Chemical abundances as population tracers
  Elemental abundance ratios as tracers of stellar populations are discussed with emphasis on F, G, and K stars providing a `fossil' record of the chemical evolution of the Galaxy. Most abundance studies have been based on homogeneous 1D model atmospheres and the assumption of local thermodynamic equilibrium (LTE), but recent works have shown that 3D non-LTE corrections can change the derived trends of abundance ratios as a function of stellar metallicity very significantly. However, when comparing stars having similar effective temperatures, surface gravities, and metallicities, 3D non-LTE corrections tend to cancel out. When applying such a differential approach to stars in the Galactic disk, bulge, and halo, abundance ratios like C/O, Na/Fe, alpha/Fe, Cu/Fe, Ba/Y, and Eu/Ba point to the existence of multiple discrete populations in each of these Galactic components. 
Poul Erik Nissen
astro-ph.GA astro-ph.SR