Ap and Bp stars

Ap and Bp stars are peculiar stars (hence the "p") of types A and B which show overabundances of some metals, such as strontium, chromium and europium; in addition, larger overabundances are often seen in praseodymium and neodymium. These stars have a much slower rotation than normal for A and B-type stars, although some exhibit rotation velocities up to about 100 kilometers per second.

Magnetic fields

They also have stronger magnetic fields than classical A- or B-type stars in the case of HD 215441, reaching 33.5 kG (3.35 T). Typically the magnetic field of these stars lies in the range of a few kG to tens of kG. In most cases a field which is modelled as a simple dipole is a good approximation and provides an explanation as to why there is an apparent periodic variation in the magnetic field, as if such a field is not aligned with the rotation axis—the field strength will change as the star rotates. In support of this theory it has been noted that the variations in magnetic field are inversely correlated with the rotation velocity. This model of a dipolar field, in which the magnetic axis is offset to the rotation axis, is known as the oblique rotator model.