Page 101 - analysis-and-interpretation-of-astronomical-sp

Analysis and Interpretation of Astronomical Spectra 101

In comparison, for the permitted Hα line, at least 12.1 eV would be required from the
ground state (n1 – n3). For this, the electrons in the nebula are by far too slow, i.e. by about
one order of magnitude (see diagram below). This explains the strong intensity of the for-
bidden-as compared to the allowed transitions. These metal ions are also called "cooler"
[237] in the context of model computations. Influenced by the highly effective line emission
they contribute significantly to the cooling of the nebula and therefore to the thermal equi-
librium. The following chart shows just the relevant small excerpts of the highly complex
term diagrams [10], [222]. For the most important metal ions, the required excitation ener-
gies and the wavelengths of the “forbidden” emissions are shown.

[eV]

7

6

5 3344
1815
4 4363 1794
7331
7319.6
7330
7318.6

3 5755
3071
2 10338 3063
10373
10278
10320
4076

4068

1 6731 6583 5007 4014
6717 6548 4959 3967
4932 3869
3729
3726

0 [N II] [O III] [Ne III] [O II]

[S II]

The following chart shows by Gieseking [222], the Maxwellian frequency distribution of
electron velocities for relatively "cool" and "hot" nebulae, calculated for Te 10,000K and
20,000K. Mapped are the two minimum rates for the excitation of the [O III] lines. The up-
per edge of the diagram I have supplemented with the values of the kinetic electron energy.
The maximum values of the two curves correspond to the average kinetic energy according
to formula {54} (0.86eV and 1.72eV).

Kinetic Energy [eV]

0 0.25 0.5 1 2 3 4 5 6 7 8 9 10 11

0.86 eV
Relative Frequency
1.72 eV
2.5 eV
5.3 eVO III 5007/4959/ 4932

O III 4363

0 200 400 600 800 1000 1200 1400 1600 1800

Electron Velocity [km/s]