Hydrogenated amorphous carbon grains in reflection nebulae

The paper presents spectroscopic data which identify the source of the photometrically detected extended red emission in the reflection nebulae NGC 2023 and NGC 7023 as emission in the form of a broad luminescence band, reaching its peak in the 6600-6800 A wavelength region. This feature is identified as luminescence from hydrogenated amorphous carbon, excited by short-wavelength photons from the central illuminating stars. In NGC 2023 the luminescence is found to be notably enhanced in a filamentary shell structure surrounding HD 39703, which is spatially coincident with a region exhibiting fluorescent molecular hydrogen emission as well as 3.3-micron emission. From BVRI surface photometry of NGC 2023 and NGC 7023 it is concluded that in regions of enhanced luminescence, presumably regions containing hot atomic hydrogen, the luminescence band shifts toward shorter wavelengths, while in more remote molecular regions more luminescent energy emerges in the long-wavelength tail of the band. This observed behavior is consistent with laboratory results concerning luminescence of hydrogenated amorphous carbon under varying conditions. 27 references