Variations of the mid-infrared emission spectrum in reflection nebulae

Using spatial-spectral data cubes of reflection nebulae obtained by ISOCAM, we have observed a shift in the central wavelength of the 7.7 mu m band within several reflection nebulae. This band, composed of components at 7.85 and 7.65 mu m, shows a centroid shift from 7.75 mu m near the edge of the nebulae to 7.65 mu m toward the center of the nebulae as the shorter wavelength component becomes relatively stronger. The behavior of the 7.7 mu m band center can be explained by assuming either that anions are the origin of the 7.85 mu m band and cations the 7.65 mu m band, or that the band center wavelength depends on the chemical nature of the polycyclic aromatic hydrocarbons (PAHs). The ratio of the 11.3/ 7.7 mu m bands also changes with distance from the central star, first rising from the center toward the edge of the nebula, then falling at the largest distances from the star, consistent with the 11.3/7.7 mu m band ratio being controlled by the PAH ionization state.