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M. M. Briley (Univ. Wisc. Oshkosh), F. Grundahl, P. E. Nissen (Aarhus Univ.), S. Feltzing (Lund Univ.)
Abundances of O, Fe, Al, Mg, Na and Li determined from high-resolution and high signal-to-noise VLT spectra from the atmospheric UV cutoff to 6800Åof 21 red giant branch (RGB) stars in the globular cluster NGC 6752 are presented. We demonstrate that the previously observed star-to-star scatter in the Strömgren c1 index is due to differences in uv NH band strengths caused by significant variations in N abundances. The c1 values also correlate extremely well with the measured CN indicies and anticorrelate with CH.
Furthermore, we show that the well known relations between O - Na and Mg - Al (as well as with CN, CH, and NH) are also present in these stars, some of which are fainter than the RGB luminosity function bump. We find that the Li (6708Å) equivalent width becomes too small to be measured as the luminosity of the stars increases above the luminosity of the RGB bump, which we interpret as the onset of deep mixng. Taken together, these data enforce the results of Gratton et al. (2001, A&A 369, 87) for turnoff and sub--giant branch stars, and suggest that in this cluster the bulk of the abundance correlations observed among the brighter giants are in place prior to appreciable RGB ascent and likely due to a ''primordial'' source, rather than due to mixing.
Based on observations obtained with the ESO Very Large Telescope. Partial support provided by the NSF under grant AST-0098489 (to MMB).