Cooling effect of CO in stellar atmospheres

We investigate the effects of the CO infrared opacity (1.5-5.0 in stellar atmospheres. For model atmospheres with effective temperatures between 3000 and 5000 K, CO lowers the boundary temperature by amounts up to 1000 K and usually back-warms the photosphere slightly. This cooling effect is smaller for dwarfs than supergiants of the same temperature and is quite strongly affected by the amount of atomic line blanketing. Evidence for this CO cooling seems to be provided by the value of the temperature minimum deduced for Arcturus. We speculate on the possible application of these results to the temperature minimum of the Sun and to grain formation in cooler stars. Subject headings: atmospheres, solar - atmospheres, stellar - late-type stars - opacities