Ap and Bp stars are
chemically peculiar star
In astrophysics, chemically peculiar stars (CP stars) are stars with distinctly unusual metal abundances, at least in their surface layers.
Classification
Chemically peculiar stars are common among hot main-sequence (hydrogen-burning) stars. Thes ...
s (hence the "p") of
spectral type
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
s A and B which show overabundances of some metals, such as
strontium,
chromium and
europium. In addition, larger overabundances are often seen in
praseodymium
Praseodymium is a chemical element with the symbol Pr and the atomic number 59. It is the third member of the lanthanide series and is considered to be one of the rare-earth metals. It is a soft, silvery, malleable and ductile metal, valued for i ...
and
neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishe ...
. 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
Ap and Bp stars have stronger
magnetic fields than classical A- or B-type stars; in the case of HD 215441, reaching 33.5 k
G (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
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
*An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system ...
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.
The origin of such high magnetic fields in Ap stars is problematic and two theories have been proposed in order to explain them. The first is the ''fossil field hypothesis'', in which the field is a relic of the initial field in the interstellar medium (ISM). There is sufficient magnetic field in the ISM to create such high magnetic fields—indeed, so much so that the theory of ]ambipolar diffusion Ambipolar diffusion (ambipolar: relating to or consisting of both electrons and positive ions moving in opposite directions) is diffusion of positive and negative species with opposite electrical charge due to their interaction via an electric field ...
has to be invoked to reduce the field in normal stars. This theory does require the field to remain stable over a long period of time, and it is unclear whether such an obliquely rotating field could do so. Another problem with this theory is to explain why only a small proportion of A-type stars exhibit these high field strengths. The other generation theory is dynamo action within rotating cores of Ap stars; however, the oblique nature of the field cannot be produced, as yet, by this model, as invariably one ends up with a field either aligned with the rotation axis, or at 90° to it. It is also unclear whether it is possible to generate such large dipole fields using this explanation, due to the slow rotation of the star. While this could be explained by invoking a fast rotating core with a high rotation gradient to the surface, it is unlikely that an ordered axisymmetric field would result.[
]
Abundance spots
The spatial locations of the chemical overabundances have been shown to be connected with the geometry of the magnetic field.
Some of these stars have shown radial velocity variations arising from pulsations of a few minutes.
For studying these stars high-resolution spectroscopy is used, together with Doppler imaging
Inhomogeneous structures on stellar surfaces, i.e. temperature differences, chemical composition or magnetic fields, create characteristic distortions in the spectral lines due to the Doppler effect. These distortions will move across spectral l ...
which uses the rotation to deduce a map of the stellar surface. These patches of overabundances are often referred to as ''abundance spots''.[
]
Rapidly oscillating Ap stars
A subset of this class of stars, called ''rapidly oscillating Ap (roAp) stars'', exhibit short-timescale, millimagnitude photometric variations and variations in radial velocities of spectral lines.
These were first observed in the highly peculiar Ap star HD 101065 ( Przybylski's star).[ These stars lie at the bottom of the ]Delta Scuti
Delta Scuti, Latinized from δ Scuti, is a variable star in the southern constellation Scutum. With an apparent visual magnitude that fluctuates around 4.72, it is the fifth-brightest star in this small and otherwise undistinguished c ...
instability strip, on the main sequence. There are currently 35 known roAp stars. The pulsation periods of these oscillators lie between 5 and 21 minutes. The stars pulsate in high overtone, non-radial, pressure modes.[
]
See also
*Peculiar star
In astrophysics, chemically peculiar stars (CP stars) are stars with distinctly unusual metal abundances, at least in their surface layers.
Classification
Chemically peculiar stars are common among hot main-sequence (hydrogen-burning) stars. The ...
* Stellar classification
*Doppler imaging
Inhomogeneous structures on stellar surfaces, i.e. temperature differences, chemical composition or magnetic fields, create characteristic distortions in the spectral lines due to the Doppler effect. These distortions will move across spectral l ...
References
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*Ap and Bp
*Ap
*Bp