The Early Evolution of Globular Clusters: The Case of NGC 2808

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Abstract
Enhancement and spread of helium among globular cluster stars have been recently suggested as a way to explain the horizontal-branch blue tails in those clusters that show a primordial spread in the abundances of C, N, O, and other elements involved in advanced CNO burning. Helium enhancement is unavoidable if the matter responsible for the abundance spreads is identified with the matter lost by massive asymptotic giant branch stars, which evolve during the early phases of globular cluster life, as we noted in 2002. In this paper we examine the implications of the hypothesis that, in many globular clusters, stars were born in two separate events: an initial burst (first generation), which gives origin to probably all high- and intermediate-mass stars and to a fraction of the cluster stars observed today, and a second, prolonged star formation phase (second generation), in which stars form directly from the ejecta of the intermediate-mass stars of the first generation. In particular, we consider in detail the morphology of the horizontal branch in NGC 2808 and argue that it unveils the early cluster evolution, from the birth of the first star generation to the end of the second phase of star formation. This framework provides a feasible interpretation for the still-unexplained dichotomy of the NGC 2808 horizontal branch, attributing the lack of stars in the RR Lyrae region to the gap in the helium content between the red clump, whose stars are considered to belong to the first stellar generation and have primordial helium, and the blue side of the horizontal branch, whose minimum helium content reflects the helium abundance in the smallest progenitor mass (~4 M☉) contributing to the second stellar generation. This scenario provides constraints on the required initial mass function, in such a way that a great many remnant neutron stars and stellar mass black holes might have been produced.
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