The temporal evolution of the 4-14 micron spectrum of V1974 Cygni (Nova Cygni 1992)

We present 4-14 micron spectra of the ONeMg nova V1974 Cygni (Nova Cygni 1992) obtained during 1992 May on the NASA Kuiper Airborne Observatory (KAO) and from the NASA 1.52 m infrared telescope at Mount Lemmon, Arizona in 1992 October/November with the HIFOGS mid-infrared spectrometer. The spectra at both epochs showed continuum emission from thermal bremstrahlung (free-free radiation) with emission lines from hydrogen and (Ne II), (Ar III), and Ne VI). During May, approximately = 80 days after outburst, the dominant emission lines in the mid-infrared spectra were a blend of three lines (Pf-alpha, Hu-beta, H11-7) near 7.5 microns and (Ne II) at 12.8 microns. By October (approximately 160 days later), the hydrogen emission had virtually disappeared, the (Ne II) 12.8 micron line had weakened considerably, and a pronounced (Ne VI) emission line had appeared at 7.6 microns. This behavior confirms that V1974 Cyg is similar to the prototypical slow ONeMg 'neon nova', Nova QU Vulpeculae (1984 No. 2). The remarkable evolution of the spectrum suggests that the ionization conditions changed drastically between 1992 May and 1992 October. We find that the ejecta of V1974 Cyg were overabundant in neon with respect to silicon by a factor of approximately 10 relative to the solar photosphere. We present new model calculations of infrared sodium forbidden line emission from (Na III) 7.319 microns (Na IV) 9.039 microns, and (Na IV) 21.29 microns that can be compared with recent model predictions of sodium synthesis in ONeMg nova outbursts. We conclude that sodium abundances in ONeMg novae can be determined by observations of infrared coronal lines of sodium that are accssible to the Infrared Space Observatory (ISO) and instruments at the NASA Infrared Telescope Facility (IRTF).