Barium Clues: Unraveling the Origins of Carbon-Enhanced Ancient Stars

T. M. Sitnova and collaborators explore a rare group of stars known as carbon-enhanced metal-poor (CEMP) stars, ancient stars in our Galaxy’s halo that contain unusually high levels of carbon. Within this family, two subclasses stand out: CEMP-s and CEMP-rs stars. Both show enrichments of heavy elements formed by neutron-capture processes, but they differ in how these elements were made. The “s” refers to the slow neutron-capture process (the s-process) that occurs in aging stars called asymptotic giant branch (AGB) stars, while “r” stands for the rapid neutron-capture process (the r-process) linked to violent cosmic events like neutron star mergers. The “rs” type, which seems to show a mix of both, has long puzzled astronomers.

Sitnova and colleagues aim to clarify this mystery by studying the ratios of barium (Ba) isotopes in ten CEMP stars. Because different nuclear processes produce distinct proportions of odd and even barium isotopes, these ratios provide a direct clue to which process dominated the star’s chemical history.

Observations and Stellar Parameters

The team selected ten CEMP stars, combining data from the European Southern Observatory and the Keck Observatory. These stars were chosen for having clear and measurable spectral lines of singly ionized barium (Ba II). The authors focused on dwarf and subgiant stars, hotter and less evolved stars, to avoid complications caused by strong molecular carbon bands that can obscure the spectral features.

Using color information from Gaia and other catalogs, Sitnova et al. determined each star’s temperature, surface gravity, and metal content. These parameters are essential for accurate modeling of stellar atmospheres and spectral lines. The team ensured their data met strict quality standards, requiring a spectral resolution above 30,000 and a signal-to-noise ratio above 30, to detect subtle isotope effects in the barium lines.

Methods and Abundance Analysis

The researchers analyzed several barium lines, particularly the resonance lines at 4554 Å and 4934 Å, which are sensitive to the isotopic composition, and the subordinate lines, which are largely unaffected by it. By comparing how much barium abundance was needed to reproduce these two types of lines, the team could estimate the fraction of odd isotopes, known as Fₒdd.

To perform these calculations, Sitnova’s group used advanced non-local thermodynamic equilibrium (NLTE) models, which simulate how atoms absorb and emit light more accurately than simpler models. They also determined europium (Eu) abundances, since the [Ba/Eu] ratio, the ratio of barium to europium, helps classify stars as CEMP-s or CEMP-rs.

Results

The analysis revealed that CEMP-s and CEMP-rs stars show distinct barium isotope ratios. The CEMP-s stars had low values of Fₒdd, between about 0.05 and 0.19, consistent with barium created mainly by the s-process. In contrast, the CEMP-rs stars showed higher Fₒdd values ranging from 0.34 to 0.57, which are incompatible with a pure s-process origin.

Furthermore, the Fₒdd values in the CEMP-rs stars could not be reproduced by mixing s-process and r-process material. Instead, they align with predictions from the intermediate neutron-capture process, or i-process, a rarer and less-understood mechanism that may occur in certain AGB stars or white dwarfs.

Discussion

Sitnova and her team compared the derived isotope ratios with theoretical expectations from all three processes. In pure s-process material, Fₒdd ≈ 0.10; in pure r-process material, Fₒdd ≈ 0.75; and the i-process predicts intermediate values near 0.6–0.8. The stars classified as CEMP-rs clearly fell closer to the i-process range, showing that their barium isotopes cannot be explained by a simple mixture of s- and r-process contributions.

This finding suggests that the i-process may play a crucial role in enriching some of the oldest stars in the Galaxy. It bridges the gap between the s- and r-processes, occurring under conditions where neutron densities are intermediate, high enough to drive rapid reactions but not as extreme as in explosive environments.

Conclusions

Sitnova et al. provide the first clear observational evidence that CEMP-rs stars bear the signature of the i-process. Their isotope ratios differ markedly from those of CEMP-s stars, revealing two distinct pathways for heavy-element formation in the early Galaxy. By measuring the faint imprints of odd barium isotopes in ancient starlight, the study opens a new window into how the Universe’s first heavy elements were forged.

Source: Sitnova