On the giant, horizontal and asymptotic branches of Galactic globular clusters – V. CCD photometry of NGC 1261 ⋆

BV CCD photometry is presented for 3352 stars (18 variables) in the globular cluster NGC 1261. The main conclusions drawn from the structure of the colour–magnitude diagram (CMD) are as follows. (i) The location and structure of the main branches agree with those found in previous studies, confirming the existence of a well-defined blue horizontal branch (HB) tail in addition to the large number of red HB stars. (ii) The mean metallicity derived from CMD indicators calibrated with the Zinn & West scale is $$[Fe/H] = - 1.4 \pm 0.2,$$ and the very small intrinsic width of the various branches yields very low upper limits $$(\Delta [Fe/H]\leq 0.06, \Delta [CNO] \leq 0.2)$$ for any chemical abundance spread. (iii) The ‘RGB bump’ is detected at V = 16.70 ± 0.05 as a clump of stars in the luminosity function of the red giant branch. (iv) $$V_{\text{HB}} = 16.70\pm 0.04$$ and $$V_{\text{TO}} = 20.14\pm 0.10$$ yield ∆$$V_{\text{HB}}^{\text{TO}} = 3.44 \pm 0.12$$.A detailed comparison with the main CMD loci of NGC 288 (a cluster that has similar metallicity but a totally blue HB) throws doubt on the hypothesis that a clear difference in age (NGC 288 being older) is the ‘second parameter’ necessary to explain the observed differences. (v) Some evidence is found for differences in the spatial distribution of the red and blue HB stars that is worthy of further analysis.