SIMBAD references

Recent observations of the globular cluster ω Centauri have shown that it has a double main sequence (MS), with a minority population of bluer and fainter MS well separated from a majority population of MS stars. Here we confirm that this special feature can only be reproduced by assuming a large range of helium abundance among several distinct populations in this cluster. We further show that the same helium enhancement required to reproduce this special feature on the MS can by itself reproduce the extreme horizontal-branch (EHB) stars observed in ω Cen, which are hotter than and separated from the majority population of normal HB stars. We also discuss the possibility that similar phenomena observed in the HB of other globular clusters are also due to the helium enhancement. If confirmed by further observations, this would establish that the third parameter that influences globular cluster color-magnitude diagram (CMD) morphology, in addition to metallicity and age, is indeed helium abundance.