Nova shells: matter-bound or radiation-bound

It is stated that the ionisation structure of nova shells is uncertain. It is usually assumed that the shell is fully ionised during the decline phase, and that the rate of decline is an indicator of the density of matter in the expanding shell. With reference to V1500 Cyg, however, it is proposed that the shell is not fully ionised, but forms a 'Stroemgren' shell. If this is so the optical light curve is determined by the decline of the energy production rate of the central object. If the expanding shell is completely ionised the size of the emitting region is determined only by the spatial extent of the shell, and the emitting region is said to be 'matter-bound'. For a completely ionised nebula the electron density is directly proportional to the local matter density, and the emission from a matter-bound shell is determined mainly by the matter density. The basic assumption that the shell of V1500 Cyg is matter-bound is here questioned, and it is proposed that the shell is radiation-bound, that is that the volume of the emitting region is determined by the balance between photoionisation and recombination. The most direct evidence that the V1500 Cyg shell was not fully ionised was the observation of neutral O emission long after maximum phase. The observed strength of OI emission is easily understood if the shell is radiation-bound. Some important consequences of the presence of an H 'Stroemgren' sphere are mentioned. (U.K.)