CARBON-MONOXIDE FUNDAMENTAL BANDS IN LATE-TYPE STARS .3. CHROMOSPHERE OR 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 altitude range of late-type stars near and above the temperature minimum in chromospheric models. Exploiting recent improvements in IR instrumentation, we observed a number of cool stars with high spectral resolution (R almost-equal-to 100,000). The analysis of the spectra was based on 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 with log g greater-than-or-equal-to 1. No direct chromospheric indicators were detected in the CO spectra. Stellar boundary (CO) temperatures were determined for the program stars and temperature profiles were constructed for alpha Tau, alpha Boo, beta Gem and beta Dra. The CO-based 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 chromospheric activity. Thermal bifurcation of the stellar surfaces is proposed to reconcile the contradicting scenarios derived based on different spectral diagnostics.