Recent observational studies of \omega Centauri by Hubble Space Telescope have discovered a double main sequence in the color magnitude diagrams ( CMDs ) of its stellar populations .
The stellar population with the blue main sequence ( bMS ) is observationally suggested to have a helium abundance much larger , by \Delta Y \sim 0.12 , than that of the red main sequence ( rMS ) .
By using somewhat idealized models in which stars of the bMS are formed from gas ejected from those of the rMS , we quantitatively investigate whether the helium overabundance of the bMS can result from self-enrichment from massive AGB stars , from mass loss of very massive young stars , or from type II supernovae within \omega Cen .
We show that as long as the helium enrichment is due to ejecta from the rMS formed earlier than the bMS , none of the above three enrichment scenarios can explain the observed properties of the bMS self-consistently for reasonable IMFs .
The common , serious problem in all cases is that the observed number fraction of the bMS can not be explained without assuming unusually top-heavy IMFs .
This failure of the self-enrichment scenarios implies that most of the helium-enriched gas necessary for the formation of the bMS originated from other external sources .
We thus suggest a new scenario that most of the second generation of stars ( i.e. , the bMS ) in \omega Cen could be formed from gas ejected from field stellar populations that surrounded \omega Cen when it was a nucleus of an ancient dwarf galaxy .