Five star-forming cores in the Galactic Ring Survey: A mid-infrared study

We have imaged five dense molecular cores, selected from the Galactic Ring Survey (GRS), in the mid-infrared with the MIRAC3 instrument. We obtained high spatial resolution (similar to1") images through narrow-band filters at 12.5 and 20.6 mum. Four of the five cores show multiple compact sources, extended structure, or both. Lower resolution observations by the Infrared Space Observatory (ISO) and the Midcourse Space Experiment (MSX) suggest that the fifth core is also surrounded by extended emission on large scales (greater than or similar to2'). The extended mid-infrared structure is well-correlated with the radio continuum morphology in each of the five cores. This similarity suggests that the hot dust traced by the mid-infrared is located within the H II region, traced by the radio continuum, and not merely in a surrounding photodissociation region or molecular cloud. If a single exciting source is assumed for each core, estimates of the zero-age main-sequence spectral types based on the infrared luminosities are typically 1-2 spectral types earlier than those based on the radio free-free emission. However, allowing for multiple exciting sources and apportioning the far-infrared and radio fluxes to the component sources according to the mid-infrared flux distribution produces better agreement between the derived spectral types, with an average difference of less than half a spectral type.