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 giant (PRG) stars has been considerably enlarged thanks to a decade-long CORAVEL radial-velocity monitoring of about 70 barium stars and 50 S stars. When account is made for the detection biases, the observed binary 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 hypothesis that they are all members of binary systems. The similarity between the orbital-period, eccentricity and massfunction distributions of Tc-poor S stars and barium stars confirms that Tc-poor S stars are the cooler analogs of barium stars. A comparative analysis of the orbital elements of the various families of PRG stars, and of a sample of chemically-normal, binary giants in open clusters, reveals several interesting features. The eccentricity – period diagram of PRG stars clearly bears the signature of dissipative processes associated with mass 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 companion being a white dwarf (WD). This lends support to the scenario of formation of the PRG star by accretion of heavy-element-rich matter transferred from the former asymptotic giant branch progenitor of the current WD. Assuming that the WD companion has a mass in the range 0.60 ± 0.04 M⊙, the masses of mild Send offprint requests to: A. Jorissen (at the address in Belgium) ⋆ This paper is dedicated to the memory of Antoine Duquennoy, who contributed many among the observations used in this study ⋆⋆ Based on observations carried out at the European Southern Observatory (ESO, La Silla, Chile), and at the Swiss telescope at Haute Provence Observatory ⋆⋆⋆ Research Associate, National Fund for Scientific Research (FNRS), Belgium and strong barium stars amount to 1.9± 0.2 and 1.5± 0.2 M⊙, respectively. Mild barium stars are not restricted 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 suggest that the difference between mild and strong barium stars is mainly one of galactic population rather than of orbital separation, in agreement with their respective kinematical properties. There are indications that metallicity may be the parameter blurring the period – Ba-anomaly correlation: at a given orbital period, increasing levels of heavy-element overabundances are found in mild barium stars, strong barium stars, and Pop.II CH stars, corresponding to a sequence of increasingly older, i.e., more metal-deficient, populations. PRG stars thus seem to be produced more efficiently in low-metallicity populations. Conversely, normal giants in barium-like binary systems may exist in more metal-rich populations. HD 160538 (DR Dra) may be such an example, and its very existence indicates at least that binarity is not a sufficient condition to produce a PRG star.