Near-infrared photometry of globular clusters - II. The metal-rich cluster 47 Tucanae (NGC 104).

Photographic photometry on the VI system is presented for a complete sample of 185 stars brighter than I = in an annulus between 3' and 8' from the centre of 47 Tucanae. Photometry is also given for 47 red giant stars nearer or further from the centre and for most of the 23 red variables now known. New spectral types are given for I I stars near the red giant tip. There is a well-defined giant branch, clearly divided into main and asymptotic branches between I = and . Above this both branches share a gap about 0m.25 wide. There is no asymptotic branch below I = , while above I = I I 8 the giant branch is wide but not resolved into two branches. A small number of stars lying to the blue of the giant branch may be cluster members. There do not appear to be any carbon stars near the top of the giant branch. Comparison with theoretical calculations indicates that excursions from the asymptotic giant branch as a result of instabilities in the He-burning shell do not extend to very high surface temperature. The observational red giant tip occurs at lower temperature than the helium flash predicted for appropriate composition. This may result from the theoretical envelopes and the colour- temperature relation predicting colours which are too blue, for which there is some independent evidence, as well as from uncertainties in the chemical composition. Star counts in annuli of various radii show that the red giants are more concentrated to the cluster centre than the integrated light of the cluster, providing a possible explanation for the reported radial change in integrated colour.