In a recent study published in the journal Astronomy & Astrophysics, a team of researchers led by Lorenzo Santarelli from the University of Bologna, Italy, has shed light on the chemical composition of some of the oldest stars in the Small Magellanic Cloud (SMC), a dwarf galaxy near our own Milky Way. The team also includes Marco Palla, Alessio Mucciarelli, Lorenzo Monaco, Deimer Antonio Alvarez Garay, Donatella Romano, and Carmela Lardo, all affiliated with various Italian research institutions.
The researchers analyzed the chemical abundances of iron, alpha-elements, and neutron-capture elements in 12 metal-poor giant stars in the SMC. These stars, with iron abundances ranging from -2.3 to -1.4 dex, are among the oldest known in the SMC, having formed within the first billion years of the galaxy’s life. The team used high-resolution spectrographs UVES/VLT and MIKE/Magellan to gather their data.
The study found that these ancient stars exhibit enhanced alpha-element abundances, but not as high as those seen in metal-poor stars in the Milky Way. This is consistent with the slower star formation rate of the SMC. More notably, the researchers observed a significant star-to-star scatter in the abundances of neutron-capture elements, which are created through processes like the rapid neutron-capture process (r-process) and the slow neutron-capture process (s-process).
The r-process elements europium (Eu) and samarium (Sm) showed abundance ratios ranging from solar to +1 dex, with three stars classified as r-II stars due to their high europium abundances. This distribution suggests that the r-process in the SMC can be highly efficient but is still influenced by the stochastic nature of its production sites and the inefficient gas mixing in the early SMC.
Interestingly, the stars richest in europium also showed high abundances of s-process elements like yttrium (Y), barium (Ba), lanthanum (La), cerium (Ce), and neodymium (Nd). All stars in the sample exhibited subsolar s-process to europium abundance ratios. The researchers attribute this to the fact that, at the metallicities of these stars, the production of neutron-capture elements is primarily driven by the r-process, as low-mass asymptotic giant branch (AGB) stars—which are responsible for the s-process—had not yet evolved and left their signature in the interstellar medium.
The team also presented stochastic chemical evolution models tailored for the SMC, which confirmed their scenario. These findings provide valuable insights into the early chemical evolution of dwarf galaxies and the processes that drive the production of heavy elements.
For the energy sector, understanding the origins and distribution of neutron-capture elements like those studied here can have practical applications. For instance, neodymium and samarium are used in the production of powerful magnets for wind turbines and other renewable energy technologies. Barium is used in drilling fluids for oil and gas exploration, while europium is used in the production of fluorescent lamps and other lighting technologies. A deeper understanding of the processes that create these elements can help inform the search for new deposits and improve the sustainability of these industries.
This article is based on research available at arXiv.