Far-infrared spectral observations of M42 and M17

Results are reported for coarse-resolution far-IR spectral observations of the H II regions M17 and M41 in the wavelength range from 42 to 115 microns. The observations were made at an altitude of 13.7 km with the 30-cm telescope of the NASA Lear jet. The spectrum of M42 is shown to be quite smooth and to fit a diluted 100-K blackbody very well. It is found that the emission from M17 falls steeply from 40 to 63 microns, then rises again to 80 microns before falling off once more toward longer wavelengths. The best fit of this spectrum for an inverse-square-wavelength emissivity law is found to require a two-temperature model with temperatures below 30 K for the cold dust component. It is concluded that the spectrum of M17 seems to require the presence of a material with a better far-IR emissivity than those for which detailed laboratory measurements have been made.