Abundances on the main sequence of the globular cluster ω Centauri

Abstract

Using the 2dF multi-fibre instrument on the Anglo-Australian Telescope, moderate resolution spectra have been obtained for a large sample of stars on the main sequence and at the turnoff in the unusual globular cluster ω Cen. We investigate the behaviour of CH, CN and SrII line strength indices as a function of overall abundance for the main sequence sample. A number of stars do not follow the relations defined by the majority. These anomalous objects can be categorized into (at least) three types. (1) Carbon enhanced stars, which represent about 5% of the sample, and which are found at all metallicities. Spectrum synthesis calculations show that the atmospheres of these stars are typically enhanced in carbon by factors of between 3 and 10. (2) Nitrogen enhanced stars, revealed for [Fe/H] [ges ]–1.3 by strong CN indices, which make up ∼40% of the cluster main sequence population above this metallicity. The stars are enhanced in nitrogen by factors of up to 100. Our data, however, provide no constraints on their relative numbers at lower [Fe/H]. (3) Stars with enhancements of the s-process element Sr by factors of 30 to 60. The possible origins for these abundance anomalies are discussed.