2003ApJ...596..501S

We present plane-parallel and spherical LTE and non-LTE (NLTE) atmospheric models of a variety of stellar parameters of the red giant star Arcturus (α Boo, HD 124897, HR 5340) and study their ability to fit the measured absolute flux distribution. Our NLTE models include tens of thousands of the strongest lines in NLTE, and we investigate separately the effects of treating the light metals and the Fe group elements Fe and Ti in NLTE. We find that the NLTE effects of Fe group elements on the model structure and F_λ distribution are much more important than the NLTE effects of all the light metals combined, and serve to substantially increase the violet and near-UV F_λ levels as a result of NLTE Fe overionization. Both the LTE and NLTE models predict significantly more flux in the blue and UV bands than is observed. We find that within the moderately metal-poor metallicity range, the effect of NLTE on the overall UV flux level decreases with decreasing metallicity. These results suggest that there may still be important UV opacity missing from the models. We find that models of solar metallicity giants of spectral type similar to that of Arcturus fit well the observed flux distributions of those stars from the red to the near-UV band. This suggests that the blue and near-UV flux discrepancy is metallicity dependent, increasing with decreasing metallicity.