signifies where a given zone at a given time is in the shadow of some interior zones that are exposed to direct irradiation.]{}

Some systematics of the shadowing process are illustrated in figure 4. The specific details reported here may be subject to the effects of finite zoning. The qualitatively significant results arise from comparing models with and without irradiation.

Figure 4 shows that the innermost disk ($\lta$ $2{\times}10^7$ cm) is always exposed to the direct irradiation. Because the outburst is initiated about 25 d prior to the optical peak, more extensive portions of the disk are exposed to direct irradiation. After maximum light, the outer disk ($>$ $2{\times}10^8$ cm) is essentially totally blocked from direct irradiation by the inner disk. With the shadowing effect, therefore, the existence of a mechanism which can reflect the radiation from the inner region is important in the current models if there are to be any effects of irradiation in the outer disk.

 Amplitude of Disk Irradiation
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[5]{} [22pc]{} [Contribution of irradiation in the disk evolution. For illustration, we choose four zones: 5 (short dash), 16 (dot), 18 (dash dot), and zone 20 (solid line). The contribution of both direct and indirect irradiation 