thesis - IRS, The Infrared Spectrograph

8 CHAPTER 1: IntroductionNGC 7027[Ar VI][Mg IV]HI (Br α)[Ne V][Ne III][Ne V][Ne II][Ne VI][Ar II]PAH[O IV][Ar III][Ar V][Si II][Ne III]PAHFigure 1.4–. ISO-SWS spectrum of PNNGC 7027. The strongforbidden lines dominatethe spectrum. Thishas been divided in twoplots to highlight thespectrum shortwards12µm (especially thedust features, PAHs).Around 80 lines havebeen measured in thisSWS spectrum. Highstages of ionization ofseveral ions are present.The spectrum from2.5 to 5 µm is richon hydrogen lines andshow as well some H 2.The H 2 lines are notseen here because ofthe scale.also occur. Since this process occurs less often the recombination lines are very weak anddifficult to measure.1.3.2 Electron density and temperature, and abundancesLine ratios in PNe are very useful to determine the physical properties of the gas. Weencourage the reader to have a look at Osterbrock (1989) for a complete description of theprocesses which are described next.To derive the temperature, line ratios that differ by several electron volts are needed sothat the relative population of the levels depends on the temperature. This is illustrated inFig. 1.5 for the O III. In the level configuration diagram the wavelengths of several lines areindicated. For instance, a ratio between any of the optical lines with any of the infrared lineswill be temperature dependent. The ratio [I(4959Å)+I(5007Å)]/I(4363) is also temperaturedependent and has been used extensively in the past because they are strong lines that canbe easily measured in the optical spectrum. In the top-right panel in Fig. 1.5 the theoreticalratio of the 5007 Å and 52 µm lines is represented at different temperatures. Therefore bymeasuring the intensities of both lines in the nebula its temperature can be inferred.