2005ApJ...626..253R

Using the Very Large Array (VLA) the C76α and C53α recombination lines (RLs) have been detected toward the ultracompact H II region (UC H II region) G35.20-1.74. We also obtained upper limits to the carbon RLs at 6 cm (C110α and C111α) and 3.6 cm (C92α) wavelengths with the VLA. In addition, continuum images of the W48A complex (which includes G35.20-1.74) are made with angular resolutions in the range 14"-2". Modeling the multiwavelength line and continuum data has provided the physical properties of the UC H II region and the photodissociation region (PDR) responsible for the carbon RL emission. The gas pressure in the PDR, estimated using the derived physical properties, is at least 4 times larger than that in the UC H II region. The dominance of stimulated emission of carbon RLs near 2 cm, as implied by our models, is used to study the relative motion of the PDR with respect to the molecular cloud and ionized gas. Our results from the kinematical study are consistent with a pressure-confined UC H II region with the ionizing star moving with respect to the molecular cloud. However, based on the existing data, other models to explain the extended lifetime and morphology of UC H II regions cannot be ruled out.