Peculiar Spectra

The spectra of some stars differ from what one would expect on the basis of their temperature and luminosity (see, e.g., Fig. 8.7). Such stars are celled peculiar. The most common peculiar spectral types will now be considered.

The Wolf-Rayet stars are very hot stars; the first examples were discovered by Charles Wolf and Georges Rayet in 1867. The spectra of Wolf-Rayet stars have broad emission lines of hydrogen and ionized helium, carbon, nitrogen and oxygen. There are hardly any absorption lines. The Wolf-Rayet stars are thought to be very massive stars that have lost their outer layers in a strong stellar wind. This has exposed the stellar interior, which gives rise to a different spectrum than the normal outer layers. Many Wolf-Rayet stars are members of binary systems.

In some O and B stars the hydrogen absorption lines have weak emission components either at the line centre or in its wings. These stars are called Be and shell stars (the letter e after the spectral type indicates that there are emission lines in the spectrum). The emission lines are formed in a rotationally flattened gas shell around the star. The shell and Be stars show irregular variations, apparently related to structural changes in the shell.

About 15% of all O and B stars have emission lines in their spectra.

The strongest emission lines are those of the P Cygni stars, which have one or more sharp absorption lines on the short wavelength side of the emission line. It is thought that the lines are formed in a thick expanding envelope. The P Cygni stars are often variable. For example, P Cygni itself has varied between three and six magnitudes during the past centuries. At present its magnitude is about 5.

The peculiar A stars or Ap stars (p = peculiar) are usually strongly magnetic stars, where the lines are split into several components by the Zeeman effect. The lines of certain elements, such as magnesium, silicon, europium, chromium and strontium, are exceptionally strong in the Ap stars. Lines of rarer elements such as mercury, gallium or krypton may also be present. Otherwise, the Ap stars are like normal main sequence stars.

The Am stars (m = metallic) also have anomalous element abundances, but not to the same extent as the Ap stars. The lines of e. g. the rare earths and the heaviest elements are strong in their spectra; those of calcium and scandium are weak.

We have already mentioned the S and C stars, which are special classes of K and M giants with anomalous element abundances. In the S stars, the normal a)

Fig. 8.7a,b. Peculiar spectra. (a) RGeminorum (above) is star is compared with one in which the zirconium lines an emission line star, with bright emission lines, indicated are unusually strong. (Mt. Wilson Observatory and Helsinki by arrows, in its spectrum; (b) the spectrum of a normal Observatory)

Fig. 8.7a,b. Peculiar spectra. (a) RGeminorum (above) is star is compared with one in which the zirconium lines an emission line star, with bright emission lines, indicated are unusually strong. (Mt. Wilson Observatory and Helsinki by arrows, in its spectrum; (b) the spectrum of a normal Observatory)

lines of titanium, scandium and vanadium oxide are replaced with oxides of heavier elements, zirconium, yttrium and barium. A large fraction of the S stars are irregular variables. The name of the C stars refers to carbon. The metal oxide lines are almost completely absent in their spectra; instead, various carbon compounds (CN, C2, CH) are strong. The abundance of carbon relative to oxygen is 4-5 times greater in the C stars than in normal stars. The C stars are divided into two groups, hotter R stars and cooler N stars.

Another type of giant stars with abundance anomalies are the barium stars. The lines of barium, strontium, rare earths and some carbon compounds are strong in their spectra. Apparently nuclear reaction products have been mixed up to the surface in these stars.

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