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2011MNRAS.418..284B - Mon. Not. R. Astron. Soc., 418, 284-319 (2011/November-3)

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


Abstract (from CDS):

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 = is the the light-s peak at N = 50 and hs = 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{Ux2605}, 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.

Abstract Copyright: 2011 The Authors Monthly Notices of the Royal Astronomical Society2011 RAS

Journal keyword(s): nuclear reactions, nucleosynthesis, abundances - stars: AGB and post-AGB - stars: carbon - stars: Population II

VizieR on-line data: <Available at CDS (J/MNRAS/418/284): table2.dat table3.dat tablea1.dat tablea2.dat refs.dat>

Simbad objects: 115

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