Abundances of RGB Stars in the Globular Cluster NGC 6752

Författare

M. M. Briley
F. Grundahl
P. E. Nissen
Sofia Feltzing

Summary, in English

Abundances of O, Fe, Al, Mg, Na and Li determined from high-resolutionand high signal-to-noise VLT spectra from the atmospheric UV cutoff to6800Å of 21 red giant branch (RGB) stars in the globular clusterNGC 6752 are presented. We demonstrate that the previously observedstar-to-star scatter in the Strömgren c<SUB>1</SUB> index is due todifferences in uv NH band strengths caused by significant variations inN abundances. The c<SUB>1</SUB> values also correlate extremely wellwith the measured CN indicies and anticorrelate with CH. Furthermore, weshow that the well known relations between O - Na and Mg - Al (as wellas with CN, CH, and NH) are also present in these stars, some of whichare fainter than the RGB luminosity function bump. We find that the Li(6708Å) equivalent width becomes too small to be measured as theluminosity of the stars increases above the luminosity of the RGB bump,which we interpret as the onset of deep mixng. Taken together, thesedata enforce the results of Gratton et al. (2001, A&A 369, 87) forturnoff and sub--giant branch stars, and suggest that in this clusterthe bulk of the abundance correlations observed among the brightergiants are in place prior to appreciable RGB ascent and likely due to a''primordial'' source, rather than due to mixing. Based on observationsobtained with the ESO Very Large Telescope. Partial support provided bythe NSF under grant AST-0098489 (to MMB).

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