On the wavelength dependency (and its variations) of the ratio disk-averaged to disk-center intensity, F/I-0 (lambda,lambda 0.385 to 10 mu m)

In the whole wavelength range, 0.385 to 10.0 mu m, the ratios F/I-0 of disk-averaged intensity F to disk-center intensity I-0 follow very closely the simple relation F/I-0 = 0.975 - 0.091 lambda(-1) - 0.00024 lambda(-5). The residuals exhibit a well-defined structure with minima and maxima in the order of +/-5 x 10(-3), which reflect the wavelength-dependency of the source function B(lambda, tau) and the continuous absorption coefficient kappa(lambda, tau). The fact that the scatter of the coefficients of the relevant limb-darkening functions is usually much larger than the scatter of the F/I-0 ratios seems to confirm the occurrence of limb-darkening variations, which however leave the F/I-0 ratios almost unchanged. Nevertheless, minor differences between different observations, which concern the details in the wavelength-dependency of the F/I-0 residuals, seem to indicate that even the F/I-0 ratios undergo minor variations.