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

par Sitnova, T. M.;Mashonkina, L.~I.;Romanovskaya, A. M.;Escate Giribaldi, Riano Isidoro ;Choplin, Arthur
Référence Astronomy & astrophysics, 704, page (A103)
Publication Publié, 2025-12-01

Article révisé par les pairs

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Résumé :

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.