THE SPACING OF THE INTERSTELLAR 6.2 AND 7.7 MICRON EMISSION FEATURES AS AN INDICATOR OF POLYCYCLIC AROMATIC HYDROCARBON SIZE

摘要

A database of astrophysically relevant, infrared spectral measurements on a wide variety of neutral as well as positively and negatively charged polycyclic aromatic hydrocarbons (PAHs), rangring in size from C_10H_8 through C_48H_20, is now available to extend the interstellar PAH model. Beyond simply indicating general characteristics of the carriers, this collection of data now makes it possible to conduct a more thorough interpretation of the details of the interstellar spectra and thereby derive deeper insights into the nature of the emitting material and conditions in the emission zones. This Letter is the first such implementation of this spectral database. The infrared spectra of PAH cations, the main PAH form in the most energetic emission zones. are usually dominated by a few strong features in the 1650- 1100 cm~(-1) (6.1 -9.1 μm) region that tend to cluster in the vicinity of the interstellar emission bands at 1610 and 1320 cm~(-1) (6.2 and 7.6 μm), but with spacings typically somewhat less than that observed in the canonical interstellar spectrum. However, the spectra in the database slow that this spacing increases steadily with molecular size. Extrapolation of this trend indicates that PAHs in the 50-80 carbon atom size range are entirely consistent with the observed interstellar spacing. Furtherimore, the profile of the l610 cm~(-1) (6.2 μm) interstellar band indicates that PAHs containing as few as 20 carbon atom contribute to this feature.