Infrared line emission from H II regions. III - Airborne observations of forbidden S III /18 microns and 33 microns/, forbidden O III /52 microns and 33 microns/, and forbidden N III /57 microns/ on M17

Abstract

High spectral resolution (approximately 0.025 kayser) observations of the fine structure lines of forbidden S III (18 and 33 microns), forbidden O III (52 and 88 microns) and forbidden N III (57 microns) have been made on up to 14 positions on M 17 with a Michelson interferometer and the 91-cm telescope of NASA's Kuiper Airborne Observatory. The results are presented and used to derive electron densities, ionic abundances, and limited information on the velocity structure. The density of 14,000/cu cm derived from the forbidden S III line ratio in the direction of the radio peak is found to be a factor of 8 higher than that derived from the forbidden O III line ratio. This result, combined with the rms densities obtained from radio continuum observations, implies a complex structure and wide range of densities in M 17. It is tentatively concluded that the ionizing spectrum is hotter than 40,000 K, but that the ionization excitation is substantially modified by the complex source structure and possibly dust absorption. The line widths are found to be narrowest in the region of the main radio peak and increase by up to a factor of 2 toward the N and E.