HORIZONTAL-BRANCH MODELS AND THE 2ND-PARAMETER PHENOMENON .2. THE CASE OF M-13 AND M-3

The problem of the age difference between the classical second-paramet er clusters M 3 and M 13 is revisited, using the ''Delta V'' and the h orizontal-branch (HE) morphology methods of age derivation. Our new ap plication of the Delta V method makes use of the colour-magnitude diag ram recently obtained by Guarnieri et al. (1993) for M 13. Besides, th e ambiguity in the determination of the HE level for M 13, due to a sc arcely populated instability strip, is minorated through computation o f detailed synthetic HE models. The method suggests that the two clust ers do not have a large age difference, with an upper limit of Delta t (M 13 - M 3) similar or equal to 3 Gyr. This result, which agrees with the independent analysis of VandenBerg et al. (1990), is shown to be in conflict with the ''age-only'' interpretation of the second-paramet er phenomenon, given that larger age differences appear necessary to m atch the observed HE morphologies, if age is the second parameter in t he halo. Evidence is presented that the super oxygen-poor stars found by Kraft et al. (1992, 1993) and others to be present in the red giant branch of M 13 (blue HE morphology), but lacking in M 3 (intermediate HE morphology), may be related to an increased mass loss rate on the red giant branch, which could be an additional parameter affecting the HE morphology.