THEORETICAL MODELING OF THE INFRARED FLUORESCENCE FROM INTERSTELLAR POLYCYCLIC AROMATIC-HYDROCARBONS

We have modeled the family of interstellar IR emission bands at 3.3, 6 .2, 7.7, 8.6, 11.3, and 12.7 mum by calculating the fluorescence from a size distribution of interstellar polycyclic aromatic hydrocarbons ( PAHs) embedded in the radiation field of a hot star. It is found that the various emission bands are dominated by distinctly different PAHs, from molecules with much less than approximately 80 C atoms for the 3 .3 mum feature, to molecules with 10(2)-10(5) C atoms for the emission in the IRAS 12 and 25 mum bands. We quantitatively describe the influ ence on the emergent spectrum of various PAH properties such as the mo lecular structure, the amount of dehydrogenation, the intrinsic streng th of the IR active modes, and the size distribution. Comparing our mo del results to the emission spectrum from the Orion Bar region, we con clude that interstellar PAHs are likely fully, or almost fully, hydrog enated. Moreover, it is found that the intrinsic strengths of the 6.2 and 7.7 mum C-C stretching modes, and the 8.6 mum C-H in-plane bending mode are 2-6 times larger than measured for neutral PAHs in the labor atory. This difference is tentatively ascribed to interstellar PAHs be ing ionized, or, alternatively, to their being highly asymmetric. It i s furthermore concluded that the earlier assignment of the 3.4 mum sub feature to the hot band of the aromatic C-H stretching mode is only ma rginally consistent with the available observational data. In some cas es an additional component appears to be required to account for its i ntensity. The hot band may be responsible for a weaker shoulder in som e cases. An assignment to the C-H stretching feature from ''superhydro genated'' PAHs, i.e., PAHs with extra H atoms attached to the peripher al carbon atoms, is considered as the additional contributor. Finally, it is pointed out how future high-quality observations of the emissio n from interstellar PAHs covering a large wavelength range, together w ith modeling of the type presented in the paper, could be used to stud y a number of important questions, such the preferred molecular struct ures of interstellar PAHs and their evolution in the interstellar medi um.