Thermal infrared imaging of ultracompact HII regions in W49A

Several compact radio continuum sources in W49A show detectable 8-20 mu m emission in MIRAC2 images obtained at the IRTF. In general, the infrared morphologies of these sources closely resemble the radio continuum emission. Spectral energy distributions indicate an infrared continuum excess above the level expected from free-free emission, consistent with thermal emission from dust grains heated to a few hundred K. The bright radio continuum sources concentrated at the western end of the ring of ultracompact Hn regions are not detected in the mid-infrared, while those at other positions in the ring are detected. This could be due to a localized region of high extinction along the line of sight. In addition, there are a few new infrared sources with no radio continuum counterparts. Finally, several infrared sources show strong 12.8 mu m [Ne II] emission, yielding neon abundances that are typically a few percent of the cosmic abundance of neon but are high considering the expected Ne++/Ne+ ratios for the range of spectral types of the ionizing sources. We conclude that the [Ne II] emission must come from shells around the ultracompact Hn regions, where the neon is able to survive as Ne+ rather than Ne++ because the radiation field has been softened by absorption of hard UV photons within the H II regions.