Peculiar star
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In astrophysics, peculiar stars have distinctly unusual metal abundances, at least in their surface layers.
Chemically peculiar stars are common among hot main sequence (hydrogen-burning) stars. These hot peculiar stars have been divided into 4 main classes on the basis of their spectra: helium-weak (He-weak), mercury-manganese (HgMn), Ap, and metallic lined (Am). The class names give a good idea of what peculiarities sets them apart. He-weak stars appear to have less helium than expected. HgMn stars have strong mercury and manganese absorption lines in their spectra. Am stars have strong heavy metal lines and weak Ca and Sc lines. Ap stars have strong magnetic fields and strong Si, Cr, Sr, Eu and other absorption lines. Some stars show characteristics found in more than one class.
It is generally thought that the pecular surface compositions observed in these hot main-sequence stars have been caused by processes that happened after the star formed, such as diffusion or magnetic effects in the outer layers of the stars. These processes cause some elements to "settle" out of the atmosphere into the layers below, while other elements are "levitated" out of the interior to the surface, resulting in the observed spectral peculiarities. It is assumed that the centers of the stars, and the bulk compositions of the entire star, have more normal chemical abundance mixtures which reflect the compositions of the gas clouds from which they formed.[1]
There are also classes of chemically peculiar cool stars (that is, stars with spectral type G or later), but these stars are typically not main sequence stars. These are usually identified by the name of their class or some further specific label. The phrase chemically peculiar star without further specification usually means a member of one of the hot main sequence types described above.
Many of the cooler chemically peculiar stars are the result of the mixing of nuclear fusion products from the interior of the star to its surface; these include most of the carbon stars and S stars. Others are the result of mass transfer in a binary star system; examples of these include the barium stars and some S stars.[2]
[edit] References
- ^ Preston, George. Annual Review of Astronomy and Astrophysics, vol 12, p 257, 1974[1]
- ^ McClure, R. Journal of the Royals Astronomical Society of Canada, vol 79, pp. 277-293, Dec. 1985
