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Paper: Atomic Diffusion in Old Stars – Helium, Lithium, and Heavy Elements
Volume: 384, 14th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun
Page: 33
Authors: Korn, A.J.; the ADIOS team
Abstract: We present strong evidence from high-resolution observations of stars in NGC 6397 that the chemical abundances in the atmospheres of unevolved halo stars are systematically affected by atomic diffusion. The abundance trends identified are in good agreement with models of stellar evolution, if effects due to gravitational settling, radiative levitation and turbulent mixing below the outer convection zone are allowed for. An element-specific diffusion signature is identified for a variety of elements (Li, Mg, Ca, Ti and Fe), for the first time in this class of stars. The observed abundance trends empirically constrain the efficiency of turbulent mixing and the amount of helium settling.

This work has far-reaching implications for the interpretation of chemical abundances and age determinations of unevolved halo stars. In particular, the lithium abundance measured in warm halo stars must be corrected upwards by about a factor to two, bringing it into much better agreement with WMAP-based predictions of Big-Bang nucleosynthesis. Also, absolute isochrone ages of field turnoff and subgiant stars will require systematic downward revisions.

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