The missing opacity and the temperature calibration of solar-type stars

The novel temperature calibration of some color indices presented for G and F dwarfs is based on the IR flux method and extends the work of Saxner and Hammarback (1985) to the Population II stars. Attention is given to the discrepancy between theoretical and empirical temperature calibrations. It is suggested that the most straightforward interpretation of the disagreement between empirical colors and some model predictions is that of the 'missing opacity' due to a veil of faint metal lines, present in the blue and UV regions of the spectra of solar-type and cooler stars, but excluded from the model.