Linking globular cluster structural parameters and their evolution:
  multiple stellar populations
  Globular clusters (GCs) are known to host multiple stellar populations
showing chemical anomalies in the content of light elements. The origin of such
anomalies observed in Galactic GCs is still debated. Here we analyse data
compiled from the Hubble Space Telescope, ground-based surveys and Gaia DR2 and
explore relationships between the structural properties of GCs and the fraction
of second population (2P) stars. Given the correlations we find, we conclude
that the main factor driving the formation/evolution of 2P stars is the cluster
mass. The existing strong correlations between the 2P fraction and the
rotational velocity and concentration parameter could derive from their
correlation with the cluster mass. Furthermore, we observe that increasing
cluster escape velocity corresponds to higher 2P fractions. Each of the
correlations found is bimodal, with a different behaviour detected for low and
high mass (or escape velocity) clusters. These correlations could be consistent
with an initial formation of more centrally concentrated 2P stars in deeper
cluster potentials, followed by a long-term tidal stripping of stars from
clusters outskirts. The latter are dominated by the more extended distributed
first population (1P) stars, and therefore stronger tidal stripping would
preferentially deplete the 1P population, raising the cluster 2P fraction. This
also suggests a tighter distribution of initial 2P fractions than observed
today. In addition, higher escape velocities allow better retention of
low-velocity material ejected from 1P stars, providing an
alternative/additional origin for the observed differences and the
distributions of 2P fractions amongst GCs.
