Only a bit more than 25 years ago, it
seemed possible to assume that all Galactic
globular clusters were chemically homogeneous.
There were indications that star-to-star Fe
abundance variations existed in ω Cen, but this
massive cluster appeared to be unique. Following
Osborn’s (1971) initial discovery, Zinn’s (1973)
observation that M92 asymptotic giant branch (AGB)
stars had weaker G-bands than subgiants with
equivalent temperatures provided the first
extensive evidence that there might be variations
in the abundances of the light elements in an
otherwise “normal” cluster. Since then
star-to-star variations in the abundances of C, N,
O, Na, Mg and Al have been observed in all cases
in which sample sizes have exceeded 5-10 stars,
e.g., in clusters such as M92, M15, M13, M3, ω
Cen, MIO and M5. Among giants in these clusters
one finds large surface O abundance differences,
and these are intimately related to differences of
other light element abundances, not only of C and
N, but also of Na, Mg and Al (cf. reviews by
Suntzeff 1993, Briley et al 1994, and Kraft 1994).
The abundances of Na and O, as well as Al and Mg,
are anticorrelated. Prime examples are found among
giants in M15 (Sneden et al 1997), M13
(Pilachowski et al 1996; Shetrone 1996a,b; and
Kraft et al 1997) and ω Cen (Norris & Da Costa
1995a,b).
These observed anticorrelations almost
certainly result from proton- capture chains that
convert C to N, 0 to N, Ne to Na and Mg to Al in
or near the hydrogen fusion layers of evolved
cluster stars. But which stars? An appealing idea
is that during the giant branch lifetimes of the
low-mass stars that we now observe, substantial
portions of the stellar envelopes have been cycled
through regions near the H-burning shell where
proton-capture nucleosynthesis can occur. This
so-called “evolutionary” scenario involving deep
envelope mixing in first ascent red giant branch
(RGB) stars has been studied by Denissenkov &
Denissenkova (1990), Langer & Hoffman (1995),
Cavallo et al (1996, 1997) and Langer et al
(1997). The mixing mechanism that brings
proton-capture products to the surface is poorly
understood (Denissenkov & Weiss 1996,
Denissenkov et al 1997, Langer et al 1997), but
deep mixing driven by angular momentum has been
suggested (Sweigart & Mengel 1979, Kraft 1994,
Langer & Hoffman 1995, Sweigart
1997).
On Low Mass AGB Stars