While chemical composition spreads are now believed to be a universal
characteristic of globular clusters (GCs), not all of them present multiple
populations in their color-magnitude diagrams (CMDs). Here we present a new
scenario for the formation of GCs, in an attempt to qualitatively explain this
otherwise intriguing observational framework. Our scenario divides GCs into
three groups, depending on the initial mass (M_I) of the progenitor structure
(PS), as follows. i) Massive PSs can retain the gas ejected by massive stars,
including the ejecta of core-collapse SNe. ii) Intermediate-mass PSs can retain
at least a fraction of the fast winds of massive stars, but none of the
core-collapse SNe ejecta. iii) Low-mass PSs can only retain the slow winds of
intermediate-mass stars. Members of the first group would include omega
Centauri (NGC 5139), M54 (NGC 6715), M22 (NGC 6656), and Terzan 5, whereas NGC
2808 (and possibly NGC 2419) would be members of the second group. The
remaining GCs which only present a spread in light elements, such as O and Na,
would be members of the third group. According to our scenario, the different
components in omega Cen should not display a sizeable spread in age. We argue
that this is consistent with the available observations. We give other simple
arguments in favor of our scenario, which can be described in terms of two main
analytical relations: i) Between the actual observed ratio between first and
second generation stars (R_SG^FG) and the fraction of first generation stars
that have been lost by the GC (S_L); and ii) Between S_L and M_I. We also
suggest a series of future improvements and empirical tests that may help
decide whether the proposed scenario properly describes the chemical evolution
of GCs.Comment: Accepted for publication in Astronomy and Astrophysic
