Extended Red Emission in IC59 and IC63
  We analysed new wide-field, wide- and narrow-band optical images of IC 59 and
IC 63, two nebulae which are externally illuminated by the early B-star
{\gamma} 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 toward far-ultraviolet (11 eV < E$_\mathrm{{photon}}$ < 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 (ISM). 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 toward potential carriers that are only marginally stable
against photo-processing 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.
