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

Abstract

We present a spectroscopic analysis of ten carbon enhanced metal-poor (CEMP) stars of type CEMP-s and CEMP-rs and determine their NLTE abundances of Ba and Eu, as well as the fractions of the odd Ba isotopes (F_odd). The Ba abundances inferred from the resonance Ba II 4554 and 4934 A lines depend on the adopted Ba isotope mixture. We perform calculations for different F_odd from 0.1 to 1.0 and determine the corresponding abundances from the Ba II resonance lines in each sample star. In addition, we determine the Ba abundances from the Ba II subordinate lines, which are almost independent of F_odd. We then compare the Ba abundances derived from the subordinate lines with those from the Ba II resonance lines. 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, in three of four stars, the possibility of Ba isotopes origin in a pure r-process with F_odd = 0.75 can be excluded. 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 compared to 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. Our analysis of CEMP-rs stars with [Ba/Eu] > 0 argues 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 is responsible for the chemical composition of these CEMP-rs stars.