The physics of type Ia supernova light curves. II. Opacity and diffusion

We examine the nature of the opacity and radiation transport in Type Ia sup ernovae. The dominant opacity arises from line transitions. We discuss the nature of line opacities and diffusion in expanding media and the appropria teness of various mean and expansion opacities used in light-curve calculat ions. Fluorescence is shown to be the dominant physical process governing t he rate at which energy escapes the supernova. We present a sample light cu rve that was obtained using a time-dependent solution of the radiative tran sport equation with a spectral resolution of 80 km s(-1) and employing an L TE equation of state. The result compares favorably with light curves and s pectra of typical supernovae and is used to illustrate the physics controll ing the evolution of the light curve and especially the secondary maxima se en in infrared photometry.