Color-Magnitude Diagram for Disk Clobular Cluster NGC 6356 Compared with Halo Clusters.

Abstract

The color-magnitude diagram for the strong metallic4ine globular cluster NGC 6356 has been obtained by using photographic plates and a photoelectric magnitude sequence made with the 200-inch Hale telescope The photometric limit of the data is V = 19 2, B = 20.2 The diagram shows a welldefined giant sequence, a stubby horixontal branch confined to stars redder than (B - V)0 = +0.3, and the beginning of a subgiant branch. rom a study of B and A stars in the cluster neighborhood, the interstellar reddening in front of the cluster is estimated to be E(B - V) = +0.5 Comparison of the C-M diagram of NGC 6356 with the diagram of halo clusters shows that the giant branch of 6356 is fainter than that of M3 (which has weak to nearly normal metallic4ine strength compared with the sun) and is fainter still than that of M92 (which has very weak metal lines) Further comparison shows that the giants in NGC 6356 are brighter than those in M67 (which have normal metallic4ine strengths) These results hold, whether the horixontal branches of M3, M92, and NGC 6356 are put at M = 000 or are placed more reasonably at M = +04 for M92, M = +06 for M3, and M = +0.9 for 6356. Reasons for the second possibility are discussed Confirmatory evidence that the brightness of giant stars in globular clusters depends on chemical composition comes from a comparison of sixteen clusters for which we have accurate photometric data. Globular clusters with high metal abundance (47 Tuc, NGC 6356, NGC 6838) have giant branches which are at least 0 8 mag. fainter than clusters with a pronounced weakening of their Fraunhofer lines This dependence of M on metal abundance makes invalid those methods of distance determination which depend upon assigning a constant absolute magnitude either to the top of the giant branch or to the nth brightest star. The photometric data for NGC 6356 suggest a true distance modulus of 15.3, a diameter of 11 pc, and an absolute magnitude of Mpg = -71 This diameter is small compared with normal halo clusters and again suggests the action of a tidal force due to the galactic nuclear bulge which limits the cluster size.