INSIGHTS INTO THE FORMATION OF BARIUM AND TC-POOR S-STARS FROM AN EXTENDED SAMPLE OF ORBITAL ELEMENTS

The set of orbital elements available for chemically-peculiar red gian t (PRG) stars has been considerably enlarged thanks to a decade-long C ORAVEL radial-velocity monitoring of about 70 barium stars and 50 S st ars. When account is made for the detection biases, the observed binar y frequency among strong barium stars, mild barium stars and Tc-poor S stars (respectively 35/37, 34/40 and 24/28) is compatible with the hy pothesis that they are all members of binary systems. The similarity b etween the orbital-period, eccentricity and mass-function distribution s of Tc-poor S stars and barium stars confirms that Tc-poor S stars ar e the cooler analogs of barium stars. A comparative analysis of the or bital elements of the various families of PRG stars, and of a sample o f chemically-normal, binary giants in open clusters, reveals several i nteresting features. The eccentricity - period diagram of PRG stars cl early bears the signature of dissipative processes associated with mas s transfer, since the maximum eccentricity observed at a given orbital period is much smaller than in the comparison sample of normal giants . The mass function distribution is compatible with the unseen compani on being a white dwarf (WD). This lends support to the scenario of for mation of the PRG star by accretion of heavy-element-rich matter trans ferred from the former asymptotic giant branch progenitor of the curre nt WD. Assuming that the WD companion has a mass in the range 0.60 +/- 0.04 M-., the masses of mild and strong barium stars amount to 1.9 +/ - 0.2 and 1.5 +/- 0.2 M-., respectively. Mild barium stars are not res tricted to long-period systems, contrarily to what is expected if the smaller accretion efficiency in wider systems were the dominant factor controlling the pollution level of the PRG star. These results sugges t that the difference between mild and strong barium stars is mainly o ne of galactic population rather than of orbital separation, in agreem ent with their respective kinematical properties. There are indication s that metallicity may be the parameter blurring the period - Ba-anoma ly correlation: at a given orbital period, increasing levels of heavy- element overabundances are found in mild barium stars, strong barium s tars, and Pop.II CH stars, corresponding to a sequence of increasingly older, i.e., more metal-deficient, populations. PRG stars thus seem t o be produced more efficiently in low-metallicity populations. Convers ely, normal giants in barium-like binary systems may exist in more met al-rich populations. HD 160538 (DR Dra) may be such an example, and it s very existence indicates at least that binarity is not a sufficient condition to produce a PRG star.