of $\sim$ 12300 kmc$^{-1}$. The formation of the dense shell starts in the optically thick medium at the optical depth of $\sim$ 8, and ends in semi-transparent medium at the optical depth of $\sim$ 1. By day 18 the shell accelerates to a velocity of $\sim$ 13400 kmc$^{-1}$ reducing the density contrast to a value of $\sim$ 2.5. The matter in the dense shell is subject to the Rayleigh-Taylor instability, which may result in the strong mixing of matter (Falk & Arnett [@fa77]). The latter, in turn, may prevent the thin shell-like structure from forming in the outer layers of the envelope.

Homologous expansion {#sec:dmd-free}
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The SN explosion causes an acceleration of the envelope matter. At late times, when the acceleration of the ejecta becomes negligible, the envelope matter expands homologously. Evidently, there is a transition from the acceleration to the homologous expansion. This transition for different mass shells occurs at different times: the deeper the layer, the later the transition (Fig. \[fig:freexp\]). Thus, the transition time of the whole expelled envelope is determined by the deepest layer. If we consider physical processes in an SN atmosphere, the relevant transition time is given by the photospheric location. It is 