A model of the physical processes in the neutral envelopes of planetary nebulae
The aim of this thesis is to predict the column densities of various neutral and ionised molecular species that are formed, or are likely to be formed, in the neutral envelope of a planetary nebula (PN). To this end a computer program has been constructed based on existing code (Abgrall et al. 1992) that considers a large set of chemical reactions covering the formation and destruction of the chemical species considered in the model. The rate coefficient of a chemical reaction will either depend on the local gas kinetic temperature if it is a gas phase reaction, or on the local radiation field spectrum if it is a photoreaction. To model the reaction network it is therefore also required to model the heating and cooling processes in the nebula to determine the kinetic temperature and also to solve the radiative transfer equation to determine the energy spectrum of ultraviolet radiation. Formation of the H2 molecule on the surface of dust grains and cosmic ray interactions are also considered. The ultraviolet absorption spectrum of the dominant molecules H2 and CO and the photodissociation rates are both functions of the rotational population. Rate coefficients for collisional cooling also depend on the rotational state. For these reasons, to model the thermal processes and the radiative transfer accurately it is also required to model the processes contributing to rotational excitation and de-excitation of H2 and CO to determine the distribution amongst their various rotational levels. Dust grains play a significant role in much of the physics occurring in the nebula, not least because they represent the catalyst for the formation of molecular hydrogen. Dust also represents the most important source of opacity for the ultraviolet radiation field and hence a significant part of the thesis is devoted to a consideration of the probable dust composition and optical properties. The results of the model are shown and a comparison is made between the predictions of the model and recent computations of molecular column densities based on astronomical observations of planetary nebulae. The probable sources of large discrepancies are discussed within the context of assumptions and possible omissions in the physical model.