The Optical Spectroscopic Evolution of V1974 Cygni (Nova Cygni 1992)

Optical observations of V1974 Cygni (Nova Cygni 1992) spanning a 4 yr period have been used to study its spectroscopic evolution. The data cover a wavelength range from ∼3200–8000 Å and follow the nebular evolutionary phase of the ejecta. We have modeled the integrated fluxes by means of the photoionization code CLOUDY. The models were run at a fixed abundance value for the most prominent elements (i.e., H, He, C, O, N, Ne, Fe, etc.) over the entire time sequence. It is possible to constrain from this simple model some of the physical conditions of the gaseous emitting region, such as temperature and density. Compared with previous studies of the gas abundances of the heavy elements, we found that smaller enhancements of S, N, and Ar, and comparable values for O and Fe, are able to reproduce the observations. The time evolution of the surface temperature of the ionizing source and the high-ionization iron lines [Fe VII] λ6087 and [Fe X] λ6374 is similar to what it is observed in the soft X-rays. The early line profiles can be reproduced using a simple kinematical model consisting of an equatorial ring and polar caps, expanding at a velocity of ∼1100 km s^-1. This simple model also approximates the structure of the resolved shell observed by HST. Considering the complicated structure of the shell, the lack of well-defined values of its gas density and our limited knowledge of the time evolution of the surface temperature of the photoionization source, the comparison between models and observations agrees remarkably well.