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The elemental abundance patterns of globular cluster stars exhibit
a rich (and often bewildering) variety. Very large star-to-star and
cluster-to-cluster abundance differences are the rule, not the exception,
and every element group seems to participate in the scatter.
The light elements of the periodic table (those elements with atomic
numbers 13 or less) have the most obvious variations. The abundances of
elements C, N, O, Na, Mg, and Al can be different in otherwise very
similar giant stars of a single globular cluster.
Fortunately, there is some order among the elemental chaos: the
abundances of these light elements do not vary at random, but instead
are interlinked in revealing correlations and anti-correlations.
Almost all cluster giants have very low contents of C, and
carbon isotopic ratios near 4-5. These two facts point to the
presence of CN-cyle hydrogen fusion in the interiors of these giants,
accompanied by efficient envelope convection. But some stars
will have relatively large abundances of O and Mg and small abundances
of Na and Al, while other stars will show the opposite abundance pattern.
A striking example is the O-Na anticorrelation plot shown in the
diagram here. A simple nucleosynthetic scenario covers most of the observed light element variations: very high temperature (up to 80 million degrees Kelvin) hydrogen fusion cycles. Thus, at the same high interior temperatures in which O can be converted to N in the ON-cycle hydrogen burning, spectroscopically undetectable) Ne can and will be turned into Na The anticorrelation between O and Na therefore occurs quite naturally, and the difference between high O, low Na stars and low O, high Na stars is that the latter stars have had a combination of higher interior temperatures and better convective envelope mixing than have the former stars. Or maybe each star in a globular cluster was born with different amounts of the various light elements. This possibility suggests that earlier generations of cluster stars (now dead) seeded the intra-cluster medium with their elemental output, and the presently observed stars gathered in this material at birth. We do not know yet the relative roles played by hydrogen ("proton-capture") nucleosynthesis in the stars we see now versus what happened in earlier times in the clusters. This current puzzle is being pursued by our group at Texas, with major collaborative efforts by astronomers at Lick, Kitt Peak, and Padova Observatories. For a recent review of our research in this area, and indications of other element groups that have unusual abundance patterns in globular clusters, please see "Abundances in Globular Clusters" (C. Sneden, 1999, Ap. & Sp. Sci., v. 265, p. 145). |
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