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Analysis and Interpretation of Astronomical Spectra 99
the ionisation stage (sect. 11) and the abundance of the generated ions. The first two pa-
rameters can be directly read from the spectrum and compared with the required ionisation
energy (see table below and [33]).
Photon Elektron
Star
Y
Te Ne
High-energy Ionisation by UV- Electron gas
Radiator Teff>25‘000K Photons λ<912Å
The kinetic energy of the electrons, released by the ionisation process, heats the nebula
particles. corresponds to the surplus energy of the UV Photons, which remains after
photo ionisation and is fully transformed to kinetic energy of the free electrons. The elec-
tron temperature and -density affect the following recombination- and collision exci-
tation processes. is directly proportional to the average kinetic energy of the free elec-
trons (Boltzmann constant ).
Formula {55} yields in Joule with the electron mass and .
The short formula {55a} gives directly in electron volts [eV].
22.5 Recombination Process Electron
If an electron hits the ion centrally, it is captured and ends up first Photon
mostly on one of the upper excited levels (terms). The energy, gen-
erated this way is emitted as a photon . It corresponds to the +ΔEn
sum of the original kinetic energy of the electron and the dis-
crete energy difference due to the distance to the Ionisation
Limit. Since the share of the kinetic energy varies widely, from
the recombination process a broadband radiation is contributed to
the anyway weak continuum emission.
22.6 Line Emission by Electron Transition Recombination
After recombination the electron “falls” either directly or via sev-
eral intermediate levels (cascade), to the lowest energy ground
state . Such transitions generate discrete line emission, ac- Photon
cording to the energy difference . Most of these photons ΔEn
leave the nebula freely – including those which end in the pixel
field of our cameras! This process cools the nebula, because the
photons remove energy, providing thereby a thermal balance to n=1
the heating process by the free electrons. This regulates the elec-
tron temperature in the nebula in a range of ca.
5,000K < < 20,000K [237].
Electron Transition
