1995A&A...294..195B

In order to determine the physical situation preceding the process of dust formation in classical novae we calculate the time dependent ionization and temperature structure of an expanding shell which is exposed to the ionizing radiation of a hot central source. Additionally the energy balance between ionizational heating and cooling processes by electron recombination, gas expansion, collisionally excited forbidden lines of several atoms and free-free radiation of hydrogen is solved. The radiation field of the classical nova is given by a synthetic spectrum calculated on the assumption of spherical symmetry and a time-independent, expanding, non-LTE atmosphere in radiative equilibrium. We show that in novae of slow and moderate speed class complete carbon ionization throughout the nova shell is not possible before day ∼50 after the outburst. In the early phases the degree of carbon ionization is significantly lower than in the corresponding blackbody model, thus enabling a complex chemistry to proceed, which may involve fast ion-neutral reactions.