The Nature of GQ Mus As Depicted From Photoionization Models of the Shell

Consequences of a detailed photoionization model study of the shell ejected by Nova Mus 1983 (GQ Mus) are presented for the period from early 1983 to 1990 (Morisset & Péquignot 1996).The drastic time variation of the emission line spectrum, including the transition from a nebular to a coronal stage in 1986, can be quantitatively understood in terms of a smooth evolution of both the expanding shell and the hot thermal source. This transition is due to a decrease of the shell density with time, and not to an increase of the source effective temperature.The model shell comprises two components so that not all directions become optically thin to ionizing radiation simultaneously. The range of density and geometrical thickness of the emitting material is of the order of a factor 2.