CARBON-MONOXIDE FUNDAMENTAL BANDS IN LATE-TYPE STARS .3. CHROMOSPHEREOR CO-MOSPHERE

The strong vibration-rotation lines of CO at 4.6 mum (DELTAv = 1) are unique diagnostics for the thermal conditions in the atmospheric altit ude range of late-type stars near and above the temperature minimum in chromospheric models. Exploiting recent improvements in IR instrument ation, we observed a number of cool stars with high spectral resolutio n (R almost-equal-to 100,000). The analysis of the spectra was based o n an earlier theoretical study which had established CO DELTAv = 1 non -LTE spectra as useful probes for stars of spectral type F, G, and K w ith log g greater-than-or-equal-to 1. No direct chromospheric indicato rs were detected in the CO spectra. Stellar boundary (CO) temperatures were determined for the program stars and temperature profiles were c onstructed for alpha Tau, alpha Boo, beta Gem and beta Dra. The CO-bas ed models feature a steady decrease in temperature at the height where the temperature increases in chromospheric models. Further comparison with chromospheric indicators shows an increasing discrepancy between the temperatures determined from CO measurements and those predicted from radiative equilibrium models, respectively, with increasing chrom ospheric activity. Thermal bifurcation of the stellar surfaces is prop osed to reconcile the contradicting scenarios derived based on differe nt spectral diagnostics.