When binaries keep track of recent nucleosynthesis

The Zr–Nb pair in extrinsic stars as an s-process diagnostic

¹ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, ULB, Campus Plaine C.P. 226, Boulevard du Triomphe, 1050 Bruxelles, Belgium
e-mail: dkarinku@ulb.ac.be
² Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ ESO, Karl Schwarzschild Straße 2, 85748 Garching bei München, Germany
⁴ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁶ Laboratoire Univers et Particules de Montpellier, Université Montpellier, CNRS, 34095 Montpellier Cedex 05, France

Received: 23 March 2018
Accepted: 28 May 2018

Abstract

Context. Barium stars are s-process enriched giants. They owe their chemical peculiarities to a past mass transfer phase. During this phase they were polluted by their binary companion, which at the time was an asymptotic giant branch (AGB) star, but is now an extinct white dwarf. Barium stars are thus ideal targets for understanding and constraining the s-process in low- and intermediate-mass AGB stars.

Aims. We derive the abundances of a large number of heavy elements in order to shed light on the conditions of operation of the neutron source responsible for the production of s-elements in the former companions of the barium stars.

Methods. Adopting a recently used methodology, we analyse a sample of eighteen highly enriched barium stars observed with the high-resolution HERMES spectrograph mounted on the Mercator telescope (La Palma). We determine the stellar parameters and abundances using MARCS model atmospheres. In particular, we derive the Nb–Zr ratio which was previously shown to be a sensitive thermometer for the s-process nucleosynthesis. Indeed, in barium stars, ⁹³Zr has fully decayed into mono-isotopic ⁹³Nb, so Nb/Zr is a measure of the temperature-sensitive ⁹³Zr/Zr isotopic ratio.

Results. HD 28159, previously classified as K5III and initially selected to serve as a reference cool K star for our abundance analysis, turns out to be enriched in s-process elements, and as such is a new barium star. Four stars are characterised by high nitrogen abundances, and among those three have high [Nb/Zr] and [hs/ls] ratios. The derived Zr and Nb abundances provide more accurate constraints on the s-process neutron source, identified to be ¹³C(α, n)¹⁶O for barium stars. The comparison with stellar evolution and nucleosynthesis models shows that the investigated barium stars were polluted by a low-mass (M ∼ 2 − 3 M_⊙) AGB star. HD 100503 is potentially identified as a high metallicity analogue of carbon-enhanced metal-poor star enriched in both r- and s-process elements (CEMP-rs).