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K. E. Kraemer (AFRL, IAR-BU), M. Kassis, J. M. Jackson, L. K. Deutsch (IAR-BU), A. Dayal (IPAC), J. L. Hora, G. G. Fazio (H-S CfA), W. F. Hoffmann (U. Ariz.), S. D. Price, M. P. Egan, S. J. Carey, T. A. Kuchar, D. R. Mizuno (AFRL)
Recent models and laboratory measurements of PAH emission suggest that the relative strengths of UIR bands can trace the ionization fraction of small dust grains. In particular, the strength of the 11.2 \mum feature is produced more strongly by neutrals, whereas the 7.7-8.6 \mum features are produced more strongly by ions. In order to examine the extent to which ionized and neutral PAHs contribute to UIR emission, we have observed massive star forming regions with the MIRAC3 CVF at 7.8 and 11.2 \mum. The structure of the ionized gas was measured by imaging each region with the CVF at 12.8 \mum, the [Ne II] fine-structure transition. We compare the UIR distributions and their ratio with the distribution of ionized gas in and around each source. To determine the large-scale distribution of the UIR features, we compare MSX band A (6.8-10.8 um) and band C (11.1-13.2) data at each source. We thus test whether or not the UIR feature strength ratio traces the ionization fraction of PAHs, as suggested by the laboratory data.