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Analysis and Interpretation of Astronomical Spectra 113

25 Identification of Spectral Lines

25.1 Task and Requirements

With the line identification, to an absorption- or emission
line with the wavelength , the responsible element or
ion is assigned. Considered purely theoretical this would
have to be relatively simple, as shown by the adjoining
excerpt of the "lineident" table, provided by the Vspec
software. In practice, however, inter alia the following
should be noted:

– The spectrum must show a high S/N ratio, further be
calibrated very precisely and adjusted by possible
Doppler shifts. Only that way we can exactly deter-
mine the wavelength of each line.

– The higher the resolution of the spectrum, the more
accurate can be determined and the fewer lines are
merging into so-called “Blends”.

25.2 Practical Problems and Solving Strategies

However the table shows, that in certain sections of the spectrum, the distances between
the individual positions are obviously very close. This happens from quantum mechanical
reasons for several of the metal lines, generating corresponding ambiguities, especially in
stellar spectra of the medium and later spectral classes.

Commonly concerned are also noble gases, as well as the so-called rare earth compounds
– eg praseodymium, lanthanum, yttrium etc. Such we find in the spectra of gas-discharge
lamps, acting here as dopants, alloy components and fluorescent agents.

Here, in most of the cases, helps the process of elimination. Most important is the knowl-
edge of the involved process temperature. For stellar spectra it is supplied by the according
spectral class. With this parameter the graphic at the end of sect. 13.11, provides on one
hand possible proposals, but excludes a priori also certain elements or corresponding ioni-
sation stages. As there already discussed, eg for normal photospheric solar spectra, Helium
He I can be excluded.

At certain stages of stellar evolution, detailed knowledge of the involved processes are
necessary. Since e.g. stars, in the final Wolf Rayet stage, first of all repel their entire outer
hydrogen shell, this element can therefore subsequently hardly be detected in such spec-
tra. Critical is here the mostly very significant He II emission at 6560.1 Å, which is often
misinterpreted by amateurs as Hα line at 6562.82 Å, see [33] tables 5 and 6.

Relatively easy is the line identification for calibration lamps with known gas filling. Thus
Vspec allows the superimposing of the corresponding emission lines, with their relative in-
tensities, directly into the calibrated lamp spectrum (see below). For such "laboratory spec-
tra" in Vspec [411] the "element" database has proven (Tools/Elements/element). For stel-
lar profiles, however, the "lineident" database is to prefer (Tools/Elements/lineident).

In cases of unknown gas filling, on a trial basis, the emission lines of the individual noble
gases He, Ne, Ar, Kr and Xe can be superimposed to the calibrated Lamp spectrum. In most
cases already the pattern of these inserted lines instantly shows, if the corresponding ele-
ment is present or not. This was also the most successful tactic for the line identification in
[32] [33] [34] [35]. However some of the noble gas emissions can be located very close to
each other such as Ar 6114.92 Å and Xe 6115.08 Å, see [33] Table 102.