Infrared spectroscopy of ultracompact H II regions

We present intermediate-resolution (lambda/Delta lambda approximate to 60) spectra of 21 ultracompact H II regions in the spectral range from 3 to 13 mu m. The 9.7 mu m silicate feature is seen in absorption, and the 12.8 mu m [Ne II] fine structure line is seen in emission toward most of the observed nebulae. The strengths of both features vary enormously from nebula to nebula, suggesting large variations in the column densities of both Ne II and silicates toward these objects. Near-IR features attributed to polycyclic aromatic hydrocarbons (PAHs) are detected in six of the sources. Spherically symmetric dust shell models were calculated to obtain the best fits to those nebulae for which distances are known, and spectral energy distributions are available in the range of 1 mm to 1 mu m. The models are used to infer properties of the dust cocoon such as the distribution of density and temperature with radius, shell thickness, outer shell radius, and dust abundances. Our results are consistent with previous models that predict large dust cocoons with central cavities, sharp temperature gradients, and approximately constant density in the outer regions.