Nebular properties from far-infrared spectrosopy

Details Nebular properties from far-infrared spectrosopy Nebular properties from far-infrared spectrosopy

Jan 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...420..772r&link_type=abstract

The Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 420, no. 2, p. 772-782

Astronomy and Astrophysics

Astronomy

72

Far Infrared Radiation, Infrared Astronomy, Infrared Spectroscopy, Interstellar Matter, Nebulae, Stellar Temperature, Abundance, Electron Density (Concentration), H Ii Regions, Metallicity, Recombination Reactions

Scientific paper

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 (Teff) 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, Ne, probing different ionization conditions and different density regimes. The more Ne-diagnostic observations made, the more reliable will be the deciphering of the actual variation in density throughout a nebula. A method to estimate Teff 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 Teff 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 micron and a hydrogen recombination line (or appropriate substitute). These techniques are applied to recent FIR data for the G333.6-0.2 H II region, including application to the recently measured (N II) 122 and 205 micron lines.

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