Luminescence from hydrogenated amorphous carbon and extended red emission from nebulae

The observation of filamentary extended red emission (ERE) structures in reflection nebulae is found to be consistent with the establishment of an equilibrium between the deactivation of small grains of hydrogenated amorphous carbon (HAC) by UV radiation and rehydrogenation by H atoms created by photodissociation of H-2 by the same UV field. Laboratory data on photodarkening and H-atom rehydrogenation rates in HAC have been used to provide a quantitative prediction of emission rates for comparison with ERE intensities. Quantitative agreement is obtained between calculated and observed ERE intensities in reflection nebulae. The luminescence efficiency of interstellar HAC is found to be similar (10(-2)-10(-3)) to that measured for laboratory samples. ERE is calculated to occur from a region with a linear dimension Delta z similar to (1-2) x 10(17) cm at the edge of a photodissociation zone. Proper motion of these emission regions may be detectable.