Stellar model chromospheres. III. Arcturus (K2 III)

We construct models for the upper photosphere and chromosphere of Arcturus based on the H, K, and IR triplets lines of Ca II and the h and k lines of Mg ii. The chromosphere model is derived from complete redistribution solutions for a five-level Ca II ion and two-level Mg II ion. A photospheric model is derived from the Ca II wings using first the ''traditional'' complete redistribution limit and then the more realistic partial redistribution approximation. In particular, the temperature and mass column densities for the temperature minimum region and the chromosphere-transition region boundary are computed and the pressure P0 in the transition region and corona estimate. We find T/sub eff/T/sub min/approx. =0.77 for Arcturus, Procyon, and the Sun and a trend of increasing mass at the temperature minimum with decreasing gravity. We find P0 to be about 1 percent of the solar value and on this basis estimate the surface brightness of the Arcturus transition region and coronal spectrum to be much less than for the Sun. Finally, the partial redistibution calculation for the Ca II K line indicates that the emission width is at least partially determined by damping rather than Doppler broadening, suggesting a reexamination of previous explanations for the Wilson-Bappu effect