THE ASTRONOMICAL JOURNAL    VOLUME 109, NUMBER 2, PAGE 650        FEBRUARY 1995

                YOUNG GLOBULAR CLUSTERS IN THE MILKY WAY: ARP 2

                          R. BUONANNO AND C. E. CORSI

 Osservatorio Astronomico di Roma, Via del Parco Mellini 84, 00136 Roma, Italy
                   Electronic mail: buonanno@astrmp.astro.it

                                 F. FUSI PECCI

                      Osservatorio Astronomico di Bologna
                  Electronic mail: flavio@astbo3.bo.astro.it

                        H. B. RICHER AND G. G. FAHLMAN

    Department of Geophysics and Astronomy, University of British Columbia
                     Electronic mail: richer@astro.ubc.ca


                                   ABSTRACT
    
    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] = 18.4 +/-
    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 Delta V^TO_HB,
    and the horizontal age parameter, Delta (B-V)^TO_RGB.  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.