Computat ion of Frequencies and Linestrengths for Triatomic Molecules of Astronomical Interest

Molecular bands have been used as a diagnostic of spectral type since the 1860's. TiO was first identified as the dominant feature in optical spectra of cool giants in 1904 (Fowler 1904). Many diatomic molecules have now been included (e.g. J0rgensen & Larsson 1990) in detailed calculations of stellar opacity and attention is starting to focus on the incorporation of accurate data for triatomics. Triatomic molecules have a much greater density of states than diatomics; bending vibrational bands-generally lower in frequency than bond-stretching modes-may contribute considerably to "filling in" spectral gaps. From the point of view of quantum chemistry, molecules can access regions which would be described classically as "chaotic" at the temperatures (usually a few thousand degrees) available in cool stellar atmospheres. Molecular transitions associated with such behaviour are to be found, in particular, in the infrared. Astronomically the study of this spectral region has recently benefitted from high resolution, sensitive spectrometers which are now available on many telescopes. Thus there is mutual advantage both for chemical physicists and for astronomers in the sharing of observational and theoretical data for cool star atmospheres. The dominant red and infrared luminosity of the underlying stellar populations of galaxies is from M giants. The dominant population in terms of number density is from M dwarfs. These M stars emit the bulk of their radiation in the red and infrared where molecules provide the dominant source of opacity in the stellar atmosphere. Detailed understanding of the evolution of M stars is compromised by the inability to construct model atmospheres without quantitative data for triatomic molecules. Two triatomic molecules are of the utmost importance in stellar atmospheres. For cool O-rich M stars, particularly the M dwarfs, the water molecule is the dominant source of opacity in the infrared (Alexander et al. 1989). For C-rich red giant stars, HCN plays an important role (3Orgensen et al. 1985). Accurate potential energy surfaces are available for both molecules. For water, we have tested four electronic ground-state potential enel;gy surfaces which have been produced in recent years