2002A&A...384..965B

We present spectra covering the wavelength range 2.28 to 2.36µm at a resolution of Δλ=0.0007µm (or R=3500) for a sample of 24 cool evolved stars. The sample comprises 8 M supergiants, 5 M giants, 3 S stars, 6 carbon stars, and 2 RV Tauri variables. The wavelengths covered include the main parts of the ¹²C¹⁶O v=2-0 and 3-1 overtone bands, as well as the v=4-2 and ¹³CO v=2-0 bandhead regions. CO lines dominate the spectrum for all the stars observed, and at this resolution most of the observed features can be identified with individual CO R- or P-branch lines or blends. The observed transitions arise from a wide range of energy levels extending from the ground state to E/k>20000K. We looked for correlations between the intensities of various CO absorption line features and other stellar properties, including IR colors and mass loss rates. Two useful CO line features are the v=2-0 R14 line, and the CO v=2-0 bandhead. The intensity of the 2-0 bandhead shows a trend with K-[12] color such that the reddest stars (K-[12]>3mag) exhibit a wide range in 2-0 bandhead depth, while the least reddened have the deepest 2-0 bandheads, with a small range of variation from star to star. Gas mass loss rates for both the AGB stars and the red supergiants in our sample correlate with the K-[12] color, consistent with other studies. The data imply that stars with {dot}(M)_gas<5x10^–7M_☉/y exhibit a much narrower range in the relative strengths of CO 2-0 band features than stars with higher mass loss rates. The range in observed spectral properties implies that there are significant differences in atmospheric structure among the stars in this sample.