CCD PHOTOMETRY FOR THE GALACTIC GLOBULAR-CLUSTER NGC-6584

We present CCD photometry in the B and V passbands for the Galactic globular cluster NGC 6584. The color-magnitude diagram reveals a horizontal branch (HB) populated on both the red and blue sides of the RR Lyrae instability strip. The cluster contains 42 confirmed RR Lyrae variables, and we find an additional 56 stars which are possible RR Lyraes. The mean magnitude of the RR Lyraes on our CCD frames is combined with that of the nonvariable stars to yield 〈V(HB)〉=16.53±0.03. Using a polynomial fit to the cluster giant stars, we derive the color of the giant branch at the level of the HB to be (B-V)g=0.89±0.02. Our photometry has also been used to isolate the red giant branch "clump," which forms when stars pause and sometimes reverse their course as they evolve up the giant branch. Its luminosity is primarily dependent upon the cluster metallicity, and we have used this fact to derive a metallicity of [Fe/H]=-1.44±0.16 for NGC 6584. Coupled with the color of the giant branch given above, this yields a cluster reddening of E(B-V)=0.07±0.04. To the extent that our photometry can be used to study the age of NGC 6584, we find it to be comparable to M3, which is not surprising given their similar HB morphologies. The color-magnitude diagram also exhibits a significant population of blue straggler stars. From a comparison of the blue straggler radial distribution to that of the subgiant stars with similar V magnitudes, we find that the blue stragglers are more centrally concentrated than the subgiants, indicative of a higher mass for the blue stragglers.