Cool luminous stars : the hybrid nature of their infrared spectra ⋆

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 1 D model photospheres. Results. We found purely empirically that there is a critical value of log W/ν ≈ −4.75 (W is the equivalent width and ν 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/ν <∼ −4.75. The carbon, oxygen, and their isotopic abundances in 23 K M giant stars obtained from such weak lines are roughly consistent with the recent evolutionary models, although the 12C/13C puzzle remains unsolved. On the other hand, it is already known that the very strong lines of log W/ν > −4.4 are contaminated by the contribution from extra molecular layers. The less strong but saturated lines of −4.75 < log W/ν <∼ −4.4 (intermediate-strength lines) cannot be understood at all with the abundance and turbulent velocity based on the weak lines. From the behavior shown by these lines and considering other observations such as the detection of H2O lines, not only in the late M giants but also in the early M and K giants, we found that the intermediate-strength lines are essentially the same as the strong lines in that they also include contamination from the extra molecular layers. Thus the behavior of the intermediate-strength lines, including those with LEP as high as 2 eV, appears to be nothing but a manifestation of the warm molecular envelope or MOLsphere. Conclusions. The infrared spectra of K M giant stars are a hybrid of at least two components originating in the photosphere and MOLsphere. Only the weak lines, mostly high excitation, are relatively free from the effect of MOLsphere and can be used to extract photospheric abundances. The strong lines and the intermediate-strength lines which actually dominate the observed infrared spectra, are badly disturbed by contamination from the MOLsphere. For this reason, they provide little information about the photosphere, but instead can be new proves of the warm molecular envelope for which little is known yet. In the interpretation and analysis of the infrared spectra of K M giant stars, it is essential to keep their hybrid nature in mind.