Ap and Bp stars

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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, or 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

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 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]

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). [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

Related Research Articles

<span class="mw-page-title-main">Przybylski's Star</span> Star in the constellation Centaurus

Przybylski's Star, or HD 101065, is a rapidly oscillating Ap star at roughly 356 light-years from the Sun in the southern constellation of Centaurus. It has a unique spectrum showing over-abundances of most rare-earth elements, including some short-lived radioactive isotopes, but under-abundances of more common elements such as iron.

<span class="mw-page-title-main">HD 37017</span> Binary star system in the constellation Orion

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<span class="mw-page-title-main">ET Andromedae</span> Star in the constellation Andromeda

ET Andromedae is a binary star system star in the northern constellation of Andromeda. It has an apparent visual magnitude of 6.48, placing it at the nominal limit for visibility with the naked eye. The distance to this system can be estimated from its annual parallax shift of 5.42 mas, which yields a value of 602 light years.

<span class="mw-page-title-main">V1291 Aquilae</span> Variable star in the constellation Aquila

V1291 Aquilae is a single star in the equatorial constellation of Aquila. It has a yellow-white hue and is dimly visible to the naked eye with an apparent visual magnitude that fluctuates around 5.65. Based on parallax measurements, it is located at a distance of approximately 278 light years from the Sun. The star it is drifting closer with a radial velocity of −22 km/s.

<span class="mw-page-title-main">BX Boötis</span> Star in the constellation Boötes

BX Boötis is a star in the northern constellation of Boötes. It is a dim star near the lower limit of visibility to the naked eye, having a nominal apparent visual magnitude of 6.35. Based upon an annual parallax shift of 10.81 mas, it is located 302 light years away. At that distance, the visual magnitude of the star is diminished by an extinction of 0.13 due to interstellar dust. It is moving closer with a heliocentric radial velocity of −11 km/s.

HR 3082 is a double star in the northern circumpolar constellation of Camelopardalis. It is faintly visible to the naked eye with an apparent visual magnitude of 5.39. The system is moving closer to the Sun with a heliocentric radial velocity of +2.7 km/s. It is currently at a distance of around 341 light years, based upon an annual parallax shift of 9.57 mas.

Rapidly oscillating Ap stars (roAp 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 23 minutes. They lie in the δ Scuti instability strip on the main sequence.

<span class="mw-page-title-main">Gamma Equulei</span> Chemically peculiar A-type main sequence star in the constellation Equuleus

Gamma Equulei, Latinized from γ Equulei, is a double star in the northern constellation of Equuleus. It is located at a distance of around 118 light-years from Earth and is visible to the naked eye with a slightly variable apparent visual magnitude of around 4.7. The star is drifting closer to the Sun with a radial velocity of −16.5 km/s.

<span class="mw-page-title-main">HD 64740</span> Star in the constellation Puppis

HD 64740 is a single star in the southern constellation Puppis, positioned near the line of sight to the Gum Nebula. It has a blue-white hue and is faintly visible to the naked eye with an apparent visual magnitude of 4.63. Parallax measurements give a distance estimate of approximately 760 light-years from the Sun, and it is drifting further away with a radial velocity of +8 km/s.

<span class="mw-page-title-main">CU Virginis</span> Star in the constellation Virgo

CU Virginis is a single star in the equatorial constellation of Virgo. It has an apparent visual magnitude of 4.99, which is bright enough to be faintly visible to the naked eye. The distance to this star can be estimated from its annual parallax shift of 13.9 mas, yielding a separation of 234 light years.

64 Serpentis is a single, blue-white hued star in Serpens Cauda, the eastern segment of the equatorial constellation of Serpens. With an apparent visual magnitude of 5.56, it is a dim star but visible to the naked eye in good seeing conditions. Based upon an annual parallax shift of 2.87±0.26 mas, it is located roughly 1,100 light years away. It is moving closer to the Sun with a heliocentric radial velocity of about −10 km/s. It is one of the brightest stars in front of the Great Rift.

HD 129899, is a solitary star located in the southern circumpolar constellation Apus, the bird-of-paradise. It has an apparent magnitude of 6.44, placing it near the limit for naked eye visibility, even under ideal conditions. The object is located relatively far at a distance of 928 light-years based on Gaia DR3 parallax measurements and it is receding with a heliocentric radial velocity of 2.5 km/s. At its current distance, HD 129899's brightness is heavily diminished by an interstellar extinction of 0.55 magnitudes and it has an absolute bolometric magnitude of −1.28.

<span class="mw-page-title-main">HD 137509</span> Star in the constellation Apus

HD 137509 is a star in the southern constellation of Apus, positioned less than a degree from the northern constellation boundary with Triangulum Australe. It has the variable star designation of NN Apodis, or NN Aps for short, and ranges in brightness from an apparent visual magnitude of 6.86 down to 6.93 with a period of 4.4916 days. The star is located at a distance of approximately 647 light years from the Sun based on parallax, and is drifting further away with a radial velocity of +0.50 km/s.

Phi Phoenicis, Latinized from φ Phoenicis, is a binary star system in the southern constellation of Phoenix. It is faintly visible to the naked eye with an apparent visual magnitude of 5.1. Based upon an annual parallax shift of 10.185 mas as seen from Earth, it is located approximately 320 light years from the Sun. It is moving away with a heliocentric radial velocity of 10.4 km/s.

<span class="mw-page-title-main">Xi Phoenicis</span> Binary star in the constellation Phoenix

Xi Phoenicis, Latinized from ξ Phoenicis, is a visual binary star system in the southern constellation of Phoenix. It is faintly visible to the naked eye, having an apparent visual magnitude of 5.70. Based upon an annual parallax shift of 14.61 mas as measured from Earth, it is located around 223 light years from the Sun. The system is moving away from the Sun with a radial velocity of about +10 km/s.

<span class="mw-page-title-main">GR Andromedae</span> Variable star in the constellation Andromeda

GR Andromedae is a variable star in the constellation Andromeda. Its apparent visual magnitude varies between 6.87 and 6.95 in a cycle of 518.2 days. It is classified as an α2 Canum Venaticorum variable.

<span class="mw-page-title-main">HN Andromedae</span> Star in the constellation Andromeda

HN Andromedae is a variable star in the constellation Andromeda. Its apparent visual magnitude varies between 6.67 and 6.76 in a cycle of 69.51 days. It is classified as an α2 Canum Venaticorum variable.

<span class="mw-page-title-main">KQ Velorum</span> Binary star system in the constellation Vela

KQ Velorum is a variable star system in the southern constellation of Vela. It has the identifier HD 94660 in the Henry Draper Catalogue; KQ Vel is the variable star designation. This appears as a sixth magnitude star with an apparent visual magnitude of 6.112, and thus is dimly visible to the naked eye under suitable viewing conditions. The system is located at a distance of approximately 373 light years from the Sun based on parallax measurements, and is drifting further away with a radial velocity of around 23 km/s.

HR 7355 is a star in the southern constellation of Sagittarius. It is faintly visible to the naked eye with an apparent visual magnitude of 6.03. The star is located at a distance of approximately 760 light years based on parallax measurements. The radial velocity of the star is poorly constrained, but it appears to be receding at the rate of +4 km/s.

<span class="mw-page-title-main">HD 125248</span> Binary star system in the constellation of Virgo

HD 125248 is a binary star system in the equatorial constellation of Virgo. It has the variable star designation CS Virginis, while HD 125248 is the designation from the Henry Draper Catalogue. This system is dimly visible to the naked eye as a point of light with an apparent visual magnitude that ranges from 5.84 down to 5.95. It is located at a distance of approximately 280 light years from the Sun based on parallax measurements, but is drifting closer with a heliocentric radial velocity of −8 km/s.

References

  1. ↑ Babcock, Horace W (1960). "The 34-KILOGAUSS Magnetic Field of HD 215441". Astrophysical Journal. 132: 521. Bibcode:1960ApJ...132..521B. doi:10.1086/146960.
  2. ↑ 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.
  3. ↑ David F. Gray (17 November 2005). The Observation and Analysis of Stellar Photospheres. Cambridge University Press. pp. 13–. ISBN   978-0-521-85186-2.
  4. ↑ 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.
  5. ↑ 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.
  6. ↑ 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 .
  7. ↑ Auriere, M.; Wade, G. A.; Konstantinova-Antova, R.; Charbonnel, C.; Catala, C.; Weiss, W. W.; Roudier, T.; Petit, P.; Donati, J.-F.; Alecian, E.; Cabanac, R.; Van Eck, S.; Folsom, C. P.; Power, J. (September 2009). "Discovery of a weak magnetic field in the photosphere of the single giant Pollux". Astronomy & Astrophysics. 504 (1): 231–237. arXiv: 0907.1423 . Bibcode:2009A&A...504..231A. doi:10.1051/0004-6361/200912050. ISSN   0004-6361.
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