The mass-ratio and eccentricity distributions of barium and S stars, and red giants in open clusters^⋆

¹ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, Campus Plaine C.P. 226, Bd du Triomphe, 1050 Bruxelles, Belgium
e-mail: ajorisse@ulb.ac.be; svaneck@ulb.ac.be
² ESO, Alonso de Córdova 3107, Casilla 19001, Santiago, Chile
e-mail: hboffin@eso.org
³ ESO, Karl Schwarzschild Strasse 2, 85748 Garching, Germany

Received: 6 May 2016
Accepted: 11 August 2016

Abstract

Context. A complete set of orbital parameters for barium stars, including the longest orbits, has recently been obtained thanks to a radial-velocity monitoring with the HERMES spectrograph installed on the Flemish Mercator telescope. Barium stars are supposed to belong to post-mass-transfer systems.

Aims. In order to identify diagnostics distinguishing between pre- and post-mass-transfer systems, the properties of barium stars (more precisely their mass-function distribution and their period–eccentricity (P−e) diagram) are compared to those of binary red giants in open clusters. As a side product, we aim to identify possible post-mass-transfer systems among the cluster giants from the presence of s-process overabundances. We investigate the relation between the s-process enrichment, the location in the (P−e) diagram, and the cluster metallicity and turn-off mass.

Methods. To invert the mass-function distribution and derive the mass-ratio distribution, we used the method pioneered by Boffin et al. (1992) that relies on a Richardson-Lucy deconvolution algorithm. The derivation of s-process abundances in the open-cluster giants was performed through spectral synthesis with MARCS model atmospheres.

Results. A fraction of 22% of post-mass-transfer systems is found among the cluster binary giants (with companion masses between 0.58 and 0.87 M_⊙, typical for white dwarfs), and these systems occupy a wider area than barium stars in the (P−e) diagram. Barium stars have on average lower eccentricities at a given orbital period. When the sample of binary giant stars in clusters is restricted to the subsample of systems occupying the same locus as the barium stars in the (P−e) diagram, and with a mass function compatible with a WD companion, 33% (=4/12) show a chemical signature of mass transfer in the form of s-process overabundances (from rather moderate – about 0.3 dex – to more extreme – about 1 dex). The only strong barium star in our sample is found in the cluster with the lowest metallicity in the sample (i.e. star 173 in NGC 2420, with [Fe/H] = −0.26), whereas the barium stars with mild s-process abundance anomalies (from 0.25 to ~ 0.6 dex) are found in the clusters with slightly subsolar metallicities. Our finding confirms the classical prediction that the s-process nucleosynthesis is more efficient at low metallicities, since the s-process overabundance is not clearly correlated with the cluster turn-off (TO) mass; such a correlation would instead hint at the importance of the dilution factor. We also find a mild barium star in NGC 2335, a cluster with a large TO mass of 4.3 M_⊙, which implies that asymptotic giant branch stars that massive still operate the s-process and the third dredge-up.