... seventies

We refer to Gratton et al. (2004) and Sneden (2005) for extensive reviews of the very broad literature related to the chemical properties of GCs; see also references in Prantzos & Charbonnel (2006, hereafter PC06).

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... homogeneous

This is not the case of $\omega$ Cen which is not a typical GC but may be considered as the surviving remnant of a larger system (e.g. Smith 2004 and references therein).

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... stars

Note that the corresponding large star-to-star abundance variations have never been observed among field stars with comparable metallicity [Fe/H], age, and evolutionary status (Hanson et al. 1998; Gratton et al. 2000; Palacios et al. 2002; Mishenina et al. 2006). Environment, therefore, seems to have a crucial influence on the occurrence of these chemical patterns.

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... reactions

In low-mass turnoff stars the temperature is not high enough to allow the proton-captures reactions in the NeNa- and MgAl-chains as will be discussed later on in more details.

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...2005)

Carretta et al. (2007) present the largest sample of O and Na abundance determinations to date in NGC 6752. For seek of homogeneity we chose however to use the smaller sample of 2005 for which the abundances of C and N were simultaneously determined. This choice does not affect the present discussion.

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... material

Note however that Ventura & D'Antona (2002a) have managed to keep the total C+N+O abundance within a factor of $\sim$3 in their AGB models computed with the FST model for convection and with some "extra-mixing'' at the base of the stellar convective envelope.

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... rotation

By fast rotation we mean an initial rotation velocity such that the star reaches the so-called critical velocity early on the main sequence. As an example, this corresponds to 400 km s^-1 on the zero age main sequence for a 60 $M_{\odot}$ star.

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