The central Blue Straggler population in four outer-halo globular
  clusters
  Using HST/WFPC2 data, we have performed a comparative study of the Blue
Straggler Star (BSS) populations in the central regions of the globular
clusters AM 1, Eridanus, Palomar 3, and Palomar 4. Located at distances RGC >
50 kpc from the Galactic Centre, these are (together with Palomar 14 and NGC
2419) the most distant clusters in the Halo. We determine their
colour-magnitude diagrams and centres of gravity. The four clusters turn out to
have similar ages (10.5-11 Gyr), significantly smaller than those of the
inner-Halo globulars, and similar metallicities. By exploiting wide field
ground based data, we build the most extended radial density profiles from
resolved star counts ever published for these systems. These are well
reproduced by isotropic King models of relatively low concentration. BSSs
appear to be significantly more centrally segregated than red giants in all
globular clusters, in agreement with the estimated core and half-mass
relaxation times which are smaller than the cluster ages. Assuming that this is
a signature of mass segregation, we conclude that AM 1 and Eridanus are
slightly dynamically more evolved than Pal 3 and Pal 4.
