thesis - IRS, The Infrared Spectrograph

18 CHAPTER 2: The ISO–SWS Spectrum of Planetary Nebula NGC 7027[Mg IV][Ar VI][Ne VI][Ar III]a)HI (Br α)[Ar V]PAH[Ne III][O IV]b)[Ne V][Ne V][Ne III]Figure 2.1–. SWS spectrum of NGC 7027. As can be seen strong lines of high ionization stages ofsome ions are seen. Hydrogen lines are abundant. Also noticeable the strong continuum at longerwavelength and PAHs at 6.6, 7.7 and 8.8 µm.2.5 The visual and the ultraviolet spectrumIn addition to the large number of infrared lines measured, also some optical and ultravioletlines were used taken from the paper of Keyes et al. (1990). The information on suchimportant ions as C, N, O is mainly derived from optical and ultraviolet data.The lines used are listed in Table 2.3. To correct optical fluxes for interstellar extinction