CCD and Infrared Array Photometry of Galactic Globular Clusters - Part One - BVJK Photometry of M69

Details CCD and Infrared Array Photometry of Galactic Globular Clusters - Part One - BVJK Photometry of M69 CCD and Infrared Array Photometry of Galactic Globular Clusters - Part One - BVJK Photometry of M69

Feb 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994mnras.266..829f&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Vol. 266, NO. 4/FEB15, P. 829, 1994

Astronomy and Astrophysics

Astronomy

29

Techniques: Photometric - Stars: Abundances - Stars: Evolution - Hertzsprung-Russell (Hr) Diagram - Globular Clusters: Individual: M69

Scientific paper

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 VHB = 15.85±0.05 (0.3 mag brighter than in previous studies) and at KHB = 13 40±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 [Fe/H] = -0.75±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±0.05 and K 13.55±0.05 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 YP 0.23±0.04, as obtained for other well-observed clusters.

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