Atomic diffusion and mixing in old stars IV. Weak abundance trends in the globular cluster NGC6752

• Context. Atomic diffusion in stars can create systematic trends of surface abundances with evolutionary stage. Globular clusters off er 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 NGC6397 at a metallicity of [Fe/H] similar to -2.1. We investigate possible abundance trends in Li, Mg, Ca, Ti, Sc, and Fe with evolutionary stage in another globular cluster NGC6752 at a metallicity of [Fe/H] similar to -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 NGC6397 and only significant at the 1-sigma level. However, the combined trend shows a significance on the 2-sigma level. The trends are best explained by stellar-structure models including atomic diffusion with more efficient additional mixing than needed in NGC6397. 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 NGC6752. The trends are weaker than those observed in NGC6397, 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 epsilon(Li) = 2.58 +/- 0.10, in agreement with WMAP-calibrated Big-Bang nucleosynthesis predictions within the mutual 1-sigma uncertainties.

Nyckelord

stars: abundances

stars: atmospheres

stars: fundamental parameters

globular clusters: individual: NGC 6752

techniques: spectroscopic

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