Proposed models for the IR emission from M17 are reconsidered in the light of recent middle-IR and far-IR data which allow a more accurate determination of the nebula's spatial structure. The data include a 100-micron map of the source, large-beam observations at 12 and 21 microns, and high-sensitivity high-resolution maps of the brightest regions at 10 microns. These observations show that the middle-IR, 100-micron, and radio continuum sources for the southern component of the H II region peak at the same position; a compact source with a deep silicate absorption feature lies near the center of the southern component; the spectra of the entire source and the central two arcmin of the southern component are extremely broad; and the total luminosity of the source is 5.6 million times the solar luminosity. It is concluded that: (1) most of the IR luminosity is generated in or very close to the ionized region, (2) there is no compelling evidence for strong depletion of dust throughout the ionized region, (3) large grains are responsible for most of the IR luminosity, and (4) the structure and evolution of the H II region may be strongly influenced by spatial variations in the density which were present when the exciting stars were formed.
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Harper et al. (1976) studied this question.