Unidentified cometary emission lines as the photoluminescence of frozen hydrocarbon particles

We discuss the possible nature of unidentified cometary emission lines. We propose a model of the ice particles in cometary halos as a mixture of frozen polycyclic aromatic hydrocarbons (PAHs) and acyclic hydrocarbons. We describe the general properties of frozen hydrocarbon particles (FHPs) and suggest interpreting some of the unidentified cometary emission lines as the photoluminescence of FHPs. We compare the positions of unidentified emission lines in the spectrum of Comet 122P/de Vico with the positions of quasi-lines in the photoluminescence spectrum of PAHs that were dissolved in acyclic hydrocarbons at a temperature of 77 K and that constituted a polycrystalline solution. We estimate the detectability of FHP photoluminescence in cometary spectra.