3D hydrodynamical model atmospheres of metal-poor stars - Evidence for a low primordial Li abundance

Realistic 3-dimensional (3D), radiative hydrodynamical surface convection s imulations of the metal-poor halo stars HD 140283 and HD 84937 have been pe rformed. Due to the dominance of adiabatic cooling over radiative heating v ery low atmospheric temperatures are encountered. The lack of spectral line s in these metal-poor stars thus causes much steeper temperature gradients than in classical 1D hydrostatic model atmospheres where the temperature of the optically thin layers is determined by radiative equilibrium. The modi fied atmospheric structures cause changes in the emergent stellar spectra. In particular, the primordial Li abundances may have been overestimated by 0.2-0.35 dex with 1D model atmospheres. However, we caution that our result assumes local thermodynamic equilibrium (LTE), while the steep temperature gradients may be prone to e.g, over-ionization.