The Chemical Evolution of R-process Elements in Stars (CERES) project is diving deep into the mysteries of how heavy elements formed in the early galaxy. This third installment by Lombardo and collaborators focuses on neutron-capture elements, particularly from barium (Ba) to europium (Eu). These elements, forged in environments like neutron star mergers and supernovae, are critical to understanding the evolution of the cosmos. The team's work centers on a group of 52 ancient, metal-poor stars—stars that formed in the universe's infancy when heavy element production followed different processes compared to today.

The Stellar Sample and Tools of the Trade

The CERES project utilized high-resolution spectra from the Very Large Telescope's UVES instrument to analyze these stars. Metal-poor stars ([Fe/H] < -1.5), which formed before the galaxy was enriched with heavy elements, were chosen for their potential to preserve the chemical fingerprints of ancient nucleosynthesis. The team applied rigorous methods, using detailed models of stellar atmospheres, to determine the abundances of barium (Ba), lanthanum (La), cerium (Ce), and other heavy elements. By comparing observed spectra with synthetic models, they could pinpoint how much of each element was present.

Findings: Heavy Elements in Early Stars

The team found intriguing patterns in how elements like Ba and Eu vary with metallicity. At very low metallicities ([Fe/H] < -2.4), the r-process—a rapid neutron-capture process occurring in explosive environments—appears to dominate. For slightly higher metallicities, elements produced by the slower s-process, likely from asymptotic giant branch (AGB) stars, begin to contribute. However, the timeline fits the theory that early galaxies had not yet hosted many of these long-lived stars.

The study also revealed significant scatter in the abundance ratios of heavy elements, such as [Ba/Fe], at lower metallicities. This variability suggests that these stars were born in gas clouds influenced by diverse nucleosynthesis events, each with unique contributions from r- and s-processes.

Decoding the r-Process Signature

When comparing stars with a pure r-process signature to those with mixed contributions, Lombardo’s team found no significant differences in their environments of formation. This hints that both types of stars could form under similar conditions, despite differences in their elemental make-up. Moreover, corrections for effects like non-local thermodynamic equilibrium (NLTE) confirmed that the r-process dominated at early times.

The Broader Picture: Galactic Trends and Beyond

The observed trends in this study align well with theoretical predictions from Galactic chemical evolution (GCE) models, which simulate how the galaxy's elemental composition changes over time. These models suggest that events like neutron star mergers and magneto-rotational supernovae were pivotal for early r-process production. The team also explored lighter neutron-capture elements like strontium (Sr) and compared their production mechanisms, highlighting the complex interplay of astrophysical processes that enriched the young galaxy.

Why This Matters

Understanding how stars like these ancient giants formed heavy elements not only tells us about the early galaxy but also about the origins of the elements we see in our Solar System. The CERES project demonstrates the power of homogeneous data analysis in unveiling cosmic history and sets the stage for future studies to refine our models of the universe's earliest times.

Source: Lombardo