Atomic diffusion and mixing in old stars

IV. Weak abundance trends in the globular cluster NGC 6752^⋆,⋆⋆

¹ Department of Physics and Astronomy, Division of Astronomy and Space Physics, Uppsala University, Box 516, 75120 Uppsala, Sweden
e-mail: pieter.gruyters@physics.uu.se
² Stellar Astrophysics Centre, Department of Physics and Astronomy, Århus University, Ny Munkegade 120, 8000 Århus C, Denmark
³ LUPM, Université Montpellier II, CNRS, CC072, Place E. Bataillon, 34095 Montpellier Cedex, France
⁴ Institute of Astronomy, Russian Academy of Science, 119017 Moscow, Russia
⁵ Research School of Astronomy & Astrophysics, Cotter Road, Weston ACT 2611, Australia
⁶ Centre for Star and Planet Formation, Natural History Museum of Denmark / Niels Bohr Institute, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen, Denmark

Received: 29 November 2012
Accepted: 3 May 2013

Abstract

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 ε(Li) = 2.58 ± 0.10, in agreement with WMAP-calibrated Big-Bang nucleosynthesis predictions within the mutual 1-σ uncertainties.