NEW INSIGHTS INTO GLOBULAR CLUSTERS FROM MAIN-SEQUENCE PHOTOMETRY

Abstract

CCD photometry of main-sequence stars is presented for four globular clusters, NGC 288, NGC 362, NGC 6752, and NGC 7099. Analysis of (B -V) color distributions of stars on the main sequences of the clusters indicates a high degree of chemical homogeneity among the stars in a cluster. The most stringent limit is for the cluster NGC 362, for which an upper limit on the standard deviation of the [M/H] distribution of a = 0.08 dex is set. The incidence of photometrically-detectable binary stars in NGC 362 and NGC 7099 is very low (< 0.05%). In NGC 288 there appears to be a second sequence to the redward of the main sequence composed of 14 objects. This sequence is 0.75 mag above the main sequence and may be comprised of equal-mass binary systems. Distances have been determined to NGC 288, NGC 362, and NGC 7099 by fitting the lower main sequences of these clusters to the local subdwarfs using the recently-revised parallax values and Lutz-Kelker corrections. The distance moduli are measured to be (m M ) = 14.65 ±0.2 for NGC 288 and NGC 362, and (m -M) = 14.50 ±0.2 for NGC 7099. The implied values for Mv