Molecular Band Intensities in G and K Stars

Abstract

Intensities of CO, CH, C2, and CN bands are studied for G and K stars with various surface gravities and metal content, assuming different C, N, and 0 compositions. Atmospheric models are based on a scaled, solar T(r) relation A simple method predicts the molecular intensities accurately, and it is therefore not necessary to work out the equilibria of all possible C, N, and 0 compounds in detail. The dependences of the computed band intensities on the metal content, Teff, and g are discussed and found to be in agreement with the observations. The possibilities of determining CNO abundances from ob- served band intensities are also discussed. The molecular equilibria are dominated by the association of CO, which is complete for stars later than KO. Two independent spectral features are required to deter- mine the C and 0 abundances, like the G-band and an [Oi] line in oxygen-rich giants. In oxygen-rich dwarfs, there is oniy one easily observable feature, the G-band, which yields a relation between 0 and C. In carbon-rich stars the C2 and CH intensities are not independent since each is a function of the abun- dance difference between carbon and oxygen. For oxygen-rich dwarfs and carbon-rich stars the second datum required to derive 0 and C can only be obtained from infrared CO observations. The nitrogen abundance may be determined from CN bands for carbon-rich stars, if the C - 0 abundance difference is known, and for oxygen-rich stars, if both the carbon and the oxygen abundances are known. The division of late-type stars into oxygen-rich and carbon-rich is extremely sharp; differences of more than 1 per cent from the mathematical equality 0 = C places a star in either class. All bands of C and N compounds are formed near r 0.1 in carbon-rich stars and, except for CO, near r 1 or 2 in oxygen-rich stars. Especially in the latter stars, the optical depth of formation (r) is a function of the CNO composition, and since the carbon depletion is a critical function of both r and the CNO composi- tion, an a priori assumption of (r) in determining abundances may lead to erroneous results