2002AJ....124..541D

We review the systematic variation between optical- and infrared-wavelength angular diameters reported for stars in the approximate range of spectral types K0-M6. We show that there is a correlation between the ratio of angular diameters and the depth of TiO absorption, in the sense that the optical diameters are larger. We argue that this validates a recent proposal by Houdashelt et al. that TiO absorption affects certain, but not all, optical-wavelength angular diameters significantly. Those authors pointed out that the infrared angular diameters appear to yield better effective temperatures than do the optical diameters, even though the latter are of higher precision. The observed angular diameter differences may arise either from limb darkening, atmospheric extension, or a combination of these two processes. Model atmosphere calculations of limb-darkening coefficients are needed to see whether the diameter discrepancy may be resolved. These models need to contain the correct opacity sources and a realistic estimate of the atmospheric geometry and dynamics. A comparison with observations such as those described in this paper will be useful for testing the validity of atmosphere models.