Upper photospheric temperature models of K giants - A comparison of super-metal-rich giants with normal giants

The LTE version of K-line synthesis that takes partial coherency effects into account is applied to the Ca II K-line wings in the spectra of four normal K giants (Beta Gem, Nu Cyg, Rho Oph, Alpha Ari) and two super-metal-rich (SMR) K giants (Alpha Ser, Beta Oph) in order to derive upper-photospheric temperature models. A model-atmosphere flux-calibration procedure that precludes the necessity of a measured angular diameter is used in the analysis of the K-line wing profiles, on the basis of which the SMR and normal K giants are compared quantitatively. The results show that temperature enhancements of 200 to 300 K in excess of radiative-equilibrium models are required in the upper photospheres of all six stars and that the upper photospheres of the SMR star models are up to 180 K cooler than those of the normal models. The temperature enhancements are interpreted as departures from radiative equilibrium, and the differential cooling in the SMR stars is tentatively attributed to enhanced CN and CO line absorption.