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A grid of S stars MARCS model atmospheres

par Van Eck, Sophie ;Neyskens, Pieter ;Plez, Bertrand;Jorissen, Alain ;Edvardsson, Bengt;Eriksson, Kjell;Gustafsson, Bengt;Jørgensen, Uffe Gråe;Nordlund, Åke A.
Référence Journal of physics. Conference series, 328, 1, 012009
Publication Publié, 2011

Article révisé par les pairs

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Résumé :

S stars are cool stars of temperatures similar to those of M giants, but their atmospheres are enriched in carbon and s-process elements because of either extrinsic pollution by a binary companion or intrinsic nucleosynthesis and dredge-up on the thermally-pulsing AGB. Despite numerous attempts to link phenomenological spectral classification criteria to physical parameters (T eff, gravity, C/O, [s/Fe], [Fe/H]), the parameter space of S stars is poorly known and this has prevented accurate abundance analysis of S stars until now. Here we present a large grid of S-star model atmospheres. ZrO and TiO band strength indices as well as VJHKL photometry are needed to disentangle the effective temperature, C/O and [s/Fe]. The stellar parameters derived on the basis of low-resolution spectra and photometry are shown to be fairly accurate when compared to high-resolution data of the same stars. The C/O ratio of S stars is found to be between the solar value (0.5) and 0.99, and not 1 as often claimed in the literature. Consistently with stellar evolution expectations, the C/O ratio increases as the effective temperature decreases.