Unearthing Ancient Stars: The Discovery of Two Metal-Poor R-Process Enriched Stars
Some of the oldest stars in our Milky Way serve as cosmic fossils, holding clues about how the elements in the universe were forged billions of years ago. In particular, metal-poor stars enriched in neutron-capture elements provide unique insights into the elusive r-process—a nuclear reaction responsible for forming some of the heaviest elements in the periodic table, like gold and uranium. In this study, astronomers A. R. da Silva and R. Smiljanic investigate two newly identified stars with strong r-process enrichment, shedding light on their origins and what they reveal about the history of our galaxy.
Methods: A Deep Dive into Stellar Chemistry
The two stars in focus, BPS CS 29529-0089 and TYC 9219-2422-1, were initially flagged as unusual in the GALAH survey, a large-scale project that catalogs chemical abundances in stars. To analyze them in greater detail, the researchers observed them using the Ultraviolet and Visual Echelle Spectrograph (UVES) at the Very Large Telescope (VLT). The study involved determining the stars’ temperatures, surface gravities, and precise chemical compositions. By examining the light absorbed by various elements in the stars' atmospheres, they measured the abundances of 44 different elements, including 27 that were created through neutron-capture processes.
Results: The Curious Case of Two R-Process Stars
The chemical compositions of these two stars suggest they originated in very different parts of the galaxy. BPS CS 29529-0089 is classified as an r-II star, meaning it has an extreme excess of europium ([Eu/Fe] = +1.79). Surprisingly, its motion suggests it is part of the thick disk of the Milky Way, making it one of the most metal-poor r-process enhanced stars found in this region. Unlike most r-II stars, it is not rich in carbon, setting it apart from similar discoveries.
On the other hand, TYC 9219-2422-1 is classified as an r-I star, with moderate r-process enrichment ([Eu/Fe] = +0.54). The way it moves through space suggests it was once part of the Gaia-Sausage-Enceladus, a dwarf galaxy that merged with the Milky Way billions of years ago. This confirms that some r-process stars originated in smaller galaxies before being absorbed into the Milky Way’s halo.
Discussion: How Were These Stars Formed?
One of the key mysteries in astrophysics is identifying the source of r-process elements. Scientists have long debated whether they come from neutron star mergers, supernovae, or even exotic objects like collapsars (massive stars that collapse directly into black holes). To explore this, the authors compared the stars' chemical abundances to theoretical models of nucleosynthesis. Their findings suggest that both stars could have been enriched by neutron star mergers occurring over 13 billion years ago. Interestingly, their chemical fingerprints also match what would be expected from stars that formed from gas enriched by a single Population III supernova—one of the first stars in the universe.
Conclusion: Unlocking the Secrets of Galactic Evolution
The discovery of these two stars adds valuable pieces to the puzzle of how heavy elements formed and spread across the universe. BPS CS 29529-0089 challenges existing ideas about where highly r-process enriched stars can be found. If it truly formed in the proto-disk of the Milky Way, it could mean that some of the first r-process events happened within our galaxy rather than in small external systems. Meanwhile, TYC 9219-2422-1 serves as a classic example of how the Milky Way grew by merging with smaller galaxies. Together, these stars help astronomers refine their understanding of the cosmic origins of the elements that shape our universe today.
Source: da Silva
