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 frequenc y of 15 GHz using combined multiple Very Large Array configuration dat a. The images show a remarkably complex structure. The edge-brightened arclike core of the H Ir region is embedded in low-level emission ext ending over several tens of arcseconds (1 '' approximate to 0.036 pc). Filamentary-like structures are observed east of the core, some perpe ndicular to the symmetry axis of the arclike core. These radio continu um observations are inconsistent with predictions of current models fo r ultracompact H II regions. Near-infrared images have also been obtai ned 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 continuu m 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 hi gh 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 regio n expands freely into a less dense but still highly anisotropic ambien t medium, thus creating the observed filamentary structure.