Two Young Galactic Globular Clusters: Terzan 7 and ARP 2

The two globular clusters, Terzan 7 and Arp 2, which lie close to each other on the sky, are found to be significantly younger than most other globular clusters of similar metal abundance. Terzan 7, with an (Fe/H) of -0.49 as inferred from a measurement of its Ca II line strength, seems to be approximately 4 Gyr younger than 47 Tuc while Arp 2, with (Fe/H) = -1.73, also measured from Ca ll lines, appears to be approximately 1 Gyr older than Ruprecht 106 which is known to be approximately 4 Gyr younger than other metal-poor clusters. These conclusions are supported by a differential comparison of the principal stellar sequences in the cluster color magnitude diagrams. They are thus independent of cluster distances and reddening estimates and are only weakly affected by uncertainties in the cluster metal abundances. The four Galactic globular clusters, Arp 2, Pal 12, Ruprecht 106, and Terzan 7, all of which are unequivocally younger than other globular clusters of similar metallicity, appear to lie along a great circle in the sky and thus conceivably are on similar orbits. This suggests that they may have been captured by the Milky Way Galaxy.