Reddenings, metallicities, and possible abundance anomalies in young globular clusters

We present new photometry in the VI passbands for the ''young'' globular clusters Rup 106, Ter 7, and Arp 2. After formulating the simultaneous reddening and metallicity (SRM) method of Sarajedini [AJ, 107, 618 (1994)] in the BV passbands, we apply it, along with the SRM method in VI, to the red giant branches (RGBs) of these clusters using B - V photometry from the literature and the V-I data presented herein. We find [Fe/H]=-1.90+/-0.10,E(B-V)=0.18+/-0.02 for Rup 106, [Fe/H]=-0.82+/-0.15,E(B-V)=0.07+/-0.03 for Ter 7, and [Fe/H]=-1.84+/-0.09,E(B-V)=0.10+/-0.02 for Arp 2. Furthermore, in light of this new abundance for Ter 7 and recent work on the luminosity of the red horizontal branch, we rederive the age of Ter 7 finding it to be some 6 Gyr younger than 47 Tuc. We show that the SRM method is insensitive to age for clusters with purely red HBs and ages as young as similar to 5 Gyr; for clusters with bluer HBs, the SRM method is only mildly sensitive to age differences between such clusters and the calibrating (standard) clusters. From these metallicity estimates, we conclude that the photometric abundances of the program clusters based on the properties of the RGB are systematically lower (Delta[Fe/H] = 0.1-0.4 dex) than those derived using other indicators, in particular the Ca II triplet method. We note that the young globular cluster Palomar 12 also exhibits this behavior. We suggest that this discrepancy is due to systematic differences in the [alpha/Fe] ratios between the young clusters and the ''normal'' Galactic globulars used to calibrate the abundance determination methods. However, we are unable to completely reconcile all the observations of Rup 106 using this approach. Systematic differences in [alpha/Fe] between the young clusters and the rest of the Galactic globulars may indicate differences in their chemical enrichment histories, perhaps due to differing environments at the times of their formation. Interestingly, both Ter 7 and Arp 2 are believed to be members of the Sagittarius dwarf galaxy. (C) 1997 American Astronomical Society.