Pecular star

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In astrophysics , stars are called peculiar stars or chemical peculiar stars ( Latin peculiaris: special, peculiar ) or CP stars for short , which have unusual metal abundances at least in the near-surface layer of their star atmosphere , the photosphere .

The discovery of chemically peculiar stars goes back to Antonia Maury , American astronomer at the Harvard College Observatory , who observed stellar spectra and published a catalog of star classifications in 1897 . She first discovered deviations in the spectra of some stars. The existence of strong magnetic fields in CP stars was established in 1948 by Horace W. Babcock with the Zeeman effect . The first spectroscopic detection of CP stars in the Magellanic Clouds was achieved in 2010.

Overview

These hot peculiar stars have been divided into 4 main classes based on their spectra, although sometimes only 2 classification systems are used:

Class name Alternative Description Abbreviation comment
Metal line star At the star CP1 often strong and sometimes variable absorption spectra of zinc, strontium, zirconium, barium. Lack of other metals such as B. Calcium and Scandium
Ap and Bp - CP2 Ap and Bp stars show frequencies of metals such as strontium, chromium and europium; additionally also praseodymium and neodymium
Mercury Manganese Star HgMn star CP3 Mercury-manganese stars with conspicuous spectral lines due to ionized mercury. Excess chemical elements such as phosphorus, manganese, gallium, strontium, yttrium, zirconium, platinum and mercury in the stellar atmosphere
Low helium star - CP4 low helium; weak He lines

The class name results from the peculiarities of the respective class compared to the main sequence stars.

properties

These chemically peculiar stars are widespread among the hot hydrogen-fusing stars and belong to the main sequence . However, cool CP stars (spectral type G and later) as well as unusual compositions in carbon stars and stars of the spectral type S were also observed .

Metal line stars

The Am stars (CP1 stars) show only weak spectral lines of simply ionized calcium and / or scandium , but an abundance of heavy metals such as zinc , strontium , zirconium and barium . They tend to rotate at slow speeds and have effective temperatures between 7000  K and 10000 K.

Ap and Bp stars

The Ap and Bp stars (CP2 stars) typically have characteristically strong magnetic fields , an abundance of elements such as silicon , chromium , strontium and europium , as well as praseodymium and neodymium, and the majority of them are also slowly rotating stars. The effective temperatures of this type are between 8000 K and 15000 K, whereby the determination of the effective temperature is generally difficult for specular stars due to their complex atmospheric structures.

Mercury Manganese Stars

The HgMn stars (CP3 stars) are also classically listed under the Ap stars, but do not have those strong magnetic fields like these. As their name suggests, they show strong spectral lines of simply ionized mercury (Hg) and manganese (Mn). They have a very slow rotation speed, even compared to the standard of the other types of CP stars. The effective temperature range for the mercury-manganese stars is between 10,000 K and 15,000 K.

Low helium stars

The helium-poor stars (CP4 stars) show significantly weaker helium lines than one might expect from their classic photometry in Johnson's UBV system .

Summary

It is generally assumed that the special surface compositions that we observe in these CP stars are only caused by processes after star formation ; On the one hand, this would be the diffusion, on the other hand, effects brought about by the magnetic field in the outer layers of the stellar atmosphere.

These processes cause some elements , especially helium , nitrogen and oxygen , to sink into deeper layers of the stellar atmosphere, while other elements such as manganese, strontium, yttrium and zirconium are lifted upwards from the more central areas in the star, which then lead to the special stellar spectra. It is assumed that the centers of these stars, like most of the rest of the star, have perfectly normal abundances of the elements. The frequencies that the gas clouds from which they originally arose also exhibit. In order for this frequency distribution and stratification to remain stable over a longer period of time due to the processes mentioned, the atmospheres of these stars themselves must remain stable enough so that no convection can occur, which in turn would make the mixing in the star too strong. And here the unusually strong magnetic field that can be observed in these types of stars could take on the necessary stabilizing role.

However, there are also cooler stars (of the spectral type G or later) with chemical peculiarities. However, these are typically not main sequence stars. Usually these stars are then referred to by their class name or other advanced specification. If chemically peculiar stars are mentioned without additional specifications, they are actually always the hot main sequence stars described above.

Many of the cooler CP stars are the result of a mixture of nuclear fusion products from the inner star region and its photosphere; this includes both carbon stars and stars of the spectral type S. Others are the result of mass transfer in close binary star systems ; Examples are the barium stars and some stars of the spectral type S.

See also

Individual evidence

  1. ^ Antonia C. Maury, Edward C. Pickering: Spectra of bright stars photographed with the 11-inch Draper Telescope as part of the Henry Draper Memorial . In: Annals of Harvard College Observatory. 1897, bibcode : 1897AnHar..28 .... 1M .
  2. ^ Horace W. Babcock: The Magnetic Field of γ Equulei . In: Astrophysical Journal . 1948, doi: 10.1086 / 145063 , bibcode : 1948ApJ ... 108..191B .
  3. ^ A b George Preston: The Chemically Peculiar Stars of the Upper Main Sequence . In: Annual Review of Astronomy and Astrophysics. Volume 12, 1974, S, 257, doi: 10.1146 / annurev.aa.12.090174.001353 .
  4. ↑ Stars and Space . Verlag Bibliographisches Institut., 2001, ISSN  0039-1263 , OCLC 1643045 , p. 120 ( limited preview in Google Book search).
  5. ^ Georges Michaud: Diffusion Processes in Peculiar A Stars . In: Astrophysical Journal. Volume 160, 1970, p. 641, bibcode : 1970ApJ ... 160..641M
  6. ^ RD McClure: The carbon and related stars. In: JRASC . tape 79 , December 1985, p. 277-293 , bibcode : 1985JRASC..79..277M .