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Analysis and Interpretation of Astronomical Spectra 103

22.11 Practical Aspects of Plasma Diagnostics

The main focus for amateurs is here the determination of the excitation class of emission
nebulae. Their diagnostic lines are relatively intense and quite close together. Moreover,
some of them are located in an area, where the difference between the original and
pseudo-continuum is relatively low – see early spectral classes in the last graph of sect.
8.10. This allows for galactic objects, even at the raw profile, ie without any extinction- or

other corrections, a reasonable classification with accuracy of about 1 class. Due to the

slightly greater distance of the He II diagnosis line (λ 4686), at middle and high excitation
classes the classification may result in up to one step to low. For emission lines, the propor-

tional-radiometric correction procedures, presented in sect. 8.8 – 8.11, are ineffective for

the correction of the continuum related measurements ( and ). For more precise

analyses, the intensity of the individual lines to be analysed, should be corrected according
to formula {53}, see comments sect. 21.4.

The measurement of the attenuated emission lines must be performed in an intensity-
normalised profile according to sect. 8.9 (see comments in sect. 20.5). Emission nebulae
usually generate an extremely weak and diffuse continuum. Thus these objects do not allow
any reliable determination of the continuum-based or values. Therefore the direct
measurement of the values in arbitrary units is preferable.

Typical galactic decrement values are in the range of D ≈ 3.0–3.5 [10]. However, there are
stark outliers like NGC 7027 with D ≈ 7.4 [10]! Since the classification lines lie close to-
gether, for a rough determination of the excitation class even this effect can usually be ne-
glected. In such extreme cases, a substantial part of the extinction can also be caused by
massive dust clouds around the star itself. E.g. at the extremely young T-Tauri objects the
measured deviation from the Balmer-Decrement is even used as a classification criterion,
see [33] sect. 13.2!

Special cases are here the faint, not two dimensional, but rather star-like appearing Plane-
tary Nebulae. In contrast to M27 and M57 they generally require relatively short exposure
times. Further they cannot be recorded on a specific area within the nebula but only in the
total light, integrated within the slit of the spectrograph. Since within these tiny discs, lar-
ger intensity differences at individual emissions occur. So even the measured Balmer-
Decrement may be distorted, aggravated by possible shifts in the slit position during re-
cording due to bad seeing and/or poor autoguiding. However, even this influence on the
determination of the excitation class has been found as low.

The determination of additional plasma parameters such as the electron temperature Te
and the electron density Ne specifically requires low-noise spectra with high resolution, re-
garding these faint objects, a real challenge with amateur equipments. In addition, some of
the used diagnostic lines are extremely weak and the error rate is correspondingly high.
Even between values in professional publications, often major deviations are noted!