YOUNG GLOBULAR-CLUSTERS IN THE MILKY-WAY - ARP-2

We have obtained the first B,V CCD color-magnitude diagram (CMD) of the galactic globular cluster Arp 2. About 1600 stars have been measured between the tip of the red giant branch at V∼15.5 and V∼23.5, about two magnitudes below the main sequence turnoff. A centrally concentrated population of blue stragglers has been detected. The slope of the red giant branch and the overall CMD morphology are consistent with that of a metal-poor cluster, with [Fe/H]=-1.84±0.25. A preliminary spectroscopic measurement based on the Ca II triplet yields [Fe/H]=-1.73±0.05. The comparison of the CMD of Arp 2 with that of other clusters favors a value more metal-poor than that indicated by the Ca II triplet. In this respect, Arp 2 is similar to Ruprecht 106 [Buonanno et al. AJ, 100, 1811 (1990), AJ, 105, 184 (1993)]. Differential ages between Arp 2 and a number of reference clusters are obtained from the vertical age parameter, ΔVTOHB, and the horizontal age parameter, Δ(B - V)TORGB. By requiring both age estimators to give consistent results, we find that Arp 2 is ∼3 Gyr younger than the group of the metal-poor clusters and slightly older than Ruprecht 106. The detection of young metal-poor clusters ([Fe/H]≤-1.8) implies a complex scenario for the origin of the galactic halo, possibly involving interactions with satellite galaxies and their cluster systems.