Observations of Infrared Fine-Structure Lines: [S III]

Extract

Recent observations of the 18.7 ym fine-structure line of S++ in NGC 7027 and BD+30°3639 (Greenberg, Dyal and Geballe, 1977 Ap.J.(Letters), 213, L74) allow the first determination of an ionic column density in ionized nebulae. The line ratios 18.7 μm/A9532 and λ6312/λ9532, besides yielding both electron density and temperature in the S++ region, have been used to indicate that the fine-structure levels of S++ are collisionally saturated. In this case the 18.7 ym surface brightness directly measures the column density of S++ ions with little dependence upon nebular structure, the major uncertainty being the experimental error. This research has been partially supported by NASA Grants NGR 05-003-511 and NGL 05-003-272.