2002AJ....124.1636K

Mosaics of the M17 southwest photodissociation region (PDR) at the mid-infrared wavelengths 9.8, 10.53, 11.7, and 20.6 µm are presented. Maps of hydrocarbon and ionized gas emission were derived by subtracting the continuum at 9.8 µm from the 10.5 and 11.7 µm maps, respectively. Comparisons of the emission from hydrocarbons and ionized gas to the 9.8 µm/20.6 µm color temperature and 9.8 µm opacity maps show that regions of hotter 9.8 µm/20.6 µm color temperatures have increased emission from ionized gas, while the hydrocarbons are found in cooler regions with high 9.8 µm opacities. Emission at 20.6 µm is spatially correlated with 11.3 µm hydrocarbon emission. Two bright point sources found in the southwest PDR are high-mass young stellar objects caught at distinctly different early evolutionary phases. M17-UC1 appears to be older than IRS 5 because it has less maser emission and has an observable H II region in radio continuum. Both sources are modeled with a radiative transfer code, which takes advantage of both the spatial and spectral information available. Models that best match the profiles and spectral energy distribution of the ultracompact H II region, M17-UC1, have central stars with a luminosity and temperature equal to that of a zero-age main-sequence (ZAMS) B0 star, an outer radius less than 0.032 pc, an inner radius about the same size as the radio observed H II region, and a total shell mass of 0.6-3.4 M_☉. The best-fit models underestimate the total observed flux found at near-infrared wavelengths, which suggests that a small, hot disk may be present in M17-UC1. A second source, identified with the near-infrared source IRS 5, has comparable mid-infrared emission to M17-UC1. The best-fitting model to IRS 5 has a central star with a luminosity and temperature equal to that of a ZAMS B0 star, an outer radius on the order of 1 pc, a small inner radius (0.001 pc), and a total shell mass of 70 M_☉.