Explaining the multiple populations in globular clusters by multiple
  episodes of star formation and enrichment without gas expulsion from massive
  star feedback
  In order to investigate the origin of multiple stellar populations found in
globular clusters (GCs) in the halo and bulge of the Milky Way, we have
constructed chemical evolution models for their putative low-mass progenitors.
In light of recent theoretical developments, we assume that supernova blast
waves undergo blowout without expelling the pre-enriched ambient gas, while
relatively slow winds of massive stars, together with the winds and ejecta from
low to high mass asymptotic-giant-branch stars, are all locally retained in
these less massive systems. Interestingly, we find that the observed Na-O
anti-correlations in metal-poor GCs can be reproduced when multiple episodes of
starburst and enrichment are allowed to continue in these subsystems. A
specific form of star formation history with decreasing time intervals between
the successive stellar generations, however, is required to obtain this result,
which is in good agreement with the parameters obtained from synthetic
horizontal-branch models. The "mass budget problem" is also much alleviated by
our models without ad-hoc assumptions on star formation efficiency, initial
mass function, and the preferential loss of first-generation stars. We also
applied these models to investigate the origin of super-He-rich red clump stars
in the metal-rich bulge suggested by Lee et al. (2015). We find that chemical
enrichment by the winds of massive stars can naturally reproduce the required
strong He enhancement in metal-rich subsystems. Our results further underscore
that gas expulsion or retention is a key factor in understanding the multiple
populations in GCs.
