CCD photometry of the globular cluster M5 .1. The color-magnitude diagram and luminosity functions

We present new BVI photometry for the halo globular cluster M5 (NGC 5904 = C1516 + 022) and examine the B- and I-band luminosity functions (LFs), based on over 20,000 stars-one of the largest samples ever gathered for a cluster luminosity function. Extensive artificial star tests have been conducted to quantify incompleteness as a function of magnitude and cluster radius. We do not see evidence in the LF of a ''subgiant excess'' or of a discrepancy in the relative numbers of stars on the red giant branch (RGB) and main sequence, both of which have been claimed in more metal-poor clusters. Enhancements of alpha-element have been taken into account in our analysis. This improves the agreement between the observed and the predicted positions of the ''red giant bump.'' Depending on the average alpha-element enhancement among globular clusters and the distance calibration, the observed discrepancy between the theoretical and the observed position for a large number of clusters can be almost completely removed. The helium abundance of M5, as determined by the population ratio R, is found to be Y = 0.19 +/- 0.02. However, there is no other indication that the helium abundance is different from other clusters of similar metallicity, and values calculated for other helium indicators are consistent with Y approximate to 0.23. The relative ages of M5, Palomar 5, M4, NGC 288, NGC 362, NGC 1261, NGC 1851, and NGC 2808 are derived via the Delta V-TO(HB) method using M5's horizontal branch (HB) as a bridge to compare clusters with very different HB morphology. We conclude that, at the level of similar to 1.5 Gyr, these clusters of comparable metallicity are the same age, with the possible exception of NGC 288 (older by 3.5 +/- 1.5 if the reddest NGC 288 HB stars are on the zero-age horizontal branch) and Palomar 5 (which may be marginally younger). Even with NGC 288 set aside, there is a large range in HE morphology between the remaining clusters that appears to eliminate age as the sole second parameter determining HE morphology in the case of constant mass loss between the RGB and the HE (although a Reimers mass-loss relation weakens this statement considerably). We are unable to choose between the competing values for M5's (absolute) metallicity: the Zinn & West value of [Fe/H] = -1.40 and the Sneden et al. high-dispersion spectroscopic measurement of -1.17. This level of discrepancy has a significant effect on the derivation of the distance modulus and absolute age of M5. From subdwarf fitting to the main sequence of the cluster, we find an apparent distance modulus (m - M)(v) = 14.41 +/- 0.07 for [Fe/H](M5) = -1.40, and 14.50 +/- 0.07 if [Fe/H](M5) -1.17. From comparisons with theoretical isochrones and luminosity functions, we find an absolute age for M5 of 13.5 +/- 1 Gyr (internal error, assuming perfect models and no [M/H] error) for the Zinn & West abundance scale and 11 +/- 1 Gyr for the higher abundance value.