On the band-to-continuum intensity ratio in the infrared spectra of interstellar carbonaceous dust

Published interpretations of the relative intensity variations of the unidentified infrared bands (UIBs) and their underlying continuum are discussed. An alternative model is proposed, in which a single carrier for both emits (a) mostly a continuum when it is electronically excited by photons (visible or UV), or (b) exclusively the UIBs, when only chemical energy is deposited by H capture on its surface, inducing only nuclear vibrations. The bands will dominate in atomic H regions but will be overcome by thermal continuum radiation when the ambient field is strong but lacks dissociating photons (900–1100 Å). The model applies to photodissociation regions as well as to limbs of molecular clouds in the interstellar medium and agrees quantitatively with recent satellite observations. It gives indications on atomic H density and UIB intensity provided the ambient radiation field is known. It invokes no chemical, electronic, structural or size change to interpret the observed intensity variations.