Ap and Bp stars

Ap star (sometimes Bp / Ap stars) are hot stars with a surface temperature of about 10,000 Kelvin ( spectral type A and B) and a strongly deviating from the majority of the early stars chemical composition. They show their sharp line-like spectra exceptionally strong lines of chromium, manganese and silicon or strontium as well as the normal stars barely detectable absorption lines of some rare earths.

The name " Ap " is composed of the spectral class and the "p" of peculiar ( particularly for English ). Often, the lines vary periodically or quasi- periodically, so the AP are also known as star spectrum variables. Sometimes these stellar class is also divided according to the specificities and the stars are referred to as Si stars, Mn star, stars, etc. Cr.

Properties

Ap stars have a magnetic field with a magnetic flux density of a few kilo - Gauss. In addition, they are often pulsating variable stars with low amplitudes and radial velocities with periods of 5 to 21 minutes. They are called ROAP -star for proceed rapidly oscillating Ap stars. ROAP - stars occur only in a temperature range of 6400-8400 Kelvin. They oscillate non- radially in high harmonics with periods of 5 to 21 minutes, the restoring force of the pressure in the atmosphere of the star is. The region of the Hertzsprung -Russell diagram in which lie the roAps is populated by non- variable Ap stars. Between the two groups there seems to be no differences in mass, chemical composition, age, or magnetic field strength. As a further mechanism for brightness variations in Ap stars variable absorption by magnetospheric clouds and Rotationsveränderlichkeit by patches in the photosphere are suspected. The periods of the spectra changes are not constant in most Ap stars, which both increases - and decreases the rotational speed were measured. This period changes are associated with changes in the radius due to the evolution of the stars from the main sequence, with the reduction of torque through a stellar wind with or without interaction with the stellar magnetic field, the precession of the axis of rotation and a light-time effect due to a companion in combination.

The spectra of Ap star vary with the period of rotation, wherein the Ap -star to rotate much slower than the normal A- B and stars. This is explained with the model of the oblique rotator, after which the star for this characteristic metals are predominantly enriched at the magnetic poles and can be visible or invisible by the rotation of the observer. The cause is likely to be in differential chemical diffusion which can selectively drop some chemical elements with a small cross section, whereas the chemical elements with a large cross section to accumulate as a result of the radiation pressure in the atmosphere. Some striking chemical abundances, as the overabundance of lithium could also be caused by Spallationsprozesse near the magnetic poles of Ap or Bp stars.

With polarimetischen measurements Stokes parameters can be derived, from which the geometry of the magnetic fields can be closed. The A-stars show this rather complex multipole magnetic fields than simple dipoles with only a north and south pole.

Formation

The rotation speed of Vorhauptreihensternen are significantly faster than in the Ap stars. These precursors are called Herbig and have a typical rotation period of one day. It is probably an interaction of the magnetic field of the star with the ionized inner zone in the protoplanetary disk, which removes most of the original torque. In order to reduce the rotational velocities to observed values of up to one month would then continue to reduce torque a stellar wind with turned froze magnetic field lines.