tokens: and carbon-normal metal-poor stars (red crosses and blue dots, respectively). Because our model is computed for , we restrict our observed sample to stars with iron abundance within a factor of five from the model value, . This selection leaves us with CEMP, CEMP-, and CEMP- stars, that is more than of our entire carbon-rich sample. The theoretical and observed abundances differ in many aspects. 1. Observed carbon-rich and carbon-normal stars appear as two distinct and clearly separated populations. We compute the linear regression of the observed europium and barium abundances weighted with the observational errors, and we determine the following equations for the ratio of barium to europium in carbon-normal and CEMP- stars, respectively, represented in Fig. 2 with blue-dashed and magenta-solid lines. If we compute the linear fit for CEMP- stars with and CEMP- stars separately we find almost identical results as in Eq. ( 2 ). Equations ( 1 ) and ( 2 ) show that the empirical relation between [Eu/Fe] and [Ba/Fe] has almost the same slope in the two classes of stars, and CEMP- stars have [Ba/Fe] on average dex higher than carbon-normal stars at the same [Eu/Fe]. The relations described by Eqs. ( 1 ) and ( 2 ) are likely consequences of the barium-to-europium ratios produced in the - and in the -process, respectively. In contrast, in our simulation there is not a clear separation between the two groups (colour distributions in Fig. 2 ). Because - and -enrichments are independent, the initial [Eu/Fe] and [Ba/Fe] follow approximately the relation described by Eq. ( 1 ), whereas the final abundances depend on the amount of material transferred from the AGB primary stars onto their secondary companions. Higher accreted masses cause stronger abundance enhancements of carbon and barium. Because AGB stars generally do not produce high europium abundances, [Eu/Fe] is essentially determined by its initial value, except for stars with initial [Eu/Fe] less than about . Consequently, for each [Eu/Fe], which is given by the initial -enrichment, we find stars with all barium abundances up to two dex higher than predicted by a pure -process, and the correlation between [Eu/Fe] and [Ba/Fe] described by Eq. ( 2 ) is completely washed out. Lugaro et al. (2009 , 2012) and Abate et al. (2015c) criticise this scenario of independent - and -process contributions with analogous arguments. Fig. 2 [Ba/Fe] vs. [Eu/Fe] in carbon-normal and carbon-rich metal-poor stars (blue dots and red crosses, respectively). Stars enriched in -elements occupy the region above the horizontal and diagonal solid black lines, corresponding to zones A and B of the plot. According to our adopted definitions, CEMP- stars are in zone B. In zone C there are stars with , , and . CEMP- stars and carbon-normal stars with and are in zone D. The dashed-blue and solid-magenta lines are linear fits of the carbon-normal stars and CEMP- stars, respectively. The red and blue distributions represent the
ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O