Star-to-star abundance variations among bright giants in the metal-poor globular cluster M15

We find significant star-to-star variations in the abundances of both the light elements oxygen, sodium, magnesium, and aluminum, and the neutron capture elements barium and europium in a sample of eighteen bright M15 giants. The oxygen and sodium abundances are strongly anticorrelated as are the magnesium and aluminum abundances. Sodium and aluminum are correlated. Hence, there are M15 giants with large sodium and aluminum ''enhancements'' and smaller oxygen and magnesium ''depletions'' as well as giants with more ''normal'' abundances of these light elements. The correlations are very similar to those found earlier in M13, M10, and M92. The CNO abundance sum is approximately the same in the five M15 giants for which all three elements have been observed. This abundance pattern is most likely the result of proton-capture nucleosynthesis and most easily explained if it takes place within the stars that we have observed. The barium and europium variations are unlikely to be the result of nucleosynthesis within the stars that we have observed. They provide strong evidence for ''primordial'' neutron-capture abundance variations in a cluster other than omega Cen. The range in both [Eu/Fe] and [Ba/Fe] is about 0.6 dex. The abundances are strongly correlated and appear to fall into two groups, one with larger Eu and Ba abundances, the other with values of [Eu/Fe] and [Ba/Fe] that are about 0.35 dex smaller. The mean value of [Ba/Eu] is the same for both groups, and falls between the pure r-process value and the solar system r+s process mix. The light element abundances apparently do not correlate with the neutron-capture abundances even though both vary over a significant (often large) range. We find no evidence for star-to-star variations in either [Fe/H] or [Ca/Fe]. We have looked for small differences in [Fe/H] and [Ca/Fe] between stars with large and small values of [Eu/Fe] and [Ba/Fe]; the average values of [Fe/H] and [Ca/Fe] are the same for both. Hence, we find that there are large variations in the abundances of a number of elements (but not all of them) in M15, a cluster only recently thought to have a uniform composition. The abundances of the light elements (C, N, O, Na, Mg, and Al) are strongly correlated as are the abundances of the neutron-capture elements (Eu, Ba), but variations in the abundances of light and neutron-capture elements seem to be independent of each other, and both appear to be independent of the Fe and Ca abundances. (C) 1997 American Astronomical Society.