  Star-to-star abundance variations of C, N, O, Na and Al in globular-cluster
red giants have been recently supplemented by the finding that [Mg/Fe] is
depleted in stars with extremely large [Al/Fe] (Shetrone 1996a). This and other
new spectroscopic results allow us to test current models of stellar evolution
and nucleosynthesis, as well as those of the formation and chemical enrichment
of globular clusters. In an effort to explain self-consistently these
observations we have considered two possibilities: (1) a deep mixing scenario
which assumes that in red giants some kind of (extra)mixing transports products
of nuclear reactions from the hydrogen burning shell (HBS) to the base of the
convective envelope; and (2) a combination of primordial and deep mixing
scenarios. It is shown that (1) cannot account for the anticorrelation of
[Mg/Fe] vs. [Al/Fe] without additional ad hoc assumptions, among which we
identify a strong but still undetected low energy resonance in the reaction
24Mg(p,gamma)25Al, and episodical increases of the HBS temperature up to the
value T approx. 74x10^6 K. In (2) intermediate mass AGB stars are assumed to
produce the decreased 24Mg and increased 25Mg initial abundances in some
globular-cluster low mass stars and Al is synthesized at the expense of 25Mg in
the HBS and transported to the surface of the red giant by extramixing. We
discuss advantages and deficiencies of both scenarios and propose some
observational tests.
