Strong radial segregation between sub-populations of evolutionary
  homogeneous stars in the Galactic globular cluster NGC 6752
  We investigate the new and still poorly studied matter of so-called multiple
stellar populations (MSPs) in Galactic globular clusters (GGCs). Studying MSPs
and their accumulated data can shed more light on the formation and evolution
of GGCs and other closely related fundamental problems. We focus on the strong
relation between the radial distribution of evolutionary homogeneous stars and
their U-based photometric characteristics in the nearby GGC NGC 6752 and
compare this with a similar relation we found in NGC 3201 and NGC 1261. We use
our new multi-color photometry in a fairly wide field of NGC 6752, with
particular emphasis on the U band and our recent and already published
photometry made in NGC 3201 and NGC 1261. We found and report here for the
first time a strong difference in the radial distribution between the
sub-populations of red giant branch (RGB) stars that are bluer and redder in
color U-B, as well as between sub-giant branch (SGB) stars brighter and fainter
in the U-magnitude in NGC 6752. Moreover, the fainter SGB and redder RGB stars
are similarly much more centrally concentrated than their respective brighter
and bluer counterparts. Virtually the same applies to NGC 3201. We find
evidence in NGC 6752 as in NGC 3201 that a dramatic change in the proportion of
the two sub-populations of SGB and RGB stars occurs at a radial distance close
to the half-mass radius, R_h, of the cluster. These results are the first
detections of the radial trend of the particular photometric properties of
stellar populations in GGCs. They imply a radial dependence of the main
characteristics of the stellar populations in these GGCs, primarily of the
abundance, and (indirectly) presumably of the kinematics.
