Atomic diffusion and mixing in old stars. IV. Weak abundance trends in the globular cluster NGC 6752

Context. Atomic diffusion in stars can create systematic trends of surface abundances with evolutionary stage. Globular clusters offer useful laboratories to put observational constraints on this theory as one needs to compare abundances in unevolved and evolved stars, all drawn from the same stellar population.
Aims: Atomic diffusion and additional mixing has been shown to be at work in the globular cluster NGC 6397 at a metallicity of [Fe/H] ~ -2.1. We investigate possible abundance trends in Li, Mg, Ca, Ti, Sc, and Fe with evolutionary stage in another globular cluster NGC 6752 at a metallicity of [Fe/H] ~ -1.6. This in order to better constrain stellar structure models including atomic diffusion and additional mixing.
Methods: We performed a differential abundance analysis on VLT/FLAMES-UVES data of 16 stars in four groups between the turnoff point and the red giant branch. Continuum normalisation of the stellar spectra was performed in an automated way using DAOSPEC. Differential abundances relative to the sun were derived by fitting synthetic spectra to individual lines in the stellar spectrum.
Results: We find weak systematic abundance trends with evolutionary phase for Fe, Sc, Ti, and Ca. The individual trends are weaker than the trends in NGC 6397 and only significant at the 1-σ level. However, the combined trend shows a significance on the 2-σ level. The trends are best explained by stellar-structure models including atomic diffusion with more efficient additional mixing than needed in NGC 6397. The model allows to correct for sub-primordial stellar lithium abundances of the stars on the Spite plateau.
Conclusions: Abundance trends for groups of elements, differently affected by atomic diffusion and additional mixing, are identified. Although the significance of the trends is weak, they all seem to indicate that atomic diffusion is operational along the evolutionary sequence of NGC 6752. The trends are weaker than those observed in NGC 6397, which is perhaps due to more efficient mixing. Using models of atomic diffusion including efficient additional mixing, we find a diffusion-corrected primordial lithium abundance of log ɛ(Li) = 2.58 ± 0.10, in agreement with WMAP-calibrated Big-Bang nucleosynthesis predictions within the mutual 1-σ uncertainties.

Based on data collected at the ESO telescopes under programmes 079.D-0645 (A) and 81.D-0253 (A).Tables 7 and 8 are available in electronic form at http://www.aanda.org