The heating of nova ejecta by radioactive decays of the β-unstable nuclei

Recent nucleosynthesis and hydrodynamic calculations of the consequences of accretion onto massive ONeMg white dwarf stars show that under certain circumstances significant amounts of the β-unstable nuclei can be produced and ejected by the resulting explosion. We use these calculations as a guide in order to obtain the conditions under which the heating of the ejected material by the nonthermal electrons and positrons produced by the decays of the β-unstable nuclei is sufficient to overcome the cooling from adiabatic expansion and lead to the production of X-ray-emitting coronal gas. These conditions are as follows: 1. A mass fraction for ²²Na of the order of 10^-3 or greater. 2. An expansion velocity in the range ∼ 10²-10³ km s^-1. 3. A photospheric radius of ∼ 10¹⁴ cm. 4. If the density distribution in the atmosphere satisfies a power law, then the exponent must be less than 3 for heating to overcome adiabatic cooling. Both the simulations of the outburst and the model atmosphere fits to the observed energy distributions, however, imply that the exponent is ≥ 3 during the early phases of the outburst. Nevertheless, for a value of the exponent of 2, we predict the time when hot coronal gas can form during the expansion phases of the envelope.