NEBULAR PROPERTIES FROM FAR-INFRARED SPECTROSCOPY

We describe a semiempirical methodology-based on measurements of far-infrared (FIR) lines-that yields information on electron densities in regions where various ionic species exist, effective temperatures (T-eff) for stars ionizing H II regions, and gas-phase heavy element abundances. Although this capability has long been available via optical data, the special features of FIR lines-relative insensitivity to extinction and electron temperature variations-extend the analysis ability. Several line ratios serve as diagnostics of electron density, N-e, probing different ionization conditions and different density regimes. The more N-e-diagnostic observations made, the more reliable will be the deciphering of the actual variation in density throughout a nebula. A method to estimate T-eff from the FIR [N III]/[N II] line ratio requires that the nebula be ionization bounded and that substantially all of the flux from the relevant lines be observed. However, to estimate T-eff by a second method that uses the ratio of FIR [S III]/[O III] lines, an ionization-bounded nebula is a sufficient, but not necessary, condition. These restrictions are unnecessary for estimating densities and heavy element abundances. We show that a fairly general determination of metallicity, via the S/H ratio, may be made for H II regions with observations of just two lines-[S III] 19 mu m and a hydrogen recombination line (or appropriate substitute). These techniques are applied to recent FIR data for the G333.6-0.2 H It region, including application to the recently measured [N II] 122 and 205 mu m lines.