Mid-Infrared Emission Features in the Interstellar Medium: Feature-to-Feature Flux Ratios

Using a limited, but representative sample of sources in the ISM of ourGalaxy with published spectra from the Infrared Space Observatory, we analyzeflux ratios between the major mid-IR emission features (EFs) centered around6.2, 7.7, 8.6 and 11.3 microns, respectively. In a flux ratio-to-flux ratioplot of EF(6.2)/EF(7.7) as a function of EF(11.3)/EF(7.7), the sample sourcesform roughly a $\Lambda$-shaped locus which appear to trace, on an overallbasis, the hardness of a local heating radiation field. But some drivingparameters other than the radiation field may also be required for a fullinterpretation of this trend. On the other hand, the flux ratio ofEF(8.6)/EF(7.7) shows little variation over the sample sources, except for twoHII regions which have much higher values for this ratio due to an ``EF(8.6\um)anomaly,'' a phenomenon clearly associated with environments of an intensefar-UV radiation field. If further confirmed on a larger database, these trendsshould provide crucial information on how the EF carriers collectively respondto a changing environment.Comment: 16 pages, 1 figure, 1 table; accepted for publication in ApJ Letter