The spread in CO absorption and effective temperature among the giants in omega Centauri

Abstract

Infrared photometric observations of 82 stars on the upper giant branch of ω Centauri are presented. The data show that ω Cen differs markedly from other globular clusters studied in the infrared in two respects. First, at a given V - K color, there is a wide spread in CO absorption (≳ 0.1 mag) in the 3 mag interval below the red giant tip; this range is approximately 5 times that found for other clusters. Second, at a given luminosity, V — K has a spread of up to 1 mag; this range is also much larger than that found for other clusters. The latter result leads us to conclude, in agreement with recent findings by other authors, that there are star-to-star variations in heavy-metal abundance within ω Cen of a factor of 30 or more. Comparison of the spread in CO absorption with that in V — K reveals two sequences of stars, one which has “enhanced” CO absorption and one which has “normal” CO absorption when compared to the run of CO with V — K among stars in other globular clusters. The proportion of stars having “enhanced” CO exceeds 50%. Many of the strong CN stars in the cluster also have “enhanced” CO. Explanations for the origin of these sequences which involve primordial abundance variations as well as mixing phenomena are discussed briefly. Implications for the identification of the “second parameter” as the CNO abundance are also discussed. Effective temperatures and bolometric luminosities are given for the stars.