Enhanced abundances of nitrogen and s-process elements in red giant stars in omega Centauri

Abstract

Analysis of spectra of a number of red giant stars in the metal-poor globular cluster w Centauri reveals overabundances of nitrogen, barium, and probably strontium by factors of up to 40 in some stars. Most of the stars under discussion lie along the cool edge of the giant branch in the color- magnitude diagram. The greatest barium enhancement occurs in the star RGO 371 which, however, lies well to the red of the cool edge and is substantially fainter than the tip of the branch. This star should probably be classified as a Population II barium star. The greatest nitrogen enhancement occurs in the star RGO 253 which does not occupy an unusual position in the color- magnitude diagram. Absence of the Swan bands of molecular carbon in all the stars discussed indicates there is little, if any, carbon enhancement (i.e., C/O < 1). The stars are presumably in the asymptotic branch phase of evolution in which mixing into the atmosphere during helium shell flashes from regions which have undergone nucleosynthesis in the star has produced the observed abundances. Only a slight nitrogen enhancement and no s-process enhancement has yet been predicted for low-mass stellar models. An explanation is given for the great width of the giant branch in the color-magnitude diagram, in terms of abundance and the mass differences across the branch. At a given Mv, the stars with redder B - V are believed to be less massive and more metal-rich. Subject headings: clusters: globular - nucleosynthesis - stars: abundances - stars: Ba II - stars: Population II