Photometry of southern globular clusters. Pt. 1

The techniques used for observing globular cluster stars are described, and several sources of error are discussed. Photoelectric UBV data are presented for 126 stars brighter than V = 16.5 in ω Cen. These data are compared with those of previous photoelectric and photographic studies. There are four principal conclusions, (i) In the colour–magnitude (C–M) diagram the ω Cen giant branch has an exceptionally large scatter, of which only a small part can apparently be due to errors, field stars, and differential reddening. There is no obvious bifurcation of the red giant branch into normal and asymptotic branches. The large intrinsic scatter may be caused by the presence of normal and asymptotic branches which are then blurred out by a small amount of differential reddening, (ii) The large variation in ultra-violet excess with magnitude which was found by E. H. Geyer seems to be largely due to errors in his ultra-violet calibration; the variation found here is approximately that predicted from expected changes in surface gravity. (iii) Most of the reddest giants are small-amplitude variables; there may be a boundary between variables and non-variables at about ( B−V ) 0 = 1.5. (iv) After correction for small (< 0.1 mag) systematic errors, the extensive Royal Greenwich Observatory photographic photometry has the RGO quoted accuracy for V < 15.5. Several observational programmes are suggested, particularly with a view to confirming that the giant branch scatter is intrinsic.