The Spectrum of an Emission Nebula from a Single Ionic Species
Joachim Köppen Kiel/Strasbourg/Illkirch April 2000
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in state of preparation
Interstellar gas is most conspicous when ionized by a hot star in the neighbourhood, such as in a H II region, planetary nebulae, the nuclei of active galaxies, and quasars. This plasma - at temperatures around 10000 K - produces a spectrum with strong emission lines of recombination lines of hydrogen and helium, and lines of heavy elements (C, N, O, Ne, S, Ar, Cl, ...) excited by collisions with the electrons. The nebula is optically thin in almost all these lines, the processes of line excitation are well known and the atomic data is quite accurate. Therefore, one can reliably analyse the spectra and derive the chemical composition in the gas.
This applet permits to compute the intensities of all the lines of a given ion relative to the intensity of one of its lines. One specifies the temperature and the density of the emitting gas, as well as the extinction measure c at the Hbeta line.
This allows you to estimate on the basis of one lines observed from a certain ion which other lines could be observable under given physical conditions and interstellar extinction.
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