AAS 198th Meeting, June 2001
Session 59. Galactic ISM
Display, Wednesday, June 6, 2001, 10:00am-7:00pm, Exhibit Hall

## [59.21] Energetics of the Interstellar Infrared Emission Features

J. Pizagno (Department of Astronomy, The Ohio State University), K. I. Uchida (RS Science Center, Center for Radiophysics & Space Research, Cornell University), K. Sellgren (Department of Astronomy, The Ohio State University), M. W. Werner (Jet Propulsion Laboratory)

We examine the energetics of exciting the aromatic mid-infrared emission features (at 7.7, 8.6, 11.3, and 12.7 \mum) by quantifying the absorption properties of amorphous carbon or polycyclic aromatic hydrocarbon (PAH) molecular candidate carriers. A model is presented that calculates the amount of flux absorbed by a candidate carrier from an illuminating star. This flux absorbed in the visible and ultraviolet is assumed to be re-radiated back into the infrared as the mid-infrared emission features, and is normalized to the total flux absorbed and re-radiated by all the dust in the region. The visible and ultraviolet candidate carrier absorption properties are presented from previously measured laboratory cross sections for 8 amorphous carbons, 5 neutral PAHs, and one ionized PAH. We also explored theoretical visual and ultraviolet cross-sections for a size distribution of neutral and ionized PAHs. The model is then compared to previously published IRAS observations of 24 visual reflection nebulae illuminated by stars with effective temperatures in the range of 3,000 to 33,000 K. The results imply that PAH molecules fit the observations better than any amorphous carbon candidates. The fit for PAH molecules depends on the size distribution and ionization state of the PAHs. We determined that the overall best fit came from a size distribution of PAHs, with different size distributions for neutral versus ionized PAHs. Work is still in progress to determine the size distribution for ionized and neutral PAHs that best fits the observations.