The C, N, and O abundances of giant stars in ω Centauri

Abstract

Indices measuring the strength of CH, C_2, CN, and NH bands and Ca ii and Ca i lines have been determined for 72 stars in ω Cen. These data, and narrow-band photometry of the 2.4 µm CO bands, are calibrated by using theoretical indices derived from synthetic spectra which are computed via a grid of model atmospheres of appropriate parameters. In addition to the four C stars, we find two groups of giants in co Cen. Both groups include stars spanning a wide range of abundances of the heavy elements, Z. The N-enhanced stars have N/Z enhanced by a factor of 5 to 15, while both C/Z and O/Z are depleted by a factor which may be as much as 10. This depletion allows the CH and CO features to be fitted satisfactorily and yields enough nitrogen to fit CN and NH features. The N-normal stars have N/Z and C/Z within factor of 2 of solar. The range of variation of C and N is typical of those seen in other globular clusters

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