2000A&A...359..397P

The galaxy M 31 is interesting, in particular, because its diffuse interstellar medium emits the Unidentified Infrared Bands although it is devoid of far UV radiation. This compels us to seek an alternative to stochastic, transient, heating of dust by absorption of far UV photons. The alternative proposed here is chemiluminescence, or the excitation of the vibrations of chemical bonds of carbonaceous dust by impact of gaseous H atoms. This mechanism was studied numerically on several molecules, using standard chemical codes to simulate the molecular dynamics upon impact. It was found that this process selectively excites several IR-active modes, which fall in the astronomical IR bands of interest. After a brief transient redistribution, these vibrational modes no longer exchange energy on the long term and are expected to decay only through IR emission. The efficiency of this process was found to be ∼0.3. The relative intensities of the IR features do not depend on the ambient radiation field. The far UV enters only indirectly, in as much as it increases the density of ambient H atoms. A chemical kinetic model was built on this basis for the interactions of dust, H atoms, H₂ molecules and far UV photons in the H1 regions of M 31. It is in very good numerical agreement with the observations of Pagani et al. (1999A&A...351..447P).