On dust irradiation in planetary nebulae in the context of survivability of ices

Abstract

A large number of molecules are observed in planetary nebulae, including simple and, - the most common (H₂, CO and OH), more complex (H₂O, SiO, HCN, HNC, HCO⁺), and even the polycyclic aromatic hydrocarbons and fullerenes containing a few dozen and more atoms. Water molecules are observed, as a rule, in the young objects, in the gas phase (water "fountains" and related water masers) and solid phase (emission of crystalline ice particles). On the other hand, the results of calculations by the Cloudy computer program, given in this paper, show that the abundance of water ice in planetary nebulae, other conditions being equal, depends on the ionization rate of hydrogen, which depends in turn on the flux of energetic particles (protons and alpha particles) in the range of MeV energies and higher. Calculated water ice column densities reach values of up to ${10}^{19}-{10}^{20}c{m}^{-2}$ at the usual average ISM H₂ ionisation rate of ${10}^{-16}{s}^{-1}$ and sharply decrease at rates that are a thousand times larger. The possibility of an increased flux of energetic particles in planetary nebulae under conditions of the standard interacting stellar winds scenario is discussed, and it is concluded that the flux may locally exceed by 1–3 orders of magnitude that of galactic cosmic rays. This may have important implications for the chemistry of complex compounds under conditions of planetary nebulae, in particular, for models of the origin of fullerenes.