21. The Peculiar A Stars
The most frequently encountered of the peculiar A stars are the ``silicon,'' ``strontium,'' and ``manganese'' groups and the so-called ``metallic-line'' stars. The spectra of the last-named consist essentially of features which seem to belong to two different spectral types and are considered separately.
The silicon and strontium stars can be identified on spectrograms of fairly low dispersion, but a satisfactory description of the details can be made only from medium- or high-dispersion spectra. Some of the brighter of the peculiar stars whose spectra can be used as prototypes are described below.
a And. - B9p. Manganese. The lines of Mn II are abnormally strong. On considerably widened, fine-grain spectrograms having a dispersion of 125 A per mm at H a number of peculiar faint lines are visible, which are sufficient to distinguish this type of spectrum from others.
Lib. - B9p. Silicon. The K line is very faint. The appearance of the wings of the H lines indicates that the star is brighter than the ordinary main-sequence stars.
Aur. - A0p. Silicon. The K line is exceedingly faint. The lines of Cr II vary in intensity. The star appears to be of luminosity class III and is brighter than the main sequence. The absolute magnitude is probably around - 1 to -2.
a CVn (brighter). - A0p. Silicon-Europium. The spectrum is exceedingly complex and requires the highest dispersion for adequate study. The lines of Si II and Eu II are both strong. Many spectral lines vary in intensity. The appearance of the wings of the hydrogen lines indicates that the star is more luminous than an ordinary A dwarf. The absolute magnitude is probably around - 1 to -2.
UMa. - A0p. A number of peculiar features which distinguish the spectrum of 78 Vir are present but are in general fainter. The Si II lines are not abnormally strong. The K line and a number of other spectral features vary in intensity within a period of a few days. This star is the brightest of the ``spectrum variables.''
17 Com. - A2p. Chromium-Europium. The spectrum is similar to 78 Vir. The K line is weak. The star is a member of the Coma cluster.
78 Vir. - Chromium-Europium. The general level of excitation corresponds roughly to an A2 star. There may be a faint, broad K line superposed over the sharp component. The blended feature at 4171, indicative of strong Cr II, is outstanding on spectrograms of low dispersion. Si II is weak; the blend at 4128 - 4132 is not due principally to Si II but is indicative of a ``europium star.'' The K line is weak. 78 Vir is a member of the Ursa Major cluster.
73 Dra. - Ap. Strontium-Europium-Chromium. A number of the lines, including 4077 and 4215, are variable in intensity. The K line is about as strong as in a normal B8 spectrum. The effective excitation is considerably lower than in a CVn and the spectrum is crowded with metallic lines.
Cas. - A5p. Strontium.
Cap. - Strontium. The spectrum can be classified as near F0 III. The strontium line at 4077 is abnormally strong but not so strong as in Equ. In both spectra the line is stronger than in any normal luminosity class at F0. There is no well-marked absolute-magnitude effect for 4077 at F0; this is near the place at which the effect changes from a negative one (early A-stars) to the strongly positive one observed in the F5-M stars.
Equ. - Strontium-Europium. The type is near F0, but the spectrum is so peculiar that a luminosity class cannot be determined. The Sr II lines 4077 and 4215 are stronger than in any other F0 star observed at Yerkes. This should not, however, be taken as evidence of high luminosity, since Sr II is insensitive to luminosity changes near F0 and more sensitive lines do not indicate that the star is a supergiant. The blend at 4128-4132 is strong, but this is not due to Si II. In stars later than A0 it appears to be indicative of the presence of Eu II.
CrB. - Chromium-europium. The spectral type is near F0, but the spectrum is so peculiar that no luminosity class can be estimated. The blend at 4128-4132 is very strong; this appears to be indicative of strong Eu II and not of abnormal strength of the Si II doublet. The blend at 4171 is strong; this is an indication of abnormal strength of Cr II. A considerable amount of the intensity of the line near 4077 is due to blended lines of Cr II. The lines of Eu II may be stronger than in any other bright star, with the possible exception of the spectrum-variable HR 5355.
|High resolution image|
Generalities. - The manganese stars appear to be present at B8-B9, the silicon stars at B9-A0, the europium stars at A0-F0, and the strontium stars at A0-F0. These groups can all be identified on low-dispersion spectrograms, but any kind of detailed discussion requires higher dispersion. The bright silicon stars observed at Yerkes appear to be around 1 or 2 mag. above the main sequence at B9 and A0. All the peculiar groups of stars lie near class A, and an association with the maximum intensity of the hydrogen lines is suggested.