Extended red emission in IC59 and IC63

Abstract

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 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 midinfrared emission in the unidentified infrared bands, supporting earlier findings that point toward far-ultraviolet (11 eV < 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 (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.

Are the carriers of diffuse interstellar bands and extended red emission the same?

“…Fortunately, the reflection nebula IC 63 gives us an opportunity to observe the two phenomena simultaneously in the same line of sight. The existence of ERE in IC 63 was first reported by Witt & Boroson (1990) and recently verified by Lai et al (2017). As shown in Fig.…”

“…Star #46, denoted by the arrow, appears right behind the strongest ERE emission in the PDR, produced by radiation from the early B-star γ Cas, incoming from the SW direction. The peak intensities of ERE are found immediately behind the hydrogen ionization fronts in regions also occupied by UV-pumped rotationally excited molecular hydrogen (for detail, see Lai et al (2017)).…”

Extended red emission: observational constraints for models

“…Even though the ERE in nebulae can be comparable in intensity to that of the scattered light underlying the ERE band (e.g. Lai et al 2017) as is the case in the diffuse ISM, the exciting spectrum in typical nebulae rises steeply in intensity from the optical to the far-UV by about two orders of magnitude, while the exciting ISRF spectrum in the diffuse ISM is comparatively flat in the UV-optical wavelength range. The reason that ERE is found to be brighter in nebulae than in the diffuse ISM is simply due to the much higher radiation density of the exciting photons, not as a result of a high quantum yield.…”

“…Also, the models are generally not mutually exclusive, although each model purports to provide the full explanation. It is not the aim of this review to discuss individual models in detail; such discussions can be found in the review by Witt and Vijh (2004) and in Lai et al (2017), but we will provide some guidelines for possible progress.…”

“…Following Lai et al (2017), we can divide the range of published ERE models into three categories; classical PL models, two-step processes, and recurrent fluorescence by isolated molecules and molecular ions. Most classical PL models are based on reports on laboratory experiments where ERE-like luminescence or fluorescence spectra were observed from substances illuminated at wavelengths just short of the ERE band.…”