In astrophysics, peculiar stars have distinctly unusual metal abundances, at least in their surface layers.
Chemically peculiar stars (CP 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, although two classification systems are sometimes used: metallic-lined (Am, CP1), Ap (CP2), mercury-manganese (HgMn, CP3) and helium-weak (He-weak, CP4). The class names provide a good idea of the peculiarities that set them apart from other stars on the main sequence.
The Am stars (CP1 stars) show weak lines of singly ionized Ca and/or Sc, but show enhanced abundances of heavy metals. They also tend to be slow rotators and have an effective temperature between 7000 K and 10 000 K. The Ap stars (CP2 stars) are characterized by strong magnetic fields, enhanced abundances of elements such as Si, Cr, Sr and Eu, and are also generally slow rotators. The effective temperature of these stars is stated to be between 8000 K and 15 000 K, but the issue of calculating effective temperatures in such peculiar stars is complicated by atmospheric structure. The HgMn stars (CP3 stars) are also classically placed within the Ap category, but they do not show the strong magnetic fields associated with classical Ap stars. As the name implies, these stars show increased abundances of singly ionized Hg and Mn. These stars are also very slow rotators, even by the standards of CP stars. The effective temperature range for these stars is quoted at between 10 000 K and 15 000 K. The He-weak stars (CP4 stars) show weaker He lines than would be expected classically from their observed Johnson UBV colours.
It is generally thought that the peculiar 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, particularly He, N and O, to "settle" out in the atmosphere into the layers below, while other elements such as Mn, Sr, Y and Zr 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. In order for such diffusion and levitation to occur and the resulting layers to remain intact, the atmosphere of such a star must be stable enough to convection that convective mixing does not occur. The proposed mechanism causing this stability is the unusually large magnetic field that is generally observed in stars of this type.
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-type stars. Others are the result of mass transfer in a binary star system; examples of these include the barium stars and some S stars.
See also 
- George Preston, "The Chemically Peculiar Stars of the Upper Main Sequence", Annual Review of Astronomy and Astrophysics, vol 12, p 257, 1974
- Georges Michaud, "Diffusion Processes in Peculiar A Stars", Astrophysical Journal, vol 160, p 641, 1970]
- McClure, R. Journal of the Royal Astronomical Society of Canada, vol 79, pp. 277–293, Dec. 1985