Barium and Tc-poor S Stars: Binary Masqueraders among Carbon Stars

Our current understanding of the origin of barium and S stars is briefly reviewed, based on new orbital elements and binary frequencies. 1. The relation of barium and S stars to carbon stars Since the last conference (IAU Coll. 106, Evolution of Peculiar Red Giants, Johnson & Zuckermann eds., 1989) devoted to chemically-peculiar red giants (PRGs), much progress has been made in understanding how barium and S stars relate to the other PRGs. The discovery of the binary nature of barium stars (McClure et al. 1980; McClure 1983) suggested from the beginning that mass transfer was likely to play a key role in the formation of the barium syndrome. As far as S stars are concerned, it has become clear that Tc-rich and Tc-poor S stars form two separate families with similar chemical peculiarities albeit of very different origins (Iben & Renzini 1983; Little-Marenin et al. 1987; Jorissen & Mayor 1988; Smith & Lambert 1988; Brown et al. 1990; Johnson 1992; Jorissen & Mayor 1992; Groenewegen 1993; Johnson et al. 1993; Jorissen et al. 1993; Ake, this conference). Tcrich (or ‘intrinsic’) S stars are genuine thermally-pulsing AGB stars where the s-process operates in relation with the thermal pulses, and where the third dredge-up brings the freshly synthesized s-elements (including Tc) to the surface (e.g. Iben & Renzini 1983; Sackmann & Boothroyd 1991). By contrast, Tc-poor (or ‘extrinsic’) S stars are believed to be the cool descendants of barium stars. Figure 1 summarizes our current understanding of the relationship between the different families of PRG stars. This general picture raises several 2 A. JORISSEN AND S. VAN ECK