FTS Observation of CO Fundamental Bands in Cool Stellar Atmospheres

The infrared ro-vibration spectrum of carbon monoxide provides a powerful observational diagnostic for the investigation of late-type stellar atmospheres. At temperatures of several thousands of degrees, CO vibrational states up to v=7 and rotational states up to j≈100 are populated. The corresponding lines are formed over, and therefore probe, a large range in altitude. The strongest fundamental lines (Δv=1) originate in the upper photosphere and in the chromosphere, regions which are only inadequately described by present stellar atmosphere theories. Numerical simulations have shown that CO affects the stellar atmospheric structure through cooling in optically thin lines (e.g. Johnson, 1973). CO observations are important, because the model predictions can be tested reliably only from observations of species that are intimately involved. This is particularly true in view of the dilemma posed by the contradicting results of 'conventional' chromospheric diagnostics and first CO Δv=1 observations on the Sun (Ayres and Testerman, 1981) and Arcturus (Heasley et al., 1978). The present study has been conducted to establish CO fundamental bands as an observational diagnostic for the higher layers of cool stellar atmospheres.