Frank O. Clark

Destruction of small IR continuum emitters

Abstract 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.

Cirrus Color Variations Due to Enhanced Radiation Fields

We have investigated the variations in 12/100, 25/100, 60/100, and 12/25 μm colors for seven main-sequence B stars and three F and G supergiants associated with infrared cirrus. All sources displayed an increase in 60/100 color above the background cirrus color. In two of the sources, Apodis and HR 890, the 12/100 and 25/100 colors decline toward the embedded star in a similar fashion to the IR colors of S264 and the Rosette Nebula. Current grain models composed of equilibrium-heated submicron grains, transiently heated small grains, and polycyclic aromatic hydrocarbons cannot account for the color variations observed around Aps and HR 890. The supergiants exhibited 12/100 and 25/100 increases, suggesting that the color deficits observed for the B stars are due to an enhancement in the soft UV component of the radiation field only. A candidate explanation for the color variations is a conglomerate small grain component, composed of very small grains and/or large molecules, that is fragmented in the enhanced radiation field around Aps and HR 890.

The Composition and Distribution of Dust in Galactic H II Regions

We have modeled the far infrared emission of two resolved H II regions, the Rosette Nebula (NGC 2237–46) and the λ Orionis H II region (S264). Visible extinction maps and radio continuum observations are combined with infrared data from the Infrared Astronomical Satellite (IRAS) to determine the composition and distribution of dust associated with these nebulae.

A Molecular Conglomerate Model of Small Interstellar Dust

We analyze emission of the interstellar dust from the vicinities of three heating sources. We find that as the radiation sources are approached, relative emission in both IRAS short wavelength bands expressed as I(12)/I(100) and I(25)/I(100) decline together. This result runs counter to existing models. We propose a molecular conglomerate model of the small interstellar dust grains to explain these data.