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The Density Structure of Highly Compact H II Regions

We report the density structure of the ultracompact (UC) H II regions G35.20-1.74, G9.62+0.19-E, and G75.78+0.34-H2O. The density profiles are derived from radio continuum emission at wavelengths from 6 to 0.3 cm. In the case of G35.20-1.74, a cometary UC H II region with a core and a tail, the spectrum of the core varies as Sν ∝ ν0.6, implying that the density structure is ne ∝ r-2. The emission from the tail has a flatter spectrum, indicating that the density gradient is also negative but shallower. For the case of G9.62+0.19, which is an H II region complex with several components, the spectrum of the region designated component E is Sν ∝ ν0.95, corresponding to ne ∝ r-2.5. The steepest spectral index, Sν ∝ ν1.4, is for the super UC H II region G75.78+0.34-H2O; its density stratification may be as steep as ne ∝ r-4. The actual density gradient may be smaller, owing to an exponential (rather than a power-law) density distribution or to the effects of finite spatial extent. The contribution from dust emission and some of the possible implications of these density distributions are briefly discussed.