CCD PHOTOMETRY OF THE GLOBULAR CLUSTER M5 I. THE COLOR-MAGNITUDE DIAGRAM AND LUMINOSITY FUNCTIONS

摘要

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 α-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 α-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.