Extended red emission in IC59 and IC63

Abstract

We analysed new wide-field, wide- and narrow-band optical images of IC 59 and\nIC 63, two nebulae which are externally illuminated by the early B-star\n{\\gamma} Cas, with the objective of mapping the extended red emission (ERE), a\ndust-related photoluminescence process that is still poorly understood, in\nthese two clouds. The spatial distribution of the ERE relative to the direction\nof the incident radiation and relative to other emission processes, whose\ncarriers and excitation requirements are known, provides important constraints\non the excitation of the ERE. In both nebulae, we find the ERE intensity to\npeak spatially well before the more extended distribution of mid-infrared\nemission in the unidentified infrared bands, supporting earlier findings that\npoint toward far-ultraviolet (11 eV < E$_\\mathrm{{photon}}$ < 13.6 eV) photons\nas the source of ERE excitation. The band-integrated absolute intensities of\nthe ERE in IC 59 and IC 63 measured relative to the number density of photons\navailable for ERE excitation are lower by about two orders of magnitude\ncompared to ERE intensities observed in the high-latitude diffuse interstellar\nmedium (ISM). This suggests that the lifetime of the ERE carriers is\nsignificantly reduced in the more intense radiation field prevailing in IC 59\nand IC 63, pointing toward potential carriers that are only marginally stable\nagainst photo-processing under interstellar conditions. A model involving\nisolated molecules or molecular ions, capable of inverse internal conversion\nand recurrent fluorescence, appears to provide the most likely explanation for\nour observational results.\n