CCD Photometry of the Globular Cluster M5. I. The Color-Magnitude Diagram and Luminosity Functions

Abstract

We present new BV I 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 and main sequence, both of which have been claimed in more metal-poor clusters. Enhancements of �-element have been taken into account in our analysis. This improves the agreement between the observed and predicted positions of the “red-giant bump”. Depending on the average �-element enhancement among globular clusters and the distance calibration, the observed discrepancy between the theoretical and observed position for a large number of clusters (Fusi Pecci et al. 1990) 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 ≈ 0.23. The relative ages of M5, Palomar 5, M4, NGC 288, NGC 362, NGC 1261, NGC 1851 and � , , !