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

Optical observations of nova V1974 Cygni (Cygni 1992), spanning a 4 yearperiod, have been used to study its spectroscopic evolution. The data cover awavelength range from $\sim 3200 - 8000$ \AA and follows the nebularevolutionary phase of the ejecta. We have modeled the integrated fluxes bymeans of the photoionization code CLOUDY. The models were run at a {\it 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 thissimple model some of the physical conditions of the gaseous emitting region,like temperature and density. Compared with previous studies of the gasabundances of the heavy elements, we found that smaller enhancements of S, Nand Ar, and comparable values for O and Fe, are able to reproduce theobservations. The time evolution of the surface temperature of the ionizingsource and the high-ionization iron lines [Fe VII] 6087 \AA and [Fe X] 6374\AA, is similar to what it is observed in the soft X-rays. The early lineprofiles can be reproduced using a simple kinematical model consisting of anequatorial ring and polar caps, expanding at a velocity of $\sim 1100$ km/s.This simple model also approximates the structure of the resolved shellobserved by HST. Considering the complicated structure of the shell, the lackof well defined values of its gas density and our limited knowledge of the timeevolution of the surface temperature of the photoionization source, thecomparison between models and observations agrees remarkably well.Comment: 14 text pages, 12 figure