Ap and Bp star
Encyclopedia
Ap and Bp stars are peculiar star
s (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.
s 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.
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
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.
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
which uses the rotation to deduce a map of the stellar surface. These patches of overabundances are often referred to as abundance spots.
, exhibit short-timescale, millimagnitude photometric variations and variations in radial velocities of spectral lines.
These were first observed in the highly peculiar Ap star HD101065 (Przybylski's star
). These stars lie at the bottom of the delta Scuti
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.
Peculiar star
In astrophysics, peculiar stars have distinctly unusual metal abundances, at least in their surface layers.Chemically peculiar stars are common among hot main sequence stars...
s (hence the “p”) of types A and B which show overabundances of some metals, such as strontium
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
, chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
and europium
Europium
Europium is a chemical element with the symbol Eu and atomic number 63. It is named after the continent of Europe. It is a moderately hard silvery metal which readily oxidizes in air and water...
; in addition, larger overabundances are often seen in praseodymium
Praseodymium
Praseodymium is a chemical element that has the symbol Pr and atomic number 59. Praseodymium is a soft, silvery, malleable and ductile metal in the lanthanide group. It is too reactive to be found in native form, and when artificially prepared, it slowly develops a green oxide coating.The element...
and neodymium
Neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
. 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 fieldMagnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s than classical A or B type stars in the case of HD 215441, reaching 33.5 kG
Gauss (unit)
The gauss, abbreviated as G, is the cgs unit of measurement of a magnetic field B , named after the German mathematician and physicist Carl Friedrich Gauss. One gauss is defined as one maxwell per square centimeter; it equals 1 tesla...
(3.35 T
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
). 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
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
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
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
(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 diffusion is diffusion of positive and negative particles in a plasma at the same rate due to their interaction via the 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
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
variations arising from pulsations of a few minutes.
For studying these stars high-resolution spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
is used, together with Doppler imaging
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 line profiles due to the stellar rotation...
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) starsRapidly oscillating Ap stars
Rapidly oscillating Ap stars are a subtype of the Ap star class that exhibit short-timescale rapid photometric or radial velocity variations. The known periods range between 5 and 21 min. They lie in the delta Scuti instability strip on the main sequence.-Discovery:The first roAp star to be...
, exhibit short-timescale, millimagnitude photometric variations and variations in radial velocities of spectral lines.
These were first observed in the highly peculiar Ap star HD101065 (Przybylski's star
Przybylski's star
Przybylski's Star , or HD 101065, is a peculiar star that is located about 410 light years from the Sun in the constellation Centaurus....
). These stars lie at the bottom of the delta Scuti
Delta Scuti
Delta Scuti is a white, F-type giant star in the constellation Scutum. It is approximately 187 light years from Earth. Delta Scuti is the prototype of the Delta Scuti type variable stars. It is a high-amplitude δ Scuti type pulsator with light variations of about 0.15 minutes...
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.