Variations of the 8.6 and 11.3 mu m emission bands within NGC 1333: Evidence for polycyclic aromatic hydrocarbon cations

We have obtained 8-13 mu m spectra of three positions in the IR reflection nebula associated with SVS 3 in NGC 1333. These observations reveal systematic variations in the relative intensities of the IR emission features at 8.6 mu m and 11.3 mu m. In particular, the [8.6]/[11.3] ratio is highest at the position of the exciting star and decreases with increasing angular distance from the star. We attribute these variations to changes in the relative populations of ionized and neutral polycyclic aromatic hydrocarbons (PAHs), driven by the strong far-ultraviolet radiation field near the star. From experiments and quantum chemical theoretical studies, the intrinsic strength of these two bands is known to vary with the charge state of the emitting PAH. We have developed a simple model for the relative intensity of the 8.6 and 11.3 mu m bands taking the charge state of the carrier into account. This model is in good agreement with the data. We point out several observational and experimental tests of this model.