Neutron source, neutron density and the origin of barium stars⋆ Free

We present new high resolution spectra of the MgH profiles in nine barium stars, and determine, by spectral synthesis techniques, the magnesium isotopic ratios in each star. When compared with recent AGB nucleosynthesis calculations, we can emphatically rule out the operation of the ²²Ne(α, n)²⁵Mg reaction as the neutron source responsible for the s-process enhancements observed in these barium stars. Although our determined magnesium isotopic abundances are consistent with the operation of the ¹³C(α, n)¹⁶O neutron source, the neutron density associated with this source, in current stellar models, is inconsistent with the low neutron density we determine from observations of the RbI line at 7800 Å in the two barium stars HD 121447 and 178717. We identify the properties required of future AGB stellar models if the much favoured origin for the barium stars, namely an AGB mass transfer scenario, is to be made consistent with the observations reported here. Alternative evolutionary origins of the barium stars are discussed.