Distinct barium isotope ratios in CEMP-s and CEMP-rs stars^★

¹ Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya 48, 119017 Moscow, Russia
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226, 1050 Brussels, Belgium

^★★ Corresponding author: sitamih@gmail.com

Received: 29 August 2025
Accepted: 16 October 2025

Abstract

Aims. We present a spectroscopic analysis of ten carbon-enhanced metal-poor (CEMP) stars of type CEMP-s and CEMP-rs and determine their non-local thermodynamic equilibrium abundances of Ba and Eu, as well as the fractions of the odd Ba isotopes (F_odd). The determination of F_odd in stars provides unambiguous information about the s-process contribution to their total Ba abundance. We aim to obtain observational constraints on the enrichment scenarios of CEMP-rs stars.

Methods. The Ba abundances inferred from the resonance Ba II 4554 and 4934 Å lines depend on the adopted Ba isotope mixture. We took different F_odd from 0.1 to 1.0 and determined the corresponding abundances from the Ba II resonance lines in each sample star. In addition, we determined the Ba abundances from the Ba II subordinate lines, which are almost independent of F_odd. We then compared the Ba abundances derived from the subordinate lines with those from the Ba II resonance lines.

Results. We found different F_odd values in CEMP-s and CEMP-rs stars. CEMP-s stars exhibit F_odd = 0.05_−0.03^+0.07, 0.17_−0.14^+0.63, 0.19_−0.14^+0.50, and 0.19_−0.12^+0.33. The obtained values agree within the error bars with the s-process F_odd = 0.10 and the solar F_odd = 0.18. Although the uncertainties are large, the possibility of a Ba isotope origin in a pure r-process with F_odd = 0.75 can be excluded for three of the four stars. CEMP-rs stars show F_odd = 0.34_−0.21^+0.55, 0.36_−0.14^+0.23, 0.44_−0.22^+0.43, 0.53_−0.38^+0.47, and 0.57_−0.31^+0.43, which are higher than those in CEMP-s stars. Although the uncertainties are large, in four of five stars, the possibility of a pure s-process origin for the Ba isotopes can be excluded. The obtained values agree within the error bars with the predicted i-process F_odd = 0.6 to 0.8.

Conclusions. Our analysis of CEMP-rs stars with [Ba/Eu] > 0 demonstrates that their [Ba/Eu] and F_odd cannot be jointly explained by a mixture of material produced by the r- and s-processes. The obtained results argue that the i-process causes the chemical composition of these CEMP-rs stars.