Old Stellar Populations. IV. Empirical Fitting Functions for Features in the Spectra of G and K Stars

Empirical fitting functions are derived for 11 atomic and molecular features prominent in the spectra of G and K stars. The fitting behavior closely mimics that expected from the physics of stellar atmospheres, except for the index CN (4150 A), for which only minimal evidence is found for traditional gravity effects but strong evidence for evolutionary mixing of CNO products to the stellar surface. This finding is important for modeling the integrated CN indices of external stellar populations.