The anticorrelation of carbon and nitrogen on the horizontal branch of 47 Tucanae

Abstract

Spectroscopic and photometric data for 14 horizontal branch stars in 47 Tuc are presented and analyzed to yield information on the systematics of the behavior of carbon and nitrogen in these objects. The observations are consistent with a bimodal distribution of cyanogen, as has been reported for the giants in this system. There is also a one-to-one anticorrelation of the behavior of cyanogen and the CH molecule. Spectrum synthesis of a selected CN-weak, CN-strong pair of stars shows that nitrogen is overabundant by approx.0.9 dex, and carbon is underabundant by approx.0.3 dex in the CN-strong object relative to the CN-weak one. As emphasized elsewhere this anticorrelation of carbon and nitrogen is extremely difficult to understand in terms of a primordial framework involving two generations of stars in which one (the CN-strong group) has been enriched by the ejecta of the other (the CN-weak group). It is also found that the mean magnitude difference between the two horizontal branch groups is only ..delta..Vapprox.0.04, which suggests that there can be little difference in helium abundance or helium core mass between the two groups. This in turn argues against explanations of the abundance peculiarities which involve variations in helium abundance or core rotation.