Cool luminous stars: the hybrid nature of their infrared spectra ^*,**

Institute of Astronomy, School of Sciences, The University of Tokyo, Mitaka, Tokyo 181-0015, Japan e-mail: ttsuji@ioa.s.u-tokyo.ac.jp

Received: 29 March 2008
Accepted: 27 May 2008

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 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 . 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  are contaminated by the contribution from outer molecular layers. The less strong but saturated lines of (intermediate-strength lines) cannot be understood at all with the abundance and turbulent velocity based on the weak lines. By studying the behavior of 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 show that the intermediate-strength lines are essentially identical to 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 dominate the observed infrared spectra, are affected badly by contamination from the MOLsphere. For this reason, they provide little information about the photosphere, but instead can be important 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 consider their hybrid nature.