  A very high resolution (R=200,000), high signal-to-noise ratio (S/N=340)
blue-green spectrum of the very metal-poor ([Fe/H]=-2.6) red giant star
HD122563 has been obtained by us at McDonald Observatory. We measure the
asymmetries and core wavelengths of a set of unblended FeI lines covering a
wide range of line strength. Line bisectors exhibit the characteristic C-shape
signature of surface convection (granulation) and they span from about 100 m/s
in the strongest FeI features to 800 m/s in the weakest ones. Core wavelength
shifts range from about -100 to -900 m/s, depending on line strength. In
general, larger blueshifts are observed in weaker lines, but there is
increasing scatter with increasing residual flux. Assuming local thermodynamic
equilibrium (LTE), we synthesize the same set of spectral lines using a
state-of-the-art three-dimensional hydrodynamic simulation for a stellar
atmosphere of fundamental parameters similar to those of HD122563. We find good
agreement between model predictions and observations. This allows us to infer
an absolute zero-point for the line shifts and radial velocity. Moreover, it
indicates that the structure and dynamics of the simulation are realistic, thus
providing support to previous claims of large 3D-LTE corrections, based on the
hydrodynamic model used here, to elemental abundances and fundamental
parameters of very metal-poor red giant stars obtained with standard 1D-LTE
spectroscopic analyses.
