OH indices continue to rise at the coolest temperatures for stars of higher luminosity. However, the magnitude (and temperature) of the maximum in the dwarf stars differs between the CO and OH indices, which we use in the next section to define a two-dimensional classification scheme for late–type stars. We note that the OH index begins to include a contribution from the stark–broadened 4-11 Brackett line at 5949 cm$^{-1}$ creating a secondary maxiumum in the strength of this index around 10,000 K in the dwarf stars.

While the temperature and luminosity dependence of the atomic features is readily understood through application of the Saha and Boltzman equations governing the population of the ionization states and energy levels respectively, the explanation behind the behavior of the molecular features is more subtle. Two possibilities for the factor of two enhancement in the molecular bands in the giants over the dwarfs have been explored in the literature. One attributes the luminosity dependence in the molecular features to differing microturbulence in the atmospheres of dwarfs and giants. The expectation is that larger microturbulence in the lower surface gravity giants effectively broadens the opacity of the feature over a larger frequency interval in these saturated 