alpha- and neutron-capture elements in 12 metal-poor Small Magellanic Cloud (SMC) giant stars, observed with the high-resolution spectrographs UVES/VLT and MIKE/Magellan. These stars have [Fe/H] between -2.3 and -1.4 dex, 10 of them with [Fe/H]<-1.8 dex. According to theoretical age-metallicity relations for this galaxy, these stars formed in the first Gyr of life of the SMC and represent the oldest SMC stars known so far. [alpha/Fe] abundance ratios are enhanced but at a lower level than MW metal-poor stars, as expected according to the slow star formation rate of the SMC. The sample exhibits a large star-to-star scatter in all the neutron-capture elements. The two r-process elements measured in this work (Eu and Sm) have abundance ratios from solar up to +1 dex, three of them with [Eu/Fe]>+0.7 dex and labeled as r-II stars. This [r/Fe] distribution indicates that the r-process in the SMC can be extremely efficient but is still largely affected by the stochastic nature of the main sites of production and the inefficient gas mixing in the early SMC evolution. A similar scatter is observable also for the s-process elements (Y, Ba, La, Ce, Nd), with the stars richest in Eu also being rich in these s-elements. Also, all the stars exhibit subsolar [s/Eu] abundance ratios. At the metallicities of these stars, the production of neutron-capture elements is driven by r-process, because the low-mass AGB stars have not yet evolved and left their s-process signature in the interstellar medium. We also present stochastic chemical evolution models tailored for the SMC that confirm this scenario.

Figures (16)

• Figure 1: Comparison between UVES-3 and MIKE-2 spectra after reduction, normalisation, radial velocity and heliocentric corrections. The two stars share similar atmospheric parameters and iron abundance. Black ticks mark the position of observable lines.
• Figure 2: Upper panel: theoretical AMR of the SMC Pagel98, superimposed with the SMC GCs (Paper II, green circles). The white dots mark the position on the theoretical AMR of ages 11, 12 and 13 Gyr. Lower panel: average Si and Ca abundances as a function of [Fe/H] for SMC stars of this work observed with UVES (red squares) and MIKE (red triangles). The blue shaded area highlights the [Fe/H] region of the stars of our sample with $-2.2<$[Fe/H]$<-1.8$ dex.
• Figure 3: $\alpha$-elements patterns: [O/Fe], [Mg/Fe], [Si/Fe], [Ca/Fe] abundances as a function of [Fe/H] for SMC stars of this work observed with UVES (red squares) and MIKE (red triangles), together with the SMC field stars (Paper I, red small circles) and the SMC GCs (Paper II, green circles). Error bars are shown for UVES and MIKE abundances. As a reference, Gaussian KDE regressions of MW abundances built from SAGA database Suda08 (blue line) are reported, together with their 1$\sigma$ confidence interval (blue shaded area), in the background.
• Figure 4: [Eu/Fe] abundances as a function of [Fe/H]. Same symbols of Fig. \ref{['alpha']}, but the red small circles are SMC field stars observed with GIRAFFE from Paper IV.
• Figure 5: [Sm/Fe] abundances as a function of [Fe/H]. Same symbols of Fig. \ref{['alpha']}.