  By means of grid-based, 3D hydrodynamical simulations we study the formation
of second generation (SG) stars in a young globular cluster (GC) of mass 10^7
Msun, the possible progenitor of an old GC with a present mass ~(1-5) * 10^6
Msun. The cluster accretes external gas as its first generation (FG) asymptotic
giant branch (AGB) stars release their ejecta and SG stars form. We consider
two models characterised by different densities of the external gas. In both
cases, we find that a very compact SG subsystem with central density >10^5M
sun/pc^3 forms in the innermost regions of the cluster. The low-density model
forms a population of extreme SG stars with high helium enhancement, followed
by the formation of another SG group out of a mix of pristine gas and AGB
ejecta and characterised by a modest helium enhancement. On the other hand, the
high-density model forms in prevalence SG stars with modest helium enhancement.
Our simulations illustrate the dynamical processes governing the formation of
SG populations in GCs and shed light on the structural properties emerging at
the end of this phase. The newly born SG groups have different concentrations,
with more extreme SG stars more centrally concentrated than those with less
extreme chemical abundances. The very high density of the SG subsystems implies
that SG massive stars, if formed, might suffer frequent close encounters,
collisions and gas stripping, thus possibly contributing further gas to the SG
formation.
