Thermal Dust Imaging of the Ultracompact HII Region G29.96-0.02

We have obtained mid-infrared images of the ultracompact H II region G29.96-0.02 at 1" resolution. These include three broadband images at 8.5, 9.7, and 12.0 μm and three narrowband images covering the spectrum around the 12.8 μm [Ne II] emission line. The appearance of the object in all bands is basically similar to the arcuate or "cometary" shape seen in previously published high-resolution radio maps. At our shortest wavelength of 8.5 μm, the faint, diffuse emission from the cometary "tail" is enhanced compared to its brightness at other wavelengths. Taking advantage of well-matched passbands, we have used the ratio between the images at 12.0 and at 8.5 μm to construct maps of the dust color temperature Td and emission optical depth τem in the object. The optical depth is sharply peaked along a long, thin ridge near the leading edge of the cometary arc. The shape of this ridge is nearly identical to that seen in the high-resolution 2 cm maps, strongly suggesting that the dust which is warm enough to emit at 8 and 12 μm is inside or extremely close to the zone of ionized gas in G29.96-0.02. This conclusion is further bolstered by the essentially identical distribution of the [Ne II] emission and the thermal dust continuum at adjacent wavelengths. The key feature of the dust temperature map is its lack of marked variation. The great majority of the nebula shows temperatures between 190 and 240 K. The small variations that do occur present a clear pattern. At the center of the ridge of high τrmem, Td is around 210 K. Moving outward to the wings of the arc, the temperature falls slowly to about 190 K at the extreme ends. On the other hand, moving back along the symmetry axis into the nebula' s tail we find that mean temperatures increase to around 230 K, with a good deal of scatter. We have tried modeling these temperature variations using some simple concepts derived from earlier radiative transfer work on related objects, together with a published model of G29.96-0.02 as a bow shock. Unlike our previous success in applying such a framework to our observations of the ultracompact source GS.89-0.39, the models fail to match the temperature pattern in G29.96-0.02. Of several possible processes which may account for the discrepancy, an increased abundance of small grains caused by shock processing of the dust seems to present the fewest obvious difficulties.