High-resolution spectroscopic observations of a hundred metal-poor Carbon and s -rich stars ( CEMP- s ) collected from the literature are compared with the theoretical nucleosynthesis models of asymptotic giant branch ( AGB ) presented in Paper I ( M ^ { AGB } _ { ini } = 1.3 , 1.4 , 1.5 , 2 M _ { \odot } , - 3.6 \la [ Fe/H ] \la - 1.5 ) .
The s -process enhancement detected in these objects is associated to binary systems : the more massive companion evolved faster through the thermally pulsing AGB phase ( TP-AGB ) , synthesising in the inner He-intershell the s -elements , 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 mass transfer of C and s -rich material from the primary AGB . We analyse the light elements as 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 ] \ga 1.5 ( CEMP- s II ) , and mild s -process enhanced stars , [ hs/Fe ] < 1.5 ( CEMP- s I ) .
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 ] ( \sim 2 dex ) .
A degeneration of solutions is found with some exceptions : most main-sequence CEMP- s II stars with low [ Na/Fe ] can only be interpreted with M ^ { AGB } _ { ini } = 1.3 – 1.4 M _ { \odot } .
Giants having suffered the first dredge-up ( FDU ) need a dilution \ga 1 dex ( dil is defined as the mass of the convective envelope of the observed star , M ^ { obs } _ { \star } , over the material transferred from the AGB to the companion , M ^ { trans } _ { AGB } ) .
Then , AGB models with higher AGB initial masses ( M ^ { AGB } _ { ini } = 1.5 – 2 M _ { \odot } ) are adopted to interpret CEMP- s II giants .
In general , solutions with AGB models in the mass range M ^ { AGB } _ { ini } = 1.3 – 2 M _ { \odot } and different dilution factors are found for CEMP- s I 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 % , 70 % ) have to be considered .
This increases the maximum [ hs/ls ] up to \sim 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 .