Revisiting the classics: On the evolutionary origin of the "Fe II" and "He/N" spectral classes of novae

TL;DR

This study revisits the spectral evolution of novae, confirming that the 'Fe II' and 'He/N' classes are phases driven by changes in ejecta properties, with all novae passing through at least three distinct phases during eruption.

Contribution

It provides modern observational evidence that nova spectral classes are phases in a universal evolutionary sequence driven by ejecta conditions.

Findings

Novae pass through at least three spectral phases.

Spectral phases are driven by changes in ejecta opacity, ionization, and density.

Duration of phases varies with nova speed class.

Abstract

The optical spectra of novae are characterized by emission lines from the hydrogen Balmer series and either Fe II or He/N, leading to their traditional classification into two spectral classes: "Fe II" and "He/N". For decades, the origins of these spectral features were discussed in the literature in the contexts of different bodies of gas or changes in the opacity of the ejecta, particularly associated with studies by R. E. Williams and S. N. Shore. Here, we revisit these major studies with dedicated, modern data sets, covering the evolution of several novae from early rise to peak all the way to the nebular phase. Our data confirm previous suggestions in the literature that the "Fe II" and "He/N" spectral classes are phases in the spectroscopic evolution of novae driven primarily by changes in the opacity, ionization, and density of the ejecta, and most if not all novae go through at least three spectroscopic phases as their eruptions evolve: an early He/N (phase 1; observed during the early rise to visible peak and characterized by P Cygni lines of He I, N II, and N III), then an Fe II (phase 2; observed near visible peak and characterized by P Cygni lines of Fe II and O I), and then a later He/N (phase 3; observed during the decline and characterized by emission lines of He I. He II, N II, and N III), before entering the nebular phase. This spectral evolution seems to be ubiquitous across novae, regardless of their speed class; however the duration of each of these phase differs based on the speed class of the nova.