The barium stars

Cassegrain spectra of all known barium stars and coud spectra of 12 barium stars are discussed. The low dispersion plates are used to set up a system of classification, giving a temperature class and a heavy-metal abundance criterion. The coud plates are used to give elemental abundances in nine of the stars. Overabundances of the heavy elenients are satisfactorily explained by the s-process of nucleosynthesis, with the possible exception of an anomaly in the Pr abundance. In all stars the required number of neutrons per original Fe nucleus in order to build most of the heavy metal abundances is found to be 40. The fraction of Fe exposed to neutron irradiation varies from X IO- in the weakest Ba stars to x IO- in the most extreme. Overabundances of carbon by factors of 3 to 5 are indicated. Coupled with an apparent slight overabundance of Na this suggests that the peculiar abundances have arisen from the effects of carbon-burning in the interior of these stars. It is shown that the most consistent picture arises from considering mixing to have started when only I per cent of the C in the active region had been consumed. This active region may be identified with a carbon-burning shell near the surface of the core in late stages of evolution. The observed metal-to-hydrogen ratio, larger than normal, in the Ba star atmospheres is probably due to mixing of a helium shell with the hydrogen envelope. Absolute magnitudes of 3 Ba stars and 2 MS stars, derived by means of the H and K emission width method, are reasonably consistent with the hypothesis of C burning (or later nuclear energy sources) in the cores of the Ba stars. We suggest that the Ba stars represent a normal stage of development in Population I stars of masses `-4 M0. The stars of 3-4 M0 also pass through the S star stage. Carbon stars may represent a similar evolutionary stage of slightly more massive stars. Population II stars also evolve through at least the Ba star stage.