THERMAL DUST IMAGING - THE ONSET OF ASYMMETRY IN THE ULTRACOMPACT H-II REGION G5.89-0.39

We have obtained images of the compact radio and infrared nebula G5.89-0.39 with the Berkeley/IGPP mid-infrared array camera. The images have a resolution of 1."1 and cover five wavelength bands between 8 and 13-mu-m. On the basis of the structure and spectral energy distribution, we concur with previously published assertions that G5.89-0.39 is an ultracompact H II region. The infrared emission is extended over 6" x 4".5, very similar to the object's size in published high-resolution radio continuum maps. The mid-infrared nebula is arcuate in shape, taking the form of a partial shell open to the south, unlike the radio images where the shell is complete. At our shortest wavelength the structure is sharp-edged as in the radio maps, but diffuse emission is seen at the longest wavelength. A deep silicate absorption band is the principal feature of the spectrum of the object at these wavelengths. The ratio of the 12.5 to the 8.5-mu-m surface brightness indicates a gradual cooling of the dust toward the western side of the infrared nebula, and a more abrupt temperature drop at the inner boundary of a zone of cool, diffuse emission in the southeast. The radio and infrared structure of G5.89-0.39 can be understood in the context of a previously published model in which the O star is surrounded by a dust-free cavity that is in turn embedded in a dusty molecular cloud. The ionization front is trapped in the swept-up material that lines the boundary of this cavity, and the mid-infrared emission arises from warm dust in the same location. The new element which our infrared data add to this picture is a clear indication of how the source deviates from spherical symmetry. The intensities and temperatures of the dust are well fitted by a simple model that places the star off-center in the radio shell, implying that the expansion of the ionization front has been anisotropic. Correlations between specific features in the radio and mid-infrared images suggest that the expansion has been shaped by density variations in the molecular medium surrounding the star and that, as a consequence, G5.89-0.39 may be undergoing a rapid transition from a state of approximate spherical symmetry to one of fairly irregular morphology.