Missing Opacity in the Atmospheric Models of Red Giants
  Synthetic spectra of Arcturus computed with the latest theoretical stellar atmospheres disagree strongly with both the spectral energy distribution and with the spectroscopic data for $\lambda < 4000$ \AA , even though the stellar parameters are well constrained. We find that the discrepancy can be removed by adding a continuous absorption opacity that is approximately equal to that which is already included in the {\sc atlas9} code. Spectroscopy of other   K2 III stars shows that the spectrum of Arcturus in not anomalous. Therefore, missing opacity may be a common feature of the models for stars in this temperature and luminosity range. 
C. I. Short and J. B. Lester