D'Antona, Francesca; Stetson, Peter B; Ventura, P; Milone, Antonino; Piotto, G; Caloi, V
The colour-magnitude diagram (CMD) of NGC 1851 presents two subgiant branches (SGBs), probably due to the presence of two populations differing in total C+N+O (carbon+nitrogen+oxygen) content. We test the idea that a difference in total C+N+O may simulate an age difference when comparing the CMD of clusters to derive relative ages. We compare NGC 1851 with NGC 6121 (M4), a cluster of very similar [Fe/H]. We find that, with a suitable shift of the CMDs that brings the two red horizontal branches...[Show more]
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