Infrared line emission from H II regions. IV - Airborne observations of NGC 7538, W49, and M8

High spectral resolution (approx.0.025 cm/sup -1/) measurements of infrared fine structure lines are presented for several H II regions observed with a Michelson interferometer and the 91 cm telescope of NASA's Kuiper Airborne Observatory. (S III) (18.7 ..mu..m) and (O III) (52 and 88 ..mu..m) lines were measured on NGC 7538 and W49 A, as well as the (O III) (52 ..mu..m) line on M8. Densities derived from the measured (O III) lines ratios, combined with densities obtained for radio continuum and optical forbidden line data, are used to model the density structure of these sources in order to derive more accurate ionic abundances. The O/sup + +/ abundances derived from the present observations are found to be in good agreement with optical determinations for NGC 7538 and M8. Limited information on the velocity structure was also obtained. The line widths measured on W49 A are in close agreement with radio data but, for NGC 7538, are found to be systematically broader than both the radio and optical lines. Attempts to detect some of the IR lines on S106, W33, and Sgr A West were unsuccessful.