13 - Dust and molecules in nova environments

Summary

Introduction

Dust and molecule formation in the winds of classical novae is now a well-observed, if poorly understood, phenomenon. Although the presence of dust in nova ejecta was not confirmed observationally until the 1970 infrared observations of FH Ser (Geisel, Kleinmann & Low, 1970), the subject has a much longer history. Dust formation in a classical nova was first proposed by McLaughlin (1935) to explain the precipitous deep minimum in the visual light curve of DQ Her; coincidentally, DQ Her was also the first nova to display observational evidence of a molecule (CN; Wilson & Merrill, 1935).

At first sight, the hostile environment of a classical nova is not an obvious place to find molecules and dust. This is because the chemistry that leads to the formation of first diatomic, then polyatomic, molecules, and eventually dust, requires an environment that is well shielded from the increasingly hard radiation field of the stellar remnant.

In this chapter the observational evidence for the presence of dust and molecules in the environments of classical novae is reviewed, together with the current state of our theoretical understanding of chemistry in nova winds, and of the growth and processing of the dust.

Molecules in nova ejecta

Preamble

Although CN holds the distinction of being the first molecule to be observed in the spectrum of a nova, in the optical spectrum of DQ Her in 1933 (Wilson & Merrill, 1935), molecules are in general most easily detected in the near- and mid-infrared, where common diatomic molecules (such as CO, H₂, SiO) have rotational–vibrational transitions.