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Analysis and Interpretation of Astronomical Spectra 106
If the recording of a spectral profile can be limited on a defined region within the nebula, in
both equations {61} and {62} the variables become identical. can then be eliminated
by equalisation of {61} and {62}. The implicitly remaining variable anway requires finally
an iteratively solving of the equation. However this requires that the values of all diagnostic
lines, for both methods, are available in good quality.
22.15 Estimation of the Electron Density from the S II and O II Ratio
The electron density can be estimated by Osterbrock from the ratio of the two sulfur
lines [S II] λλ 6716, 6731 or the oxygen lines [O II] λλ 3729, 3726 [201]. The big advan-
tage of this method: These lines are so close together, that the extinction and instrumental
responses can’t exert any significant effect on the ratio. The disadvantage is, that the two
lines, except by SNR, are generally very weak and therefore difficult to measure.
1.6
1.4
1.2
Intensity Ratio 1.0
[S II] 6716 / 6731
0.8
0.6
[O II] 3729 / 3726
0.4
0.2
0.0 102 103 104 105
101 Electron Density [cm-3]
22.16 Distinguishing Characteristics in the Spectra of Emission Nebulae
Due to the synchrotron and bremsstrahlung SNR show, especially in the X-ray part of the
spectrum, a clear continuum see [33], Table 85. This appears especially pronounced in the
X-ray domain, so X-ray telescopes are highly valuable to distinguish SNR from the other
nebula species, particularly at very faint extragalactic objects. For all other types of Emis-
sion Nebulae the detection of a continuum radiation is difficult.
In the optical part of SNR spectra, the [S II] and [O I] lines are, relative to Hα, more intense
than at PN and H II regions. This effect is caused here by shock wave induced collision ioni-
sation, see [33], Table 85. The [S II] and [OI] emissions are very weak at PN and almost to-
tally absent in H II regions [201].
The electron density is very low in SNR, ie somewhat lower than in H II regions. It
amounts in the highly expanded, old Cirrus Nebula to about 300 cm-3:
By the still young and compact Crab Nebula it is about 1000 cm-3 [201]. By PN, gets
highest and is usually in the order of 104 cm-3 [201]. In the H II region of M42, is within
the range of 1000–2000 cm-3 [224].
In H II regions, the excitation by the O- and early B-class stars is relatively low and there-
fore the excitation class in the order of E = 1-2 only modest [33].
