The density structure of highly compact HII 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 S-nu proportional to nu (0.6), implying that the density structure is n(e) proportional to 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 S-nu proportional to nu (0.95) corresponding to n(e) proportional to r(-2.5). The steepest spectral index, S-nu proportional to nu (1.4), is for the super UC H II region G75.78+0.34-H2O; its density stratification may be as steep as n(e) proportional to 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.