optical emission lines are used to determine physical parameters, such as temperature, density and chemical composition of hii regions. however, one has to use lines from different ions as a diagnostic for electron temperature (te) and density (ne). the interpretation of these diagnostics, in particular in spectra of extragalactic hii regions, rely on the assumption that the observed lines are all emitted by the same region within the nebula. this may not be the case, which makes the interpretation ambiguous. this problem is exacerbated by the effects of extinction of optical lines. properties derived from these diagnostics depend in addition on the assumption that the nebula is simple and homogeneous. the best way to verify the validity of these assumptions is to actually measure the temperature, ionization and density structure in resolved hii regions. this can best be done in our galaxy. the only way to circumvent the ambiguity mentioned earlier, is to use far-ir lines combined with optical lines, all from the same ion, to determine ne and te simultaneously. several ions are suited for such an analysis (e.g. s iii, o iii or n ii). we want to measure the variations in strength of the lines of these ions, by performing a raster of observations covering the entire hii region. because we want to model the hii regions using the photo-ionization code cloudy we chose objects that have a relatively simple geometry. since we will be able to determine the projected 2-dimensional structure of the hii regions, we can compare this with the model calculations. the combination of these observations and the subsequent modeling will be a unique testbed, r^ant for the interpretation of spectroscopic data of extragalactic hii regions. it will furthermore provide detailed insight in the physics of hii regions. in order to do the analysis we need accurate measurements of mid- and far- infrared fine-structure-lines. these observations cannot be done from the ground. only iso provides the required signal-to-noise and spectral resolution to reach our goals.
Instrument
LWS01 , SWS02
Temporal Coverage
1997-07-24T09:09:37Z/1997-08-23T07:07:21Z
Version
1.0
Mission Description
The Infrared Space Observatory (ISO) was the worlds first true orbiting infrared observatory. Equipped with four highly-sophisticated and versatile scientific instruments, it was launched by Ariane in November 1995 and provided astronomers world-wide with a facility of unprecedented sensitivity and capabilities for a detailed exploration of the Universe at infrared wavelengths.
European Space Agency, van der Hulst et al., 1999, 'DENSITY AND IONIZATION STRUCTURE OF HII REGIONS openParPART 1closePar', 1.0, European Space Agency, https://doi.org/10.5270/esa-38vptk6
