Origin of the `He/N' and `Fe II' Spectral Classes of Novae

TL;DR

This paper investigates the origins of spectral classes in novae, linking He/N and Fe II spectra to different physical regions and proposing the binary mass ratio as a key factor influencing spectral types.

Contribution

It introduces a physical model explaining the spectral differences in novae and suggests the prevalence of hybrid spectra during the postoutburst decline.

Findings

He/N spectra originate from white dwarf ejecta

Fe II spectra form in circumbinary gas from the secondary star

The binary mass ratio influences spectral class dominance

Abstract

The spectra of postoutburst novae display either He+N or Fe II lines as the most prominent non-Balmer lines at maximum light. Spectral diagnostics indicate physical conditions for 'He/N' spectra that are consistent with their origin in the white dwarf (WD) ejecta, whereas 'Fe II' spectra point to their formation in a large circumbinary envelope of gas whose origin is the secondary star. A determining parameter for which of the two types of spectra predominates may be the binary mass ratio Msec/MWD. The increasing fraction of novae that are observed to be 'hybrid' objects, where both classes of spectra appear sequentially, is explained by changing parameters in the two emitting regions during the postoutburst decline. We argue that most novae may be hybrids that show both types of spectra during decline. The emission line intensity ratio O I {\lambda}8446/{\lambda}7773 is suggested as a good density diagnostic for the ejecta, and a finding list of emission lines identified in recent spectroscopic surveys of novae is presented as an aid to future line identification work.