The s-process in low-metallicity stars – II. Interpretation of high-resolution spectroscopic observations with asymptotic giant branch models


E-mail: (AVR); (ANO)


High-resolution spectroscopic observations of 100 metal-poor carbon and s-rich stars (CEMP-s) collected from the literature are compared with the theoretical nucleosynthesis models of the asymptotic giant branch (AGB) presented in Paper I (MAGBini= 1.3,  1.4,  1.5,  2  M, − 3.6 ≲ [ Fe/H ] ≲− 1.5). The s-process enhancement detected in these objects is associated with binary systems: the more massive companion evolved faster through the thermally pulsing AGB phase (TP-AGB), synthesizing s-elements in the inner He intershell, which are partly dredged up to the surface during the third dredge-up (TDU) episode. The secondary observed low-mass companion became CEMP-s by the mass transfer of C- and s-rich material from the primary AGB.

We analyse the light elements C, N, O, Na and Mg, as well as the two s-process indicators, [hs/ls] (where ls =〈Y, Zr〉 is the the light-s peak at N = 50 and hs =〈La, Nd, Sm〉 the heavy-s peak at N = 82) and [Pb/hs]. We distinguish between CEMP-s with high s-process enhancement, [hs/Fe] >rsim 1.5 (CEMP-sII), and mild s-process enhanced stars, [hs/Fe] < 1.5 (CEMP-sI). To interpret the observations, a range of s-process efficiencies at any given metallicity is necessary. This is confirmed by the high spread observed in [Pb/hs] (∼2 dex). A degeneration of solutions is found with some exceptions: most main-sequence CEMP-sII stars with low [Na/Fe] can only be interpreted with MAGBini= 1.3–1.4 M. Giants having suffered the first dredge-up (FDU) need a dilution >rsim1 dex (dil is defined as the mass of the convective envelope of the observed star, Mobs, over the material transferred from the AGB to the companion, MtransAGB). Then AGB models with higher AGB initial masses (MAGBini= 1.5–2 M) are adopted to interpret CEMP-sII giants. In general, solutions with AGB models in the mass range MAGBini= 1.3–2 M and different dilution factors are found for CEMP-sI stars.

About half of the CEMP-s stars with europium measurements show a high r-process enhancement (CEMP-s/r). The scenario for the origin of CEMP-s/r stars is a debated issue. We propose that the molecular cloud from which the binary system formed was previously enriched in r-process elements, most likely by local SN II pollution. This initial r-enrichment does not affect the s-process nucleosynthesis. However, for a high r-process enrichment ([r/Fe]ini= 2) the r-process contributions to solar La, Nd and Sm (30, 40 and 70 per cent) have to be considered. This increases the maximum [hs/ls] up to ∼0.3 dex. CEMP-s/r stars reflect this behaviour, showing higher [hs/ls] than observed in CEMP-s on average.

Detailed analyses for individual stars will be provided in Paper III.