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

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 (Fodd). The determination of Fodd 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 <ns0:sc>II</ns0:sc> 4554 and 4934 Å lines depend on the adopted Ba isotope mixture. We took different Fodd from 0.1 to 1.0 and determined the corresponding abundances from the Ba <ns0:sc>II</ns0:sc> resonance lines in each sample star. In addition, we determined the Ba abundances from the Ba <ns0:sc>II</ns0:sc> subordinate lines, which are almost independent of Fodd. We then compared the Ba abundances derived from the subordinate lines with those from the Ba <ns0:sc>II</ns0:sc> resonance lines. Results. We found different Fodd values in CEMP-s and CEMP-rs stars. CEMP-s stars exhibit Fodd = 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 Fodd = 0.10 and the solar Fodd = 0.18. Although the uncertainties are large, the possibility of a Ba isotope origin in a pure r-process with Fodd = 0.75 can be excluded for three of the four stars. CEMP-rs stars show Fodd = 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 Fodd = 0.6 to 0.8. Conclusions. Our analysis of CEMP-rs stars with [Ba/Eu] > 0 demonstrates that their [Ba/Eu] and Fodd 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.