2017MNRAS.469.4933L

We analysed new wide-field, wide- and narrow-band optical images of IC 59 and IC 63, two nebulae that are externally illuminated by the early B-star γ Cas, with the objective of mapping the extended red emission (ERE), a dust-related photoluminescence process that is still poorly understood, in these two clouds. The spatial distribution of the ERE relative to the direction of the incident radiation and relative to other emission processes, whose carriers and excitation requirements are known, provides important constraints on the excitation of the ERE. In both nebulae, we find the ERE intensity to peak spatially well before the more extended distribution of mid-infrared emission in the unidentified infrared bands, supporting earlier findings that point towards far-ultraviolet (11 < E_photon < 13.6 eV) photons as the source of ERE excitation. The band-integrated absolute intensities of the ERE in IC 59 and IC 63 measured relative to the number density of photons available for ERE excitation are lower by about two orders of magnitude compared to ERE intensities observed in the high-latitude diffuse interstellar medium. This suggests that the lifetime of the ERE carriers is significantly reduced in the more intense radiation field prevailing in IC 59 and IC 63, pointing towards potential carriers that are only marginally stable against photoprocessing under interstellar conditions. A model involving isolated molecules or molecular ions, capable of inverse internal conversion and recurrent fluorescence, appears to provide the most likely explanation for our observational results.