THE PUZZLING MGAL ANTICORRELATION IN GLOBULAR-CLUSTER RED GIANTS - PRIMORDIAL PLUS DEEP MIXING SCENARIO

Star-to-star abundance variations of C, N, O, Na and Al in globular-cl uster red giants have been recently supplemented by the finding that [ Mg/Fe] is depleted in stars with extremely large [Al/Fe] (Shetrone 199 6a). To find out which of the magnesium isotopes is responsible for th e observed depletion of [Mg/Fe] Shetrone (1996b) also undertook an iso topic analysis of Mg and found that it is Mg-24 which is depleted in A l-rich giants. On the other hand, Norris & Da Costa (1995) demonstrate d that even in the massive globular cluster omega Cen which has intrin sic spreads in both [Fe/H] and the abundances of the s-process element s, [O/Fe] anticorrelates with [Na/Fe] and [Al/Fe] as in ''normal'' mon ometallic clusters. These new spectroscopic results allow us to test c urrent models of stellar evolution and nucleosynthesis, as well as tho se of the formation and chemical enrichment of globular clusters. In a n effort to explain self-consistently these observations we have consi dered two possibilities: (1) a deep mixing scenario which assumes that in red giants some kind of (extra)mixing transports products of nucle ar reactions from the hydrogen burning shell (HBS) to the base of the convective envelope; and (2) a combination of primordial and deep mixi ng scenarios. It is shown that (1) cannot account for the anticorrelat ion of [Mg/Fe] vs. [Al/Fe] without additional ad hoc assumptions, amon g which we identify a strong but still undetected low energy resonance in the reaction Mg-24(p,gamma)Al-25, and episodical increases of the HBS temperature up to the value T approximate to 7410(6) K. In (2) int ermediate mass AGE stars are assumed to produce the decreased Mg-24 an d increased Mg-25 initial abundances in some globular-cluster law mass stars and Al is synthesized at the expense of Mg-25 in the HBS and tr ansported to the surface of the red giant by extramixing. We discuss a dvantages and deficiencies of both scenarios and propose some observat ional tests.