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
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2 ESO, Alonso de Córdova 3107, Casilla 19001, Santiago, Chile
3 ESO, Karl Schwarzschild Strasse 2, 85748 Garching, Germany
Received: 6 May 2016
Accepted: 11 August 2016
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.
Key words: binaries: spectroscopic / stars: abundances / white dwarfs / open clusters and associations: general
Based on observations made with the Mercator Telescope, operated on the island of La Palma by the Flemish Community, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, and on observations made with the HARPS spectrograph installed on the 3.6 m telescope at the European Southern Observatory.
© ESO 2017