Colors of reflection nebulae. II. The excitation of extended red emission.

New BVI color-difference measurements are reported for the reflection nebulae NGC 7023, NGC 2068, and CED 201. The data are obtained through BVI imaging with a CCD detector and are analyzed together with previously published measurements for the reflection nebula NGC 2023. All four nebulae are substantially redder in the (V, I) range than expected on the basis of dust scattering alone, a result of the presence of extended emission in the I band. The relative strength of the I excess reaches a maximum in nebular regions where the color in the (B, V) range is bluest. These facts are interpreted in terms of a model which explains the extended I emission as a luminescence process excited by UV radiation from the illuminating star(s). Model fits identify the wavelength region between 1800 and 2500 A of the illuminating star's spectrum as the source of the energy of excitation. It appears possible that the 2200 A band in the dust-extinction curve is a gate for the excitation energy, and it is likely that the I excess emission and the extended near-IR emission recently discovered in reflection nebulae are similar in nature and origin.