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, \hbox { \it 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 M 3 and M 5 .
Differential fits of their color-magnitude diagrams are made to each other and to M 3 and M 5 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 ( { } ^ { < } _ { \sim } 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 .