Ap and Bp stars have stronger magnetic fields than classical A- or B-type stars; in the case of HD 215441, reaching 33.5 kG (3.35 T).[1] 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.[2] 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.[3]
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 which uses the rotation to deduce a map of the stellar surface. These patches of overabundances are often referred to as abundance spots.[4]
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).[5] 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.[6]
^Babcock, Horace W (1960). "The 34-KILOGAUSS Magnetic Field of HD 215441". Astrophysical Journal. 132: 521. Bibcode:1960ApJ...132..521B. doi:10.1086/146960.
^Landstreet, J. D; Bagnulo, S; Andretta, V; Fossati, L; Mason, E; Silaj, J; Wade, G. A (2007). "Searching for links between magnetic fields and stellar evolution: II. The evolution of magnetic fields as revealed by observations of Ap stars in open clusters and associations". Astronomy and Astrophysics. 470 (2): 685. arXiv:0706.0330. Bibcode:2007A&A...470..685L. doi:10.1051/0004-6361:20077343. S2CID 15591645.
^David F. Gray (17 November 2005). The Observation and Analysis of Stellar Photospheres. Cambridge University Press. pp. 13–. ISBN978-0-521-85186-2.
^Kochukhov, Oleg (2011). "The spots on Ap stars". Physics of Sun and Star Spots. 273: 249. arXiv:1010.0264. Bibcode:2011IAUS..273..249K. doi:10.1017/S1743921311015328. S2CID 118436816.
^Kurtz, D. W (1978). "12.15 Minute Light Variations in Przybylski's Star, HD 101065". Information Bulletin on Variable Stars. 1436: 1. Bibcode:1978IBVS.1436....1K.
^Murphy, Simon J.; Saio, Hideyuki; Takada-Hidai, Masahide; Kurtz, Donald W.; Shibahashi, Hiromoto; Takata, Masao; Hey, Daniel R. (2020). "On the first δ SCT-roAp hybrid pulsator and the stability of p and g modes in chemically peculiar A/F stars". Monthly Notices of the Royal Astronomical Society. 498 (3): 4272. arXiv:2009.00730. Bibcode:2020MNRAS.498.4272M. doi:10.1093/mnras/staa2667.
April 09, 2023
stars, chemically, peculiar, stars, hence, spectral, types, which, show, overabundances, some, metals, such, strontium, chromium, europium, addition, larger, overabundances, often, seen, praseodymium, neodymium, these, stars, have, much, slower, rotation, than. Ap and Bp stars are chemically peculiar stars hence the p of spectral 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 Contents 1 Magnetic fields 2 Abundance spots 3 Rapidly oscillating Ap stars 4 See also 5 ReferencesMagnetic fields EditAp and Bp stars have stronger magnetic fields than classical A or B type stars in the case of HD 215441 reaching 33 5 kG 3 35 T 1 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 2 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 3 Abundance spots EditThe 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 which uses the rotation to deduce a map of the stellar surface These patches of overabundances are often referred to as abundance spots 4 Rapidly oscillating Ap stars EditMain article rapidly oscillating Ap star 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 5 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 6 See also EditPeculiar star Stellar classification Doppler imagingReferences Edit Babcock Horace W 1960 The 34 KILOGAUSS Magnetic Field of HD 215441 Astrophysical Journal 132 521 Bibcode 1960ApJ 132 521B doi 10 1086 146960 Landstreet J D Bagnulo S Andretta V Fossati L Mason E Silaj J Wade G A 2007 Searching for links between magnetic fields and stellar evolution II The evolution of magnetic fields as revealed by observations of Ap stars in open clusters and associations Astronomy and Astrophysics 470 2 685 arXiv 0706 0330 Bibcode 2007A amp A 470 685L doi 10 1051 0004 6361 20077343 S2CID 15591645 David F Gray 17 November 2005 The Observation and Analysis of Stellar Photospheres Cambridge University Press pp 13 ISBN 978 0 521 85186 2 Kochukhov Oleg 2011 The spots on Ap stars Physics of Sun and Star Spots 273 249 arXiv 1010 0264 Bibcode 2011IAUS 273 249K doi 10 1017 S1743921311015328 S2CID 118436816 Kurtz D W 1978 12 15 Minute Light Variations in Przybylski s Star HD 101065 Information Bulletin on Variable Stars 1436 1 Bibcode 1978IBVS 1436 1K Murphy Simon J Saio Hideyuki Takada Hidai Masahide Kurtz Donald W Shibahashi Hiromoto Takata Masao Hey Daniel R 2020 On the first d SCT roAp hybrid pulsator and the stability of p and g modes in chemically peculiar A F stars Monthly Notices of the Royal Astronomical Society 498 3 4272 arXiv 2009 00730 Bibcode 2020MNRAS 498 4272M doi 10 1093 mnras staa2667 Retrieved from https en wikipedia org w index php title Ap and Bp stars amp oldid 1113481069, wikipedia, wiki, book, books, library,
