Novae Ejecta as Colliding Shells

TL;DR

This paper provides evidence that nova ejecta consist of two distinct components from different origins, and their collision explains spectral evolution, with implications for nova outburst mechanisms.

Contribution

It introduces a model of nova ejecta as colliding shells from the white dwarf and circumbinary gas, explaining spectral changes and suggesting new outburst triggers.

Findings

Emission-line widths are narrower than P Cygni profiles.

Novae spectral evolution results from shell collision.

Circumbinary gas is more massive, affecting expansion.

Abstract

Following on our initial absorption-line analysis of fifteen novae spectra we present additional evidence for the existence of two distinct components of novae ejecta having different origins. As argued in Paper I one component is the rapidly expanding gas ejected from the outer layers of the white dwarf by the outburst. The second component is pre-existing outer, more slowly expanding circumbinary gas that represents ejecta from the secondary star or accretion disk. We present measurements of the emission-line widths that show them to be significantly narrower than the broad P Cygni profiles that immediately precede them. The emission profiles of novae in the nebular phase are distinctly rectangular, i.e., strongly suggestive of emission from a relatively thin, roughly spherical shell. We thus interpret novae spectral evolution in terms of the collision between the two components of ejecta, which converts the early absorption spectrum to an emission-line spectrum within weeks of the outburst. The narrow emission widths require the outer circumbinary gas to be much more massive than the white dwarf ejecta, thereby slowing the latter's expansion upon collision. The presence of a large reservoir of circumbinary gas at the time of outburst is suggestive that novae outbursts may sometime be triggered by collapse of gas onto the white dwarf, as occurs for dwarf novae, rather than steady mass transfer through the inner Lagrangian point.