Thermal infrared imaging of ultracompact HII regions in W49A

Several compact radio continuum sources in W49A show detectable 8-20 mu m e mission in MIRAC2 images obtained at the IRTF. In general, the infrared mor phologies of these sources closely resemble the radio continuum emission. S pectral energy distributions indicate an infrared continuum excess above th e level expected from free-free emission, consistent with thermal emission from dust grains heated to a few hundred K. The bright radio continuum sour ces 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 extinct ion along the line of sight. In addition, there are a few new infrared sour ces 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 consid ering the expected Ne++/Ne+ ratios for the range of spectral types of the i onizing sources. We conclude that the [Ne II] emission must come from shell s 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 absor ption of hard UV photons within the H II regions.