The evolution of the infrared spectra of classical novae.

We review the character of the infrared emission from classical novae, and describe the McDonald Observatory program of monitoring their near-infrared spectra. The K-band (1.9-2.5 m) is unique in containing strong emission features from ions with a wide range of ionization potentials, giving it special importance for understanding the physical conditions in the gaseous ejecta. Five novae were followed during 1986-93, including Nova Cyg 1992 (V1974 Cyg). Despite having a wide range in the speed of decline and amount of dust formed, all showed a similar overall pattern of evolution. The infrared spectrum of a nova is initially dominated by emission lines of H I and He I, but as time passes the degree of ionization increases. Eventually the spectrum becomes dominated by "coronal" lines, which require the ionization of species with IP > 100 eV. We intepret this behavior as being due to the declining density of the ejecta and rising surface temperature of the white dwarf which photoionizes the nova shell.