Nucleosynthesis during nova explosions and grain formation in ejected envelopes

Evolution of CNO elements during the hydrogen shell flash on the surface of white dwarfs is discussed in relation to nova explosions. According to the theory for shell flash of finite amplitude, its progress is determined only by two parameters, the mass of the white dwarf M sub(WD) and the mass of the accreted hydrogen-rich envelope ΔM sub(H). We examine how the relative abundances of CNO elements in the ejected shell depend on M sub(MD) and ΔM sub(H). Two typical groups of the C/O ratio result from the investigation: oxygen-rich in the case of small ΔM sub(H) and small M sub(WD), and carbon-rich in the case of large ΔM sub(H) or large M sub(WD). Under these predicted chemical compositions, the growth of grains in the expanding shell is investigated. There exists a lower mass limit of the expanding shell if infrared brightening is to be observed; the IR brightening is due to thermal emission from grains. Grains consist of graphites when M sub(WD) is greater than 0.8 Msub(solar mass) and they are composed mainly of silicates when M sub(WD) is smaller than 0.6 Msub(solar mass). However, nova explosions with the recurrence period shorter than 104 yr will not accompany the formation of grains. (author)