NEAR-INFRARED AND VERY HIGH-SENSITIVITY VLA-2 CENTIMETER CONTINUUM OBSERVATIONS OF THE H-II REGION G29.96-0.02

The ultracompact H II region G29.96-0.02 has been imaged at a frequency of 15 GHz using combined multiple Very Large Array configuration data. The images show a remarkably complex structure. The edge-brightened arclike core of the H Ir region is embedded in low-level emission extending over several tens of arcseconds (1 '' approximate to 0.036 pc). Filamentary-like structures are observed east of the core, some perpendicular to the symmetry axis of the arclike core. These radio continuum observations are inconsistent with predictions of current models for ultracompact H II regions. Near-infrared images have also been obtained with the USNO infrared camera. These images identify the position of the ionizing star and suggest that the H II region is embedded in a young stellar cluster. We suggest a model in which the radio continuum appearance of G29.96-0.02 may be explained simply by expansion in a highly anisotropic medium; more exotic models are not required. To the west, the H II region is expanding into a region with a relatively high ambient density. This denser region impedes the expansion of the H II region in that direction. Molecular material is photoevaporated off a hot molecular core and is then swept back along pressure gradients, creating the cometary appearance. On the eastern side, the H II region expands freely into a less dense but still highly anisotropic ambient medium, thus creating the observed filamentary structure.