CCD and IR-array photometry of Galactic globular clusters - I. BVJK photometry of M69

We present new, high-quality optical (B and V) and near-infrared (J and K) imaging photometry of about 1500 giant and horizontal branch stars in the low-latitude, metal-rich cluster M69 (NGC 6637). With these data we find the horizontal branch to be at $$V_{\rm HB}=15.85 \pm 0.05$$ (0.3 mag brighter than in previous studies) and at $$K_{\rm HB}=13.40 \pm 0.05$$ consistent with the results of Davidge & Simons. The cluster metallicity, as determined from different properly calibrated observables measured on various colour-magnitude diagrams, is found to be $$\rm {[Fe/H]}=-0.75 \pm 0.20$$, confirming the strong similarity with 47 Tuc, whose main loci nicely fit the M69 data if one adopts a differential distance modulus of 1.40 mag. There is no photometric evidence to support a solar metallicity as apparently suggested by some integrated-light spectroscopic features. The brightest giants in our sample are all variable and, after a revision of the distance modulus, they fit the scenario described by Frogel & Elias concerning the core He flash luminosity and variability periods. The so-called ‘RGB bump’ is detected at $$V=16.37\pm0.05$$ and $$K=13.55\pm0.05$$ in agreement with the results obtained by Fusi Pecci et al. for about a dozen clusters of various metallicities. The proper consideration of its possible influence on the star counts used to obtain estimates of the helium abundance via the so-called R-method seems able to explain the lower-than-primordial helium content claimed for some metal-rich clusters, yielding $$Y_{\rm P} \sim 0.23\pm0.04$$, as obtained for other well-observed clusters.