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Chemically peculiar stars

History and current knowledge of CP stars

The chemically peculiar (CP) stars of the upper main sequence have been targets for astrophysical studies since the discovery of these object by the American astronomer Antonia Maury in 1897. The main characteristics of the classical CP stars are: peculiar and often variable line strengths, quadrature of line variability with radial velocity changes, photometric variability with the same periodicity and coincidence of extrema. Slow rotation was inferred from the sharpness of spectral lines. Overabundances of several orders of magnitude compared to the Sun were derived for heavy elements such as Silicon, Chromium, Strontium and Europium.

Babcock (1947) discovered a global dipolar magnetic field in the star 78 Virginis followed by a catalog of similar stars (Babcock 1958). This catalog also described the variability of the field strength in many CP stars, including even a reversal of magnetic polarity, leading to the Oblique Rotator concept of slowly rotating stars with non-coincidence of the magnetic and rotational axes (Stibbs 1950). This model produces variability and reversals of the magnetic field strength similar to a light house. Due to the chemical abundance concentrations at the magnetic poles also spectral and the related photometric variabilities are easily understood, as well as radial velocity variations of the appearing and receding patches on the stellar surface.

Preston (1974) divided the CP stars into the following groups:
  • CP1: Am/Fm stars without a strong global magnetic field
  • CP2: "classical" CP stars with strong magnetic fields
  • CP3: HgMn stars, basically non-magnetic
  • CP4: He-weak stars, some of these objects show a detectable magnetic field

Later on, the group of lBootis stars were thought to be another individual subgroup. These objects are non-magnetic, late B to early F-type stars with underabundances of heavy elements whereas C, N, O and S are almost solar abundant (Paunzen et al. 2002).

The peculiar (surface) abundances for the CP stars have been explained by the following theories:
  • Diffusion of chemical elements depending on the balance between gravitational pull and uplift by the radiation field through absorption in spectral lines (Michaud 1970). This should modify the chemical surface composition in case of sufficient stability in these layers.
  • Selective Accretion from the interstellar medium (Havnes, Conti 1971) via the stellar magnetic field.

While diffusion seems to be appropriate for both the magnetic and non-magnetic stars to explain spectral peculiarity, it is not yet clear to which extent the interaction with the interstellar medium via accretion and transport of angular momentum may modify the effects of diffusion and break the stellar rotational velocities during main sequence life time.

A most important point is the origin of the global stellar magnetic fields for CP stars. Two theories have been developed in this respect (Moss 1989):
  • The dynamo theory is based on the existence of a contemporaneous dynamo operating in the convective core of the magnetic stars
  • The fossil theory has two variants: the magnetic field is either the slowly decaying relics of the frozen-in interstellar magnetic field or of the dynamo acting in the pre-main sequence phase.

The detection of magnetic fields in CP stars is based on the analysis of the Zeeman effect in the Stokes parameter profiles of spectral lines. The line intensity profile is sensitive to the mean magnetic field modulus, whereas the spectrum of the radiation observed in right and left circular polarization profile of a spectral line is sensitive to the transverse component of the magnetic field. Landstreet, Mathys (2000) have shown that the slowest rotating CP stars exhibit an almost symmetrical magnetic field whereas very fast rotating stars have very asymmetrical fields.

See also section "Evolutionary status of CP stars".