THE EVOLUTION AND CLASSIFICATION OF POSTOUTBURST NOVAE SPECTRA

The first results are presented of a long-term CTIO study of the postoutburst spectra of novae, obtained with CCDs over the wavelength interval lambda-lambda-3200-1-mu-m. For certain novae the temporal coverage has been sufficiently complete that successive stages are observed in the spectral evolution, suggesting a method by which the spectra may be classified. A classification system is proposed which defines phases in evolution in terms of the characteristics of the stronger emission lines. These phases are related to physical conditions in the shell through the ionizing radiation field and mean gas density. Certain characteristics of nova spectral evolution are apparent from our limited sample, the most important of which is the necessity of invoking the presence of strong density inhomogeneities of many orders of magnitude in the envelopes in order to explain the emission spectrum. Reliable abundance determinations of nova ejecta must incorporate these condensations in the analyses. The observations also indicate that the spectral evolution of neon novae may differ from that of other novae in not developing coronal emission lines.