THE MIDINFRARED EVOLUTION OF NOVA V1974 CYGNI

We observed nova V1974 Cygni (1992) at mid-infrared wavelengths lambda = 7.5-13 mu m nine times between 1992 April (54 days after the outbur st) and 1994 July (day 882). Low- and medium-resolution spectra record ed the evolution of hydrogen recombination lines, free-free continuum emission, and the [Ne II] 12.8 mu m, [Ne VI] 7.6 mu m, and [Mg VII] 9. 0 mu m forbidden lines through the nebular and coronal line phases of this event. We interpret the spectra in terms of a series of CLOUDY mo dels that evolve in a physically realistic way throughout the period o f observation. The models indicate the presence in the ejecta of three major components of differing densities, but with masses, geometries, and expansion velocities that are consistent with values indicated by optical and UV observations of the nova. In our models the abundances of neon and magnesium relative to hydrogen are 50 and 5 times their s olar values, respectively, and significant overabundances of other met als are also implied. We discuss many effects such as the increase of line widths with ionization and the origin of coronal line emission as they are represented by our modeling.