The significance of the star clusters NGC 2420 and 47 Tucanae for galactic evolution.

Attention is drawn to the old disk "metal-poor" cluster NGC 2420 and the "metal-rich" globular cluster 47 Tuc, a member of the spheroidal component of the Galaxy, which have approximately the same metallicity index [8(U - B) = 0.11 mag for 47 Tuc and 8(U - B) = 0.10 mag for NGC 2420]. This similarity in metal abundance allows an instructive comparison to be made of the two components of the Galaxy. The color-magnitude (C-M) diagrams of the two clusters reveal differences which cannot be explained in terms of a difference in age alone. We conclude that the two clusters differ in the abundance of chemical elements other than those which contribute directly to the ultraviolet excess, i.e., either helium or the CNO elements. Comparison with theoretical isochrones yields agreement provided that either of two conditions is satisfied: (1) the helium abundance of 47 Tuc is greater than that of NGC 2420; or (2) the CNO content of 47 Tuc is less than that of NGC 2420. The implications of these alternatives in terms of models of galactic evolution are briefly discussed. The above relative fitting procedure yields a difference between the distance moduli of the two clusters of 1.2 mag. The difficulties in obtaining the absolute distance moduli are discussed. Best agreement is obtained for (m - M)47Tuc 13.0, corresponding to an age of about 13 x 10 years. Subject headings: clusters: globular - clusters: open - stars: abundances - stars: evolution