A Quantitative Method to Investigate the Spectral Class Time Evolution of Novae

Novae are spectroscopically classified as Fe ii or He/N according to the dominance of Fe ii , or He and N emission lines in their prenebular spectra, while spectra exhibiting both are classified as hybrid. Whereas the spectral class of a nova is known to correlate with its progenitor properties, a recent detailed spectroscopic follow-up of 10 novae suggests instead a time evolution from “early He/N” to “Fe II ” and to “late He/N.” We investigate here this apparent contradiction using a spectroscopic data set of 26 novae from the Astronomical Ring for Amateur Spectroscopy (ARAS) database. For this purpose we introduce a quantitative coefficient, κ , defined as the ratio of the flux of the Fe ii λ 5169 line to the mean flux of five He and N emission lines, with κ > 1 indicating Fe ii –dominated spectra and κ < 1 indicating He/N-dominated spectra. This coefficient is shown to provide a reliable quantitative tool for tracing nova spectral evolution while reducing biases from incomplete coverage. A significant fraction of the novae in our sample exhibit at least one transition between Fe ii – and He/N-dominated phases. The time evolution of κ closely follows their optical light curves, repeatedly crossing unity in novae exhibiting jitters, which reveal nonmonotonic spectral evolution. Since the light-curve shapes and their timescales depend on intrinsic progenitor properties, particularly white dwarf mass, they also determine the duration of Fe ii – and He/N-dominated phases, introducing a bias in spectral classification when it relies on early, sparse spectra.