  We present data from mid-infrared Keck Telescope imaging of 18 radio-selected
ultra-compact HII region candidates at diffraction-limited resolution. The goal
of these observations is to determine the sizes, luminosities, and morphologies
of the mid-infrared emitting dust surrounding the stellar sources. All 18
sources were imaged at 11.7um and at 17.65um, and 10 of them were imaged also
at 24.5um. All the sources were resolved. We have generated dust temperature
and optical depth maps and combine them with radial velocity measurements and
radio data (1.4 and 5 GHz) to constrain the properties of these star-forming
regions. Half of our objects are excited by B-stars, and all our objects have
derived types that are later than an O6 star. We find a significant correlation
between infrared and radio flux densities, and a weaker one between infrared
diameters and the central source ionizing photon rates. This latter correlation
suggests that the more compact sources result from later spectral types rather
than young age. Our new data may suggest a revision to infrared color selection
criteria of ultra-compact HII regions at resolutions <1". These 18 sources are
part of a sample of 687 sources dominated by ultra-compact HII regions selected
by matching radio and infrared maps of the first Galactic quadrant by Giveon
and coworkers. The new mid-infrared images constitute a significant improvement
in resolving sub-structure at these wavelengths. If applied to all of this
sample our analysis will improve our understanding of embedded star-formation
in the Galaxy.
