Studies of Extremely Young Clusters. II. NGC 6530.

Three-color photoelectric observations of NGC 6530 to V = 16 indicate that the color-magnitude diagram, like that of NGC 2264, consists of a normal main sequence extending from OS to about A0, below which the stars lie above the main sequence. Like NGC 2264, the luminosity functions of NGC 6530 (determined to Mv +3) and of the Orion Nebula cluster (determined to Mv = +4.5) agree with that which would be observed in the solar neighborhood if no evolution of stars away from the main sequence had occurred. Also like NGC 2264 and Orion, it has been found that a large number of faint T Tauri stars occur in the cluster. The presence of the T Tauri stars and the agreement of the color-magnitude diagram and luminosity function with those of NGC 2264 indicate that the two objects are very similar in composition and lend added weight to the interpretatioll of the color-magnitude diagrams of both clusters as indicating that the clusters are so young that the fainter stars are still in the process of gravitational contraction. There is some indication that the ratio of the color excesses, EU-B/EB- v, may be abnormal for the stars in NGC 6530. As a result, definitive values for the reddening and for the distance of the cluster have not been obtained.