Dust characteristics of massive star-forming sites in the mid-infrared

Four massive star-forming regions were imaged in the mid-infrared with the MIRAC3 instrument: W51 IRS 2, Mon R2, DR 21, and S140. We obtained high spatial resolution (similar to1") images at several wavelengths from 7.8 to 13.2 mum with the circular variable filter, as well as narrow-band continuum images at 12.5 and 20.6 mum toward each region. In each massive star-forming region, one or more sources show deep silicate absorption. For at least two of the massive star-forming regions, W51 IRS 2 and Mon R2, the absorbing material is highly localized and may be circumstellar material in disks or shells. The silicate absorption occurs at least as often around massive young stars as around young stars of lower mass (which are more often observed). The estimated optical depths of the silicate features are consistent with those predicted by radiative transfer models toward ultracompact H II regions, but substantially higher than observed toward T Tauri stars and other low-mass young stellar objects. There is no consistent correspondence between silicate absorption and either the dust color temperature or the 12.5 km opacity. In W51 IRS 2, the two previously known mid-infrared sources have been resolved into at least six subsources. Infrared counterparts are newly reported for two radio-continuum sources in S140. Also, new mid-infrared sources have been detected in both W51 IRS 2 and S140. We suggest that the infrared source in the southwest of DR 21 may not be self-luminous, but may instead be heated by the three nearby radio continuum sources. The gas density in the ring at Mon R2 supports the blister scenario for the IRS 1 H II region.