Analysis of 26 barium stars

II. Contributions of s-, r-, and p-processes in the production of heavy elements

Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, 05508-900 São Paulo, Brazil e-mail: [dinah;barbuy]@astro.iag.usp.br

Received: 4 February 2006
Accepted: 16 March 2006

Abstract

Context.Barium stars show enhanced abundances for the slow neutron capture (s-process) heavy elements, so they are suitable objects for studying s-process elements.

Aims.The aim of this work is to quantify the contributions of the s-, r-, and p-processes for the total abundance of heavy elements from abundances derived for a sample of 26 barium stars. The abundance ratios between these processes and neutron exposures were studied.

Methods.The abundances of the sample stars were compared to those of normal stars, thus identifying the fraction relative to the main component of the s-process.

Results.The fittings of the curves (neutron-capture cross-section times abundance, plotted against atomic mass number) for the sample stars suggest that the material from the companion asymptotic giant branch star had approximately the solar isotopic composition as concerns fractions of abundances relative to the s-process main component. The abundance ratios of heavy elements, hs, ls, and s and the computed neutron exposure are similar to those of post-AGB stars. For some sample stars, an exponential neutron exposure fits the observed data well, whereas a single neutron exposure provides a better fit for others.

Conclusions.The comparison of barium and AGB stars supports the hypothesis of binarity for the barium star formation. Abundances of r-elements that are part of the s-process path in barium stars are usually higher than those in normal stars, so barium stars also seemed to be enriched in r-elements, although to a lower degree than s-elements. No dependence on luminosity classes was found in the abundance-ratio behaviour among the dwarfs and giants of the sample of barium stars.