Cool luminous stars: The hybrid nature of their infrared spectra

Abstract

Aims. We determine carbon, oxygen, and their isotopic abundances based on CO and OH spectra in 23 red giant stars, and identify possible origin of difficulty in abundance analysis of cool luminous stars. Methods. We apply the line-by-line analysis based on the classical micro-turbulent model and 1D model photospheres. Results. We found empirically that there is a critical value of log W/v ≈ -4.75(W is the equivalent width and the wavenumber) above which the observed lines do not follow the classical line formation theory based on the micro-turbulent model and that the classical abundance analysis can be applied only to the weak lines of log W/v ≤ -4.75. The carbon, oxygen, and their isotopic abundances in 23 K-M giant stars obtained from such weak lines are approximately consistent with evolutionary models, although the 12C/13C puzzle (observed 12C/13C ratios are too small compared with theoretical predictions) remains unsolved. It is already known that the strong lines of log W/v > -4.4 are contaminated by the contribution from outer molecular layers. The less strong but saturated lines of -4.75