Multiwavelength Carbon Recombination Line Observations with the Very Large Array toward an Ultracompact H ii Region in W48: Physical Properties and Kinematics of Neutral Material

Abstract

Using the Very Large Array (VLA) the C76alpha and C53alpha recombination lines (RLs) have been detected toward the ultracompact H II region (UC HII region) G35.20-1.74. We also obtained upper limits to the carbon RLs at 6 cm (C110alpha and C111alpha) and 3.6 cm (C92alpha) 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 HII 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 HII 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 HII 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 HII regions cannot be ruled out.