Mid-infrared emission features in the interstellar medium: Feature-to-feature flux ratios

Using a limited but representative sample of sources in the interstellar medium of our Galaxy with published spectra from the Infrared Space Observatory, we analyze the Aux ratios between the major mid-IR emission features (EFs) centered around 6.2, 7.7, 8.6, and 11.3 mu m, respectively. In a flux ratio-to-flux ratio plot of EF(6.2 mu m)/EF(7.7 mu m) as a function of EFf(11.3 mu m)/EF(7.7 mu m), the sample sources form roughly a Lambda-shaped locus that appears to trace, on an overall basis, the hardness of a local heating radiation field. But some driving parameters other than the radiation field may also be required for a full interpretation of this trend. On the other hand, the flux ratio of EF(8.6 mu m)/EF(7.7 mu m) shows little variation over the sample sources, except for two H II regions that have much higher values for this ratio because of an "EF(8.6 mu m) anomaly," a phenomenon clearly associated with environments of an intense far-UV radiation field. If further confirmed on a larger database, these trends should provide crucial information on how the EF carriers collectively respond to a changing environment.