Letter to the Editor

Abundances of RGB stars in NGC 6752 ^*,**

¹ Institute of Physics and Astronomy, Aarhus University, Ny Munkegade, 8000 Aarhus C, Denmark e-mail: fgj, pen@ifa.au.dk
² University of Wisconsin Oshkosh, Oshkosh, WI 54901, USA e-mail: mike@maxwell.phys.uwosh.edu
³ Lunds Observatory, Box 43, 221 00 Lund, Sweden e-mail: sofia@astro.lu.se

Corresponding author: F. Grundahl, fgj@ifa.au.dk

Received: 29 January 2002
Accepted: 17 February 2002

Abstract

Abundances of O, Fe, Al, Mg, Na and Li determined from high-resolution and high signal–to–noise spectra of 21 red giant branch (RGB) stars in the globular cluster NGC 6752 are presented. We demonstrate that the Strömgren c₁ index correlates extremely well with the measured NH, CH and CN indices and that variations in c₁ are due to differences in UV NH band strengths. As shown by Grundahl et al. ([CITE]), the RGB stars in all 20 globular clusters surveyed possess large star-to-star variations in their c₁ index, which we interpret as significant inhomogeneities in N abundances. The well known relations among O and Na and Mg and Al as well as with CN, CH, and NH are also present in these stars, some of which are fainter than the RGB bump. We find that the Li (6708 Å) equivalent width becomes too small to be measured as the luminosity of the stars increase above the luminosity of the RGB bump. Taken together, these data enforce the results of Gratton et al. ([CITE]) for turnoff and sub–giant branch stars, and suggest that in this cluster the abundance correlations observed among the brighter giants are in place prior to appreciable RGB ascent and likely due to previous generation(s) of stars, rather than due to mixing in the observed stars.