We present intermediate-resolution (j/*j B 60) spectra of 21 ultracompact H II regions in the spectral range from 3 to 13 km. The 9.7 km silicate feature is seen in absorption, and the 12.8 km [Ne II] Ðne 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 Ðts to those nebulae for which distances are known, and spectral energy distributions are available in the range of 1 mm to 1 km. 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.

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We have used the Very Long Baseline Array (VLBA) and the Very Large Array (VLA) to image Galactic neutral hydrogen in absorption towards four compact extragalactic radio sources with 10 milliarcsecond resolution. Previous VLBA data by Faison et al (1998) have shown the existence of prominent structures in the direction of the extragalactic source 3C 138 with scale sizes of 10-20 AU with changes in HI optical depth in excess of 0.8 ± 0.1. In this paper we confirm the small scale H I optical depth variations toward 3C 147 suggested earlier at a level up to 20 % ± 5% . The sources 3C 119, 2352+495 and 0831+557 show no significant change in H I optical depth across the sources with one sigma limits of 30%, 50%, and 100%. Of the seven sources recently investigated with the VLBA and VLA , only 3C 138 and 3C 147 show statistically significant variations in HI opacities. Deshpande (2000) have attempted to explain the observed small-scale structure as an extension of the observed power spectrum of structure on parsec size scales. The predictions of Deshpande (2000) are consistent with the VLBA H I data observed in the directions of a number of sources, including 3C 147, but are not consistent with our previous observations of the H I opacity structure toward 3C 138.