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Astron. Astrophys. 363, 555-567 (2000)

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2. Observations

The abundance data of the heavy-elements, C and O elements of MS and S stars are taken from Smith & Lambert (1985, 1986, 1990). The listed values of [N/Fe] [FORMULA]) have typical errors 0.1-0.2 dex, and N represents a kind of s-process element. Indeed, for the observations in the series papers of Smith & Lambert, the accuracy of the iron peak elements (Ti, Fe and Ni) abundances and the heavy-elements Y, Zr, Nd abundances were better. But the abundance of Nd element is only at 50[FORMULA] of s-origin, it is not reasonable to regard Nd as the representative of heavier s-elements (Busso et al. 1992; Zhang et al. 1998a), and the reasonable representative should be the mean abundance of Ba, La, Nd and Sm elements (Busso et al. 1995). In order to do this we suggest that the initial abundance of Nd have to be increased by 0.11 dex before the data of MS and S giants are compared with carbon stars (Busso et al. 1995). Y and Zr elements are the representatives of lighter s-elements. For C and O elements, the accuracy is higher at 0.1 dex in Smith & Lambert (1990). Considering that, at present, the sample with both observed C/O ratios and heavy-element abundances is rather limited and there is a close relationship between the Tc-no MS, S stars (stars that contain no Tc) and the thermal pulse AGB stars (Busso et al. 1992; Jorissen & Mayor 1992), we can usefully include the Tc-no stars in our study of evolutionary tendency of the M[FORMULA]S[FORMULA]C sequence (Busso et al. 1992).

The abundance data of heavy-elements of the 12 carbon stars (N-type) are taken from Utsumi (1985), with typical errors of 0.4 dex in [N/Ti], which are dealt like in Busso et al. (1995). The corresponding abundances of C, O elements are taken from Lambert et al. (1986).

The heavy-element abundances of 12 barium stars are taken from Zacs (1994). For [FORMULA] Cap and HR 774, the heavy-element abundances are from the same sources as in Busso et al. (1995). The sun, instead of [FORMULA] Vir, is chosen to be a comparison standard, namely, [FORMULA]. The abundances of [FORMULA] Vir to the sun are taken from Tomkin & Lambert (1986). The new solar system abundances are taken from Anders & Grevesse (1989). The overabundances of s-process elements in barium stars are characterized by a mean value of [s/Fe]=([Y/Fe]+[Ba/Fe]+[La/Fe]+[Ce/Fe]+[Nd/Fe])/5.0. The typical error is 0.2 dex (Boffin & Zacs 1994). Zr has been excluded from this mean value, since the abundance of Zr is poorly related to other heavy-elements in the sample (Boffin & Zacs 1994). The corresponding observational data of orbital elements, orbital period P and eccentricity e, are taken from McClure & Woodsworth (1990), Boffin & Zacs (1994) and Jorissen et al. (1998).

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© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000