On the Formation of Multiple Stellar Populations in Globular Clusters
  Nearly all globular clusters (GCs) studied to date show evidence for multiple
stellar populations, in stark contrast to the conventional view that GCs are a
mono-metallic, coeval population of stars. Building on earlier work, we propose
a simple physical model for the early evolution (several 10^8 yr) of GCs. We
consider the effects of stellar mass-loss, type II and prompt type Ia
supernovae, ram pressure, and accretion from the ambient ISM on the development
of a young GC's own gas reservoir. In our model, type II SNe from a first
generation of star formation clears the GC of its initial gas reservoir. Over
the next several 10^8 yr, mass lost from AGB stars and matter accreted from the
ambient ISM collect at the center of the GC. This material must remain quite
cool (T~10^2K), but does not catastrophically cool on a crossing time because
of the high Lyman-Werner flux density in young GCs. The collection of gas
within the GC must compete with ram pressure from the ambient ISM. After
several 10^8 yr, the Lyman-Werner photon flux density drops by more than three
orders of magnitude, allowing molecular hydrogen and then stars to form. After
this second generation of star formation, type II SNe from the second
generation and then prompt type Ia SNe associated with the first generation
maintain a gas-free GC, thereby ending the cycle of star formation events. Our
model makes clear predictions for the presence or absence of multiple stellar
populations within GCs as a function of GC mass and formation environment.
Analyzing intermediate-age LMC clusters, we find evidence for a mass threshold
of ~10^4 Msun below which LMC clusters appear to be truly coeval. This
threshold mass is consistent with our predictions for the mass at which ram
pressure is capable of clearing gas from clusters in the LMC at the present
epoch. (ABRIDGED)
