A treatment of line opacity in expanding medium is most crucial for the light curve (LC) modeling of Type Ia supernovae (SNe Ia). Spectral lines are the main source of opacity inside SN Ia ejecta from ultraviolet through infrared range. Here we focus on the mean opacity for the energy equation. We solve the Boltzmann equation for photons in the comoving frame for a spherically-symmetrical flow. For rectangle line profiles we find an analytical expression for frequency averaged intensity and absorptive opacity. The results differ from previously known heuristic solutions. The LCs in the I-band are in better agreement with observations.
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