On the giant, horizontal and asymptotic branches of Galactic globular clusters - IV. CCD photometry of NGC 1904

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Abstract
A new colour–magnitude diagram for the evolved stars in the Galactic globular cluster NGC 1904 (M79) is presented. 3188 objects were measured on B, V CCD frames covering most of the cluster. Beyond confirming the overall morphology of the CMD obtained in previous studies, these new data yield the following results. (i) The mean metallicity derived from CMD indices calibrated with the Zinn & West scale is |$[\text {Fe/H}]=-1.60\pm0.20$|⁠, and the very small intrinsic widths of the various branches yield very low upper limits |$(\Delta[\text {Fe/H}]) \le 0.14,\enspace \Delta[\text {CNO}]\le0.23)$| for any chemical abundance spread. (ii) The ‘RGB bump’ is well detected at |$V=16.00\pm0.05$|⁠, as a clumping of stars in the luminosity function of the red giant branch. (iii) Besides a small group of objects which populate the region typical for blue stragglers, there is some ‘indirect’ evidence for the possible existence of either a wider BS population or very blue horizontal branch objects in the central regions of the cluster. (iv) |$V_\text {HB}=16.15\pm0.10$|⁠, and the HB has a very long blue tail that extends down to a magnitude fainter than the turn-off. The stellar distribution along the HB is very asymmetric, as often found in blue horizontal branch clusters.

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