An 8.5 GHz Arecibo Survey of Carbon Recombination Lines toward Ultracompact H II Regions: Physical Properties of Dense Molecular Material

We report here on a survey of carbon recombination lines (RLs) near 8.5 GHz toward 17 ultracompact H II regions (UCHs). Carbon RLs are detected in 11 directions, indicating the presence of dense photodissociation regions (PDRs) associated with the UCHs. In this paper, we show that the carbon RLs provide important, complementary information on the kinematics and physical properties of the ambient medium near UCHs. Non-LTE models for the carbon line-forming region are developed, assuming that the PDRs surround the UCHs, and we constrained the model parameters by multifrequency RL data. Modeling shows that carbon RL emission near 8.5 GHz is dominated by stimulated emission, and hence we preferentially observe the PDR material that is in front of the UCH continuum. We find that the relative motion between ionized gas and the associated PDR is about half that estimated earlier, and it has an rms velocity difference of 3.3 km s-1. Our models also give estimates for the PDR density and pressure. We found that the neutral density of PDRs is typically >5 × 105 cm-3, and UCHs can be embedded in regions with high ambient pressure. Our results are consistent with a pressure-confined H II region model in which the stars are moving relative to the cloud core. Other models cannot be ruled out, however. Interestingly, in most cases, the PDR pressure is an order of magnitude larger than the pressure of the ionized gas. Further investigation is needed to understand this large pressure difference.