THE EVOLUTION OF THE INFRARED-SPECTRA OF CLASSICAL NOVAE

We review the character of the infrared emission from classical novae, an describe the McDonald Observatory program of monitoring their near -infrared spectra. The K-band (1.9-2.5 mum) is unique in containing st rong emission features from ions with a wide range of ionization poten tials, giving it special importance for understanding the physical con ditions 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 o verall pattern of evolution. The infrared spectrum of a nova is initia lly dominated by emission lines of H I and He I, but as time passes th e degree of ionization increases. Eventually the spectrum becomes domi nated by ''coronal'' lines, which require the ionization of species wi th IP > 100 eV. We intepret this behavior as being due to the declinin g density of the ejecta and rising surface temperature of the white dw arf which photoionizes the nova shell.