4 - Thermonuclear processes

Summary

Introduction

The classical nova outburst is one consequence of the accretion of hydrogen-rich material onto a white dwarf (WD) in a close binary system. Over long periods of time the accreting material gradually forms a layer of fuel on the WD and the bottom of this layer is gradually compressed and heated by the strong surface gravity of the WD. Ultimately, the bottom of the layer becomes electron-degenerate. The degeneracy of the material then contains the explosion so that, once nuclear burning in the layer bottom reaches thermonuclear runaway (TNR) conditions, the temperatures in the nuclear burning region will exceed 10⁸ K under almost all circumstances. As a direct result, a major fraction of the nuclei in the envelope capable of capturing a proton (C, N, O, Ne, Mg …) are transformed into β⁺-unstable nuclei, which limits nuclear energy generation on the dynamical time-scale of the runaway and yields distinctively non-solar CNO isotopic abundance ratios in the ejected gases.

Observations of the outburst show that a classical nova explosively ejects metal enriched gas and grains and this material is a source of heavy elements for the interstellar medium (ISM). The observed amount of metal enrichment demands that mixing of the accreted material with core material occur at some time during the evolution of the outburst.