Destruction of small IR continuum emitters

We have used IRAS survey detectors and new 8-22 μm IRA S Low Resolution Spectrometer data from the Rosette Nebula and λ Orionis H II regions to probe the physics of dust emission in high radiation environments. IRAS LRS spectra extracted from neutral regions near the Rosette nebula reveal only a continuum with no trace of UIR emission. As the H II regions are approached, the IRAS survey data reveal 12 and 25 μm emission declining together, beginning well outside of the ionized region. The 25 pin emission initially declines faster than the 12 μm emission. Well inside of the Rosette H II Region. the 25 μm emission suddenly reverses its decline and begins to increase. LRS spectra from the ionized material in this regime reveal only continuum emission at wavelengths above 14 μm. Dust emission models compared to the data indicate that in regimes of enhanced radiation the normal carriers of the emission detected by both 12 and 25 ,μm survey detectors in the diffuse interstellar medium suffer destruction. The sudden increase of 25 μm survey detector emission in the Rosette, together with the LRS spectra showing only continuum emission, suggest that this emission may arise from a thermal emitter that is normally difficult to detect. Neither a simple molecular model nor a runaway sublimation grain destruction model reproduce the simultaneous decline of 12 and 25 μm emission. We propose a plausible model in which the 25 μm carriers consist of imperfectly bound conglomerates of basic structural units. Some of these. basic structural units would be released intact upon destruction of the conglomerates as denser radiationn fields are encountered. Either cyclic carbon molecular structures or amorphous solid state particles are consistent as basic structural units in this model. IRAS-LRS UIR emission bands are observed to be restricted to dense dusty molecular regions excited by early B stars.