Fig. 7. Abundance profiles of He, C, N and O, at T = 9 Gyr (dashed curves ) and T = 13.5 Gyr (solid curves ), obtained with the WW95 yields (thin curves , same as e.g. in Fig. 3), and the M92 yields (thick curves ), in the framework of the model presented in Sect. 2. It can be seen that, compared to the WW95 yields, the M92 yields lead to considerably different results for C, to slightly different results for N, and to quasi-identical results for He and O. In particular, the M92 yields may explain completely the carbon abundance profile, with no need for any contribution from intermediate mass stars; on the contrary, nitrogen observations suggest that another source is mandatory.