Super-He-Rich populations in globular clusters

Recently observations of the color-magnitude diagrams (CMDs) of the massive globular cluster ω Centauri have shown that it has a striking double main sequence (MS). Here we confirm, with the most up-to-date Y² isochrones, that this special feature can only be reproduced by assuming a large variation (δ Y=0.15) of primordial helium abundance among several distinct populations in this cluster (Fig. 1). We further show that the same helium enhancement required for this special feature on the MS can by itself reproduce the extreme horizontal branch (HB) stars observed in ω Cen (Fig. 1). Similarly, the complex features on the HBs of other globular clusters, such as NGC 2808, NGC 6388 and NGC 6441, are explained by large internal variations of helium abundance (Fig. 2). The presence of super-helium-rich populations in some globular clusters suggests that a third parameter, other than metallicity and age, also influences the CMD morphology of these clusters.