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

TL;DR

The paper investigates whether Ba isotope ratios, quantified by the odd-isotope fraction $F_{ m odd}$, can distinguish s-, r-, and i-process nucleosynthesis in CEMP-s and CEMP-rs stars. Using NLTE analyses of Ba II resonance lines alongside Ba II subordinate lines, the authors derive Ba and Eu abundances for ten stars, and infer $F_{ m odd}$ by matching abundances from lines with different sensitivities to isotopic composition. They find that CEMP-s stars cluster around $F_{ m odd} oughly 0.05$–$0.19$ (consistent with s-process values near 0.10 and solar 0.18), while CEMP-rs stars show higher $F_{ m odd}$ in the range 0.34–0.57, often aligning with i-process predictions of $F_{ m odd} o 0.6$–0.8 and inconsistent with a pure r- or pure s-process origin. The results argue that i-process nucleosynthesis is responsible for the Ba isotopic composition in most CEMP-rs stars, providing a crucial isotopic constraint on early Galactic chemical enrichment and the nature of neutron-capture processes in binary AGB mass transfer scenarios.

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.

Figures (4)

• Figure 1: NLTE abundance corrections for the Ba ii lines as a function of EWs. See legend for lines designation.
• Figure 2: Differences between NLTE abundances from individual lines computed with F$_{\rm odd}$ = 0.10 (s-process) and 0.75 (r-process) in the sample stars known in the literature as CEMP-s (circles), CEMP-rs (squares), and with unclear classification (triangles). For comparison, we show data for normal VMP stars from 2025AA...699A.262S. Open and filled symbols correspond to the Ba ii 4554 Å and 4934 Å lines, respectively.
• Figure 3: Differences between NLTE abundances from the subordinate and the resonance Ba ii lines computed with F$_{\rm odd}$ = 0.10 (s-process, filled symbols) and 0.75 (r-process, open symbols) in the sample stars known in the literature as CEMP-s (circles), CEMP-rs (squares), and with unclear classification (triangles).
• Figure 4: F$_{\rm odd}$ as a function of [Ba/Eu] in the sample stars. Designations are the same as in Fig. \ref{['ba_sub_res']}. For comparison, we show theoretical predictions for r-process (dashed line), s-process (dashdotted line), s- and r-process material mixture (dotted line), and i-process (diamonds). Filled diamonds correspond to i-process models with [Fe/H] and [Ba/Fe] that fall in the parameter range of our sample stars. Blue error bar indicates i-process nuclear uncertainties associated to 1 m$_{\rm \odot}$ model with [Fe/H] = $-2.5$.