Secondary star formation within massive star clusters: Origin of
  multiple stellar populations in globular clusters
  We numerically investigate whether and how gaseous ejecta from AGB stars can
be converted into new stars within originally massive star clusters (MSCs) in
order to understand the origin of multiple stellar populations in globular
clusters (GCs). We adopt a scenario in which (i) MSCs with masses of M_s can be
formed from high-mass, high-density giant molecular clouds (GMCs) in their host
galactic building blocks embedded in dark matter halos at high redshifts and
(ii) their evolution therefore can be significantly influenced by M_s, their
initial locations, and physical properties of their hosts. Our 3D
hydrodynamical simulations show that gaseous ejecta from AGB stars can be
retained within MSCs and consequently converted into new stars very efficiently
in the central regions of MSCs, only if M_s exceed a threshold mass (M_th) of
~10^6 M_sun. The new stars can correspond to the ``second generation (SG)'' of
stars with higher Na and lower O abundances observed in GCs. Star formation
efficiencies during the formation of SG stars within MSCs with M_s > M_th can
be rather high (0.3-0.9) so that very compact new clusters within original MSCs
can be formed. M_s should be as large as 10^6-10^7 M_sun to explain the
observed large fraction of SG stars in the present ordinary Galactic GCs,
because new stars can consist of only 1-4% among all stars for the standard
IMF. Nuclear MSCs are found to retain much more effectively the AGB ejecta and
convert more efficiently the gas into new stars owing to much deeper
gravitational potential of their hosts. We suggest that both M_s and their
locations within their hosts can determine whether abundance spread can be seen
only in light elements or even in heavy ones.
