The Link between Symbiotic Stars and Chemically-peculiar Red Giants

Barium stars and technetium-poor, extrinsic S stars are binary systems with a white dwarf companion, and with orbital elements similar to those of symbiotic systems. One may thus wonder whether these various families of binary systems involving red giant stars are somehow related. This question is actually twofold: (i) Do barium and binary S stars exhibit some symbiotic activity? (ii) Do symbiotic systems exhibit overabundances of s-process elements like barium and S stars? This paper reviews the current situation regarding these two questions. 1. The zoo of red giant stars Symbiotic stars (SyS), barium stars and technetium-poor S stars are three families involving giant stars where binarity plays a key role. Thanks to the progress of UV astronomy, the binary nature of SyS now goes undisputed as their UV spectra bear the signature of a hot component, generally a white dwarf (WD), whereas the optical spectrum is dominated by the red giant (Miko lajewska, this conference). Barium stars, first identified by Bidelman & Keenan (1951), are G and K giants where carbon and elements heavier than Fe, like Ba and Sr, have surface abundances in excess of the solar value (e.g. Wallerstein at al. 1997 and references therein), i.e., [X/Fe] = log [(N(X)/N(Fe)) / (N⊙(X)/N⊙(Fe))] > 0, where N(X) stands for the abundance of element X. Heavy elements like Sr and Ba are synthesized by the so-called s-process of nucleosynthesis, a sequence of neutron captures starting on abundant seed nuclei like iron-group elements (Burbidge et al. 1957; Wallerstein et al. 1997). The operation of the s-process is commonly associated with thermal pulses (TPs) occurring on the asymptotic giant branch (AGB; e.g. Goriely & Mowlavi 2000). AGB stars have a complex internal structure, consisting of a carbon-oxygen core, heliumand hydrogenburning shells and a deep convective envelope (Olofsson & Habing 2003). TPs are a recurrent thermal instability affecting the He-burning shell. Right after a TP, the inner boundary of the convective envelope may penetrate the intershell region where the s-process operated. As a result of this mixing process (the socalled ‘third dredge-up’ – 3DUP), s-process-enriched material is brought to the stellar surface. Barium stars are, however, too warm and of too low a luminosity to be thermally-pulsing AGB (TP-AGB) stars (Scalo 1976). With the discovery