SIMBAD references

The origin of a double main sequence (MS) in ω Centauri is explored. We have shown from theoretical calculations of stellar evolution that the colors of MS stars are shifted to those of the observed blue MS if the surface layers are polluted by He-rich materials with a mass of ∼0.1 M_☉. Stars are thought to be polluted through numerous encounters with the ejecta descended from massive asymptotic giant branch (AGB) stars. Two populations of stars with different kinematics observed especially in ω Cen indicate that kinematically cooler stars are more polluted through encounters with AGB ejecta than kinematically hotter ones because the accretion rate is inversely proportional to the cube of the relative velocity. We propose that both of these factors split the MS in ω Cen. This theoretical scheme explains why only ω Cen exhibits a double MS and matches the amount of He necessary to produce the blue MS to that supplied from massive AGB stars. Furthermore, we predict that even if globular clusters (GCs) possess only one generation of stars, the velocity dispersion of stars broadens the MS in the color-magnitude diagram as long as the GCs are massive enough to retain the AGB ejecta after the burst of star formation. This view explains the broad MS recently found in the GC NGC 2808, which exhibits no scatter in [Fe/H] and thus is likely to consist of a single generation of stars, unlike the case of ω Cen.