THE FORMATION OF NOVAE SPECTRA

The early post-outburst spectra of novae can be divided into two classes based on the stronger of two groups of emission lines, either Fe II lines or lines of He and N. Novae with prominent Fe II lines evolve more slowly, have a lower level of ionization, and show P Cygni absorption components. Novae with stronger lines of He and N have larger expansion velocities and a higher level of ionization, and the lines are more flat-topped with little absorption. The spectral evolution of the "He/N" novae is very rapid, and usually results in coronal lines, very strong neon lines, or no forbidden lines at all, whereas over half of the "Fe II" novae develop a standard forbidden line spectrum. The spectra can be interpreted in terms of a two-component gas consisting of a discrete shell and a continuous wind, in which the narrower "Fe II" spectrum is formed in the wind, while the broader "He/N" spectrum is formed in the shell ejected at maximum light.