Tracing Cosmic Origins: Europium in the Small Magellanic Cloud

The study by S. Anoardo et al. explores how heavy elements are produced in one of our neighboring dwarf galaxies, the Small Magellanic Cloud (SMC). Focusing on europium (Eu), an element formed almost entirely by the rapid neutron-capture process (r-process), the authors use observations of 209 giant stars to study how this process unfolded across the SMC’s history. Their work offers the most extensive dataset of Eu abundances in the SMC to date and reveals how the galaxy’s low star formation efficiency shaped its chemical makeup compared to larger systems like the Milky Way.

Understanding r-Process Enrichment

The r-process occurs in environments where atomic nuclei rapidly capture neutrons, producing many of the universe’s heaviest elements, such as gold and europium. Its astrophysical sources include two main categories: prompt sources, like certain rare and powerful supernovae, and delayed sources, such as neutron star mergers that occur long after stars form. Because europium is almost entirely produced by the r-process, its abundance in a galaxy reveals how efficiently these sources enriched its gas over time. Anoardo and colleagues note that, unlike the Milky Way, the SMC has had fewer massive stars and slower star formation, making it an ideal testbed to understand how these enrichment processes vary across different galactic environments.

Observations and Measurements

The team used the FLAMES spectrograph on the Very Large Telescope to gather spectra from three different fields across the SMC, each centered on a globular cluster. From these spectra, they measured the strength of light absorption lines associated with europium (Eu II at 6645 Å), barium (Ba II at 6496.9 Å), and iron (Fe), among others. Combining new data with earlier studies, they derived the ratios [Eu/Fe], [Ba/Fe], and [Mg/Fe], key tracers of the r-, s-, and α-processes, respectively. These measurements were made using a customized fitting code that compared observed stellar spectra to theoretical models. The final dataset spans over 1.5 dex in metallicity, allowing the authors to trace chemical trends from the SMC’s earliest epochs to more recent times.

Chemical Trends in the SMC

Across the full metallicity range, SMC stars show enhanced europium relative to iron, with [Eu/Fe] values similar to those in Milky Way stars but remaining higher at low metallicities. The abundance declines with increasing [Fe/H], consistent with the introduction of iron from Type Ia supernovae, which produce little to no europium. Interestingly, the ratio [Eu/Mg], which compares r-process and α-element production, stays nearly constant at about +0.5 dex across all metallicities. This is significantly above the solar value, meaning the SMC produces more europium per unit of magnesium than the Milky Way does. Meanwhile, [Ba/Eu], which tracks the contribution of the slow neutron-capture (s-) process, increases with metallicity, reflecting a growing role of asymptotic giant branch (AGB) stars at later times.

Interpreting the Results

The observed scatter in [Eu/Fe] among the most metal-poor SMC stars suggests that the earliest r-process events were rare and unevenly mixed into the galaxy’s gas. As metallicity increased, the abundance ratios stabilized, pointing to more uniform enrichment. Comparing the SMC to the Large Magellanic Cloud (LMC) and Milky Way, the authors find that both Magellanic Clouds share similarly high [Eu/Mg] values, indicating more efficient r-process enrichment in dwarf galaxies. The differences between these systems likely arise from their distinct star formation histories: galaxies with slower, less efficient star formation retain signatures of delayed r-process sources more clearly than galaxies like the Milky Way, where chemical mixing proceeds faster.

Broader Implications and Conclusions

Anoardo and collaborators conclude that the SMC’s enhanced europium-to-magnesium ratio is a hallmark of its low star formation efficiency and extended chemical evolution. These findings reinforce the idea that [Eu/α] ratios can distinguish stars formed within the Milky Way from those that originated in smaller galaxies later accreted by it. Moreover, the consistent patterns across multiple dwarf systems imply that r-process enrichment from neutron star mergers or other delayed events plays a major role at low metallicities. The team’s results provide a new cornerstone for models of chemical evolution and help illuminate how the Milky Way built up its heavy elements through contributions from its galactic neighbors.

Source: Anoardo