SCATTERING AND LUMINESCENCE IN THE BUBBLE NEBULA

We present low-dispersion spectra obtained from 4000 to 9300 angstrom in several directions over the Bubble Nebula (NGC 7635), for which IRAS spectrometric observations do not show any unidentified infrared emission band. For most of the observed regions, when the nebular spectrum corrected for atomic continuum emission is divided by the spectrum of the exciting star (BD +60-degrees-2522), a broad emission band appears in the continuum, peaking around 7800 angstrom, superposed over a significant scattering component. It seems difficult to account for the red band by fluorescence from free polycyclic aromatic hydrocarbon molecules. The band might be explained by fluorescence from aromatic molecules loosely attached to the surface of grains. These grains would account for the observed scattering. A good fit of the observations is found for hydrogenated amorphous carbon grains or silicate grains embedded in organic material, with, in both cases, grain-size distributions encompassing a proportion of large grains significantly greater than that usually accepted for the interstellar medium.