BVI CCD PHOTOMETRY OF NGC-5053 - THE MOST METAL-POOR GALACTIC GLOBULAR-CLUSTER

We present (V,B-V), (V,V-I), and (V,B-I) color-magnitude diagrams for NGC 5053, which is the most metal-poor globular cluster known in the galaxy. The horizontal branch (HB) is predominantly blueward of the RR Lyrae instability strip and the slope of the red giant branch (RGB) is relatively steep, both characteristic of a metal-poor system. Monte-Carlo simulations show that both the RGB and HB are underpopulated relative to those of the metal-poor cluster M15, indicative of the relatively lower total luminosity of NGC 5053. Based on the mean magnitude of stars located at the red and blue edges of the instability strip, we find V(HB)=16.65±0.03. At this magnitude, polynomial fits to the cluster RGBs in the two measured colors yield (B-V)g=0.71±0.016 and (V-I)g=0.93±0.016. Using the BVI two-color diagram in a differential manner with respect to M68 and M15 gives a reddening of E(B-V)=0.06±0.02 for NGC 5053. We have utilized these quantities to extend the calibration of the simultaneous reddening and metallicity method [Sarajedini, AJ, 107, 618 (1994a)] to [Fe/H]=-2.4. Another feature of the color-magnitude diagram that is evident is the rich population of blue straggler stars (BSSs). We are able to recover 22 of 24 previously known BSSs, and we have added 3 more BSSs.