MOLECULAR EQUILIBRIA AND THE LUMINOSITY CLASSIFICATION OF G-STARS AND K-STARS

Two accompanying papers have used synthetic spectra to discuss the calibration of CO band indices and the C(41-42), C(42-45) and C(45-48), C(42-45) diagrams of the DDO photometric system. The band passes of the latter system are strongly influenced by lines of CN, CH, MgH, and SiH, with the SiH lines affecting the 41 pass band in dwarfs. Simple formulas which make predictions of the strengths of these lines are discussed. The results of these formulas are compared with approximate calculations of the ratio of line to continuous absorption coefficients in the stellar atmospheres. Some discussion of the depth dependence of the different molecular abundances is also given. The luminosity dependence of the CO bands is shown to be a consequence of the lower continuous absorption coefficient in giant star atmospheres as compared to dwarfs. The lesser depletion of carbon by CO formation and lower continuous opacity in giants with T(eff) < 4500 K leads to them having stronger CN and CH bands than the corresponding dwarfs. The need to include an enhanced nitrogen abundance to explain the observed CN band strengths in the hotter giants can be understood. The relative importance of CH as a continuous opacity source is considered. The SiH lines are shown to vary strongly with T(eff) and log g, in a manner similar to that of the MgH lines.