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

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.

The Close Binary Fraction of Solar-type Stars Is Strongly Anticorrelated with Metallicity

“…Companions to blue stragglers have been observed up to P ≈ 3,000 days (a ≈ 5 AU; Mathieu & Geller 2009), companions to barium stars extend to P ≈ 20,000 days (a ≈ 20 AU; Jorissen et al 1998;Van der Swaelmen et al 2017), and the widest known symbiotic, Mira, has an orbital period of P ≈ 500 years (a ≈ 80 AU; Prieur et al 2002;Sokoloski & Bildsten 2010). Future studies of blue stragglers, barium stars, cataclysmic variables, novae, and symbiotics must consider the effects of a metallicity-dependent close binary fraction.…”

Discovery of technetium- and niobium-rich S stars: The case for bitrinsic stars

“…A tentative period of 575 d and a mass function of (2.7 ± 0.2) × 10 −5 M was obtained by Jorissen et al (2019), suggestive of an orbit seen almost pole-on (i = 4.7 • ± 1 • ). BD+79 • 156 is a confirmed binary with an orbital period of 10931 days, an eccentricity of 0.461, and a mass function of 0.025 M , compatible with a WD companion (Van der Swaelmen et al 2017;Jorissen et al 2019). An additional confirmation of the presence of a WD companion around BD+79 • 156 (and o 1 Ori) comes from the UV excess seen in their spectral energy distribution (SED), well visible when comparing photometric observations with model SED (Fig.…”

The orbits of subdwarf-B + main-sequence binaries

“…9 the period-eccentricity distribution of post-AGB (pAGB) binaries are plotted. This sample contains dusty pAGB binaries with known orbital parameters from Waters et al (1993); Van Winckel et al (1995); Pollard et al (1996); Gonzalez & Wallerstein (1996); Van Winckel et al (1998, 1999; Maas et al (2002Maas et al ( , 2005 Manick et al (2017), and barium (Ba) and Tcpoor S stars for which orbital parameters were derived by Van der Swaelmen et al (2017). The Ba stars are giant stars in binary systems with a WD companion where the giant has an overabundance in s-process elements caused by mass transfer from WD progenitor during its AGB phase (Bidelman & Keenan 1951;Jorissen et al 1998).…”

“…9. The period-eccentricity distribution of different types of post-AGB binaries that have undergone a mass-loss phase in their past: dusty pAGB binaries (green squares, Waters et al 1993;Van Winckel et al 1995;Pollard et al 1996;Gonzalez & Wallerstein 1996;Van Winckel et al 1998, 1999Maas et al 2002Maas et al , 2005Van Winckel 2007;van Winckel et al 2009;Gorlova et al 2014;Gezer et al 2015;De Smedt et al 2016;Manick et al 2017), Ba stars (blue circles, Van der Swaelmen et al 2017), Tc-poor S stars (black x's, Van der Swaelmen et al 2017) and the wide sdB binaries (red crosses, this article). See section 6.2.…”