thesis - IRS, The Infrared Spectrograph

1.3. Physics in PNe 95.35 eVO IIIλ 4363A1 S 02.5 eVλ 5007A0.01 eVλ 4959A52 µ m88.8 µ m1D23P23P3 1 P 07.93 eVNe V1S 03.8 eVλ 2974A0.09 eVλ 3426A14.3 µ m24.3 µ m1D23P23P3 1 P 0Figure 1.5–. Ground level configurations for O III and Ne V (left). On the right, different line ratiosare shown as temperature (top) or the density (bottom) indicators. See Sect. 1.3 for details.To derive the density, intensity ratios of lines originating from levels close in energy areneeded so that the dependence on temperature is canceled. In that respect the infrared linesoriginating from the ground levels are very useful since their dependence on temperatureis negligible. If their radiative transition probabilities or collisional de-excitations (whichdepend on density) are different then the ratio depends on the density. This is illustratedin Fig. 1.5 for Ne V. The ratio of both optical lines (indicated in the figure) or infraredlines is density dependent. In the bottom-right panel of Fig. 1.5 the theoretical ratio ofthe infrared lines is given at different densities, therefore by measuring those lines thedensity of the gas can be determined. Note that the density can only be determined between∼ 10 4 and ∼ 3×10 5 cm −3 and that the dependence on the adopted temperature is very small.Abundances are derived relative to hydrogen. Therefore, to derive the abundance of anion (ionic abundance) besides the intensity of an ionic line and the nebular density, the intensityof a hydrogen line together with the H + density are needed. The forbidden lines are veryuseful since they are very strong and their flux can be accurately measured. For hydrogenthe intensity of Hβ is currently used. To derive the total abundance of an element (elemental