Radiative Opacity in Stellar Atmospheres. II. Effect of Ultraviolet Continuum on the Photospheric Radiation Field

Comparisons with observations are made for theoretical limb darkening as well as for intensity dis- tribution as a function of wavelength obtained from various solar models with different choices of opaci- ties. Particular attention is directed to the opacity due to metals, using the photo-ionization cross- sections recently computed by means of the quantum-defect method It is found that satisfactory agree- ment can be obtained between theory and observations if one assumes an additional opacity, amounting to a factor of about 10-100 larger than metal absorption in the spectral region between XX2500 and 3500. The notable improvement in agreement with observations for the visual and infrared regions is attributed to a considerable change in the temperature stratification throughout the atmosphere produced by the ultraviolet flux, as a direct consequence of the condition of radiative equilibrium. Various evidences are presented to support the conclusion that the necessary opacity is due not to line blanketing but to the existence of a still unknown source of absorption, unless the errors involved in the determinations of metal absorption are unrealistically large