Observations of [S III] 18.71 micron emission in galactic H II regions

Emission of [S III] 18.71 μm radiation has been detected in the Orion Nebula, M17, NGC2024, and W51, and an upper limit has been set for Sgr A. The line flux from M17 has been mapped with a resolution of 2.'7. A simple theory for predicting the [S III] flux from radio continuum observations is presented and compared with the data. It is found that Orion and M17 are in satisfactory agreement with the observations, and that NGC2024 is consistent with a low excitation source behind a reasonable amount of 18 μm dust extinction. In W51 the low observed flux may be explained either by low S++ abundance or, more likely, by dust extinction. In Sgr A attenuation by dust is the most plausible explanation