On the Composition and Distribution of Dust in Galactic H II Regions
Session 26 -- Interstellar Medium and Radiative Shocks
Oral presentation, Monday, 9, 1995, 10:00am - 11:30am

## [26.04D] On the Composition and Distribution of Dust in Galactic H II Regions

R. F. Shipman (Phillips Laboratory/Geophysics Directorate)

The dust emission and distribution from two Galactic H II regions, the Rosette Nebula and the $\lambda$ Orionis H II region, have been modeled and compared with spatially resolved IRAS observations. The grain models used in this study are consistent with observations of isolated interstellar clouds and the diffuse interstellar meduim. We exploited the wealth of published observations of these two H II regions to constrain the free parameters of the infrared emission models. These models represent a comprehensive, multi-wavelength study of each H II region. The contribution of nebular lines to the IR emission observed by IRAS was calculated from radio continuum measurements. Emission lines are found to contribute only a few percent of the total IR emission. Despite their regular appearance, both H II regions have significant deviations from spherical symmetry. In particular, the IR emission for the $\lambda$ Orionis H II region requires a nonspherical distribution of dust near the central core. $I_{\nu}(60)/I_{\nu}(100)$ is not representative of a single grain temperature within either H II region. Therefore, a grain component separate from the grains emitting at 100 $\mu m$ is required to supply the excess 60 $\mu m$ emission. These emitters could be either small (50 to 200 $\AA$) grains that experience strong temperature fluctuations or "astronomical" iron grains that attain high equilibrium temperatures. In order to match the IRAS observations, the abundance of the grains that produce the 12 and 25 $\mu m$ emission in the ISM must be reduced inside the ionized regions. This depletion occurs in radiation fields 20 to 36 times the UV component of the diffuse ISRF. The destruction mechanism for the 12 and 25 $\mu m$ carriers is not well modeled by sublimation or molecular disruption.