Ages for Globular Clusters in the Outer Galactic Halo: The Second   Parameter Clusters Palomar~3, Palomar~4 and Eridanus
  We have used the WFPC2 camera on the Hubble Space Telescope to obtain photometry of the outer-halo globular clusters Palomar 3, Palomar 4, and Eridanus. These three are classic examples of the "second parameter" anomaly because of their red horizontal-branch morphologies in combination with their low-to-intermediate metallicities. Our color-magnitude diagrams in (V, V-I) reach $V_{lim} \simeq 27.0$, clearly delineating the subgiant and turnoff regions and about three magnitudes of the unevolved main sequences. The slopes and dereddened colors of the giant branches are consistent with published [Fe/H] estimates that rank the clusters (Pal 3, Eridanus, Pal 4) in order of increasing metallicity, with all three falling near or between the abundance values of the classic nearby halo clusters M3 and M5. Differential fits of their color-magnitude diagrams are made to each other and to M3 and M5 for relative age determinations. We find that the three outer-halo cluster CMDs differ from the nearby clusters in a way that is consistent with their being younger by $\sim 1.5 - 2 $Gyr, if we have correctly estimated the clusters' chemical-abundance ratios. Conversely, the inferred age difference could be smaller ($\ltsim 1 $Gyr) if either [Fe/H] or [$\alpha$/Fe] for the outer-halo clusters is significantly lower than we have assumed. Possible age spreads of order 1$ $Gyr among both the nearby and outer-halo clusters may also be present. 
Peter B. Stetson, Michael Bolte, William E. Harris, James E. Hesser,   Sidney van den Bergh, Don A. VandenBerg, Roger A. Bel...