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Polar (star)

April 15, 2024

For other uses, see Polar (disambiguation).

In astronomy, a polar is a highly magnetic type of cataclysmic variable (CV) binary star system, originally known as an AM Herculis star after the prototype member AM Herculis. Like other CVs, polars contain two stars: an accreting white dwarf (WD), and a low-mass donor star (usually a red dwarf) which is transferring mass to the WD as a result of the WD's gravitational pull, overflowing its Roche lobe.[1] Polars are distinguished from other CVs by the presence of a very strong magnetic field in the WD. Typical magnetic field strengths of polar systems are 10 million to 80 million gauss (1000–8000 teslas).[2] The WD in the polar AN Ursae Majoris has the strongest known magnetic field among cataclysmic variables, with a field strength of 230 million gauss (23 kT).[3]

Polar variables are magnetic white dwarfs accreting material from a low mass donor, with no accretion disk due to the intense magnetic field

Accretion mechanism edit

One of the most critical consequences of the WD's magnetism is that it synchronizes the rotational period of the WD with the orbital period of the binary;[2] to first order, this means that the same side of the WD always faces the donor star. This synchronous rotation is considered a defining feature of polars.[1][2] Additionally, the WD's magnetic field captures the accretion stream from the donor star before it can develop into an accretion disk. The capture of the accretion stream is known as threading, and it occurs when the magnetic pressure from the WD matches the stream's ram pressure.[2] The captured material flows along the WD's magnetic field lines until it violently accretes onto the WD in a shock near one or more of the star's magnetic poles.[2] This accretion region covers only a fraction of the WD's surface, but it can contribute half of the system's optical light.[4] In addition to optical and near-infrared cyclotron radiation, the accretion region also produces X-rays due to the high temperature of gas within the shock, so polars are frequently brighter in X-rays than non-magnetic CVs.[1]

Whereas accretion in a non-magnetic system is governed by viscosity within the accretion disk, accretion in a polar is entirely magnetic. Additionally, while an accretion disk can be crudely envisioned as a two-dimensional structure with no significant thickness, the accretion flow in a polar has complex three-dimensional structure because the magnetic field lines lift it out of the orbital plane.[2] Indeed, in some polars, the vertical extent of the accretion flow enables it to regularly pass in front of the WD's accretion spot as seen from Earth, causing a temporary decrease in the system's observed brightness.[4]

Polars derive their name from the linearly and circularly polarized light that they produce.[1] Information about the accretion geometry of a polar can be found by studying its polarization.

Asynchronous polars edit

The 1:1 ratio of the WD rotational period and the binary orbital period is a fundamental property of polars, but in four polars (V1500 Cyg, BY Cam, V1432 Aql, and CD Ind), these two periods are different by ~1% or less.[5] The most common explanation for the WD's asynchronous rotation is that each of these systems had been synchronous until a nova eruption broke the synchronization by changing the WD's rotation period.[6] The first known asynchronous polar, V1500 Cyg, underwent a nova in 1975, and its asynchronous rotation was discovered after the nova faded, providing the best observational evidence of this scenario.[6] In V1500 Cyg, BY Cam, and V1432 Aql, there is observational evidence that the WD is resynchronizing its spin period with the orbital period, and these systems are expected to become synchronous on a timescale of centuries.[5]

Due to the slight difference between the orbital and WD rotation periods, the WD and its magnetosphere slowly rotate as seen from the donor star. Critically, this asynchronous rotation causes the accretion stream to interact with different magnetic field lines. Since the accretion flow travels along the field lines which have captured it, it will follow different trajectories when it interacts with different field lines. As a concrete example, the accretion flow in the eclipsing polar V1432 Aql sometimes threads onto field lines which carry it so far above the orbital plane that the flow is not obscured when the donor star eclipses the WD, but at other times, it threads onto field lines with less vertical extent, causing the accretion flow to be much more fully eclipsed.[7] The corresponding variations in eclipse depth were shown to depend very strongly on the orientation of the WD's magnetic field with respect to the donor star.[7] For comparison, in a synchronous polar, the WD does not rotate with respect to the donor star, and the stream always interacts with the same field lines, resulting in a stable accretion geometry.

There is also evidence in each of the four asynchronous polars that the accretion stream is able to travel much deeper into the WD's magnetosphere than in synchronous systems, implying an unusually high rate of mass transfer from the donor star or a low magnetic field strength, but this has not been studied in detail.[7]

Intermediate polars edit

Main article: Intermediate polar

Another class of cataclysmic variables with magnetic white dwarfs accreting material from a main sequence donor star are the intermediate polars. These have less strong magnetic fields and the rotation of the white dwarf is not synchronised with the orbital period. It has been proposed that intermediate polars may evolve into polars as the donor is depleted and the orbit shrinks.[2]

References edit

  1. ^ a b c d Hellier, Coel (2001). Cataclysmic Variable Stars. Springer.
  2. ^ a b c d e f g Cropper, Mark (1990-12-01). "The polars". Space Science Reviews. 54 (3–4): 195–295. Bibcode:1990SSRv...54..195C. doi:10.1007/BF00177799. ISSN 0038-6308. S2CID 189786424.
  3. ^ Krzeminski, W. & Serkowski, K. (August 1977). "Extremely high circular polarization of AN Ursae Majoris". The Astrophysical Journal Letters. 216: L45. Bibcode:1977ApJ...216L..45K. doi:10.1086/182506.
  4. ^ a b Harrop-Allin, M. K.; Cropper, M.; Hakala, P. J.; Hellier, C.; Ramseyer, T. (1999-09-23). "Indirect imaging of the accretion stream in eclipsing polars — II. HU Aquarii". Monthly Notices of the Royal Astronomical Society. 308 (3): 807–817. Bibcode:1999MNRAS.308..807H. doi:10.1046/j.1365-8711.1999.02780.x. ISSN 0035-8711.
  5. ^ a b Warner, Brian (2003). "General Properties of Quiescent Novae". AIP Conference Proceedings. 637: 3–15. arXiv:astro-ph/0206452. doi:10.1063/1.1518170. S2CID 43999382.
  6. ^ a b Stockman, H. S.; Schmidt, Gary D.; Lamb, D. Q. (1988-09-01). "V1500 Cygni - Discovery of a magnetic nova". The Astrophysical Journal. 332: 282. Bibcode:1988ApJ...332..282S. doi:10.1086/166652.
  7. ^ a b c Littlefield, Colin; Mukai, Koji; Mumme, Raymond; Cain, Ryan; Magno, Katrina C.; Corpuz, Taylor; Sandefur, Davis; Boyd, David; Cook, Michael (2015-05-21). "Periodic eclipse variations in asynchronous polar V1432 Aql: evidence of a shifting threading region". Monthly Notices of the Royal Astronomical Society. 449 (3): 3107–3120. Bibcode:2015MNRAS.449.3107L. doi:10.1093/mnras/stv462. ISSN 0035-8711.

Further reading edit

  • Coel Hellier (2001). Cataclysmic Variable Stars: How and Why They Vary. Springer Praxis. ISBN 978-1-85233-211-2.

April 15, 2024
polar, star, other, uses, polar, disambiguation, astronomy, polar, highly, magnetic, type, cataclysmic, variable, binary, star, system, originally, known, herculis, star, after, prototype, member, herculis, like, other, polars, contain, stars, accreting, white. For other uses see Polar disambiguation In astronomy a polar is a highly magnetic type of cataclysmic variable CV binary star system originally known as an AM Herculis star after the prototype member AM Herculis Like other CVs polars contain two stars an accreting white dwarf WD and a low mass donor star usually a red dwarf which is transferring mass to the WD as a result of the WD s gravitational pull overflowing its Roche lobe 1 Polars are distinguished from other CVs by the presence of a very strong magnetic field in the WD Typical magnetic field strengths of polar systems are 10 million to 80 million gauss 1000 8000 teslas 2 The WD in the polar AN Ursae Majoris has the strongest known magnetic field among cataclysmic variables with a field strength of 230 million gauss 23 kT 3 Polar variables are magnetic white dwarfs accreting material from a low mass donor with no accretion disk due to the intense magnetic field Contents 1 Accretion mechanism 2 Asynchronous polars 3 Intermediate polars 4 References 5 Further readingAccretion mechanism editOne of the most critical consequences of the WD s magnetism is that it synchronizes the rotational period of the WD with the orbital period of the binary 2 to first order this means that the same side of the WD always faces the donor star This synchronous rotation is considered a defining feature of polars 1 2 Additionally the WD s magnetic field captures the accretion stream from the donor star before it can develop into an accretion disk The capture of the accretion stream is known as threading and it occurs when the magnetic pressure from the WD matches the stream s ram pressure 2 The captured material flows along the WD s magnetic field lines until it violently accretes onto the WD in a shock near one or more of the star s magnetic poles 2 This accretion region covers only a fraction of the WD s surface but it can contribute half of the system s optical light 4 In addition to optical and near infrared cyclotron radiation the accretion region also produces X rays due to the high temperature of gas within the shock so polars are frequently brighter in X rays than non magnetic CVs 1 Whereas accretion in a non magnetic system is governed by viscosity within the accretion disk accretion in a polar is entirely magnetic Additionally while an accretion disk can be crudely envisioned as a two dimensional structure with no significant thickness the accretion flow in a polar has complex three dimensional structure because the magnetic field lines lift it out of the orbital plane 2 Indeed in some polars the vertical extent of the accretion flow enables it to regularly pass in front of the WD s accretion spot as seen from Earth causing a temporary decrease in the system s observed brightness 4 Polars derive their name from the linearly and circularly polarized light that they produce 1 Information about the accretion geometry of a polar can be found by studying its polarization Asynchronous polars editThe 1 1 ratio of the WD rotational period and the binary orbital period is a fundamental property of polars but in four polars V1500 Cyg BY Cam V1432 Aql and CD Ind these two periods are different by 1 or less 5 The most common explanation for the WD s asynchronous rotation is that each of these systems had been synchronous until a nova eruption broke the synchronization by changing the WD s rotation period 6 The first known asynchronous polar V1500 Cyg underwent a nova in 1975 and its asynchronous rotation was discovered after the nova faded providing the best observational evidence of this scenario 6 In V1500 Cyg BY Cam and V1432 Aql there is observational evidence that the WD is resynchronizing its spin period with the orbital period and these systems are expected to become synchronous on a timescale of centuries 5 Due to the slight difference between the orbital and WD rotation periods the WD and its magnetosphere slowly rotate as seen from the donor star Critically this asynchronous rotation causes the accretion stream to interact with different magnetic field lines Since the accretion flow travels along the field lines which have captured it it will follow different trajectories when it interacts with different field lines As a concrete example the accretion flow in the eclipsing polar V1432 Aql sometimes threads onto field lines which carry it so far above the orbital plane that the flow is not obscured when the donor star eclipses the WD but at other times it threads onto field lines with less vertical extent causing the accretion flow to be much more fully eclipsed 7 The corresponding variations in eclipse depth were shown to depend very strongly on the orientation of the WD s magnetic field with respect to the donor star 7 For comparison in a synchronous polar the WD does not rotate with respect to the donor star and the stream always interacts with the same field lines resulting in a stable accretion geometry There is also evidence in each of the four asynchronous polars that the accretion stream is able to travel much deeper into the WD s magnetosphere than in synchronous systems implying an unusually high rate of mass transfer from the donor star or a low magnetic field strength but this has not been studied in detail 7 Intermediate polars editMain article Intermediate polar Another class of cataclysmic variables with magnetic white dwarfs accreting material from a main sequence donor star are the intermediate polars These have less strong magnetic fields and the rotation of the white dwarf is not synchronised with the orbital period It has been proposed that intermediate polars may evolve into polars as the donor is depleted and the orbit shrinks 2 References edit a b c d Hellier Coel 2001 Cataclysmic Variable Stars Springer a b c d e f g Cropper Mark 1990 12 01 The polars Space Science Reviews 54 3 4 195 295 Bibcode 1990SSRv 54 195C doi 10 1007 BF00177799 ISSN 0038 6308 S2CID 189786424 Krzeminski W amp Serkowski K August 1977 Extremely high circular polarization of AN Ursae Majoris The Astrophysical Journal Letters 216 L45 Bibcode 1977ApJ 216L 45K doi 10 1086 182506 a b Harrop Allin M K Cropper M Hakala P J Hellier C Ramseyer T 1999 09 23 Indirect imaging of the accretion stream in eclipsing polars II HU Aquarii Monthly Notices of the Royal Astronomical Society 308 3 807 817 Bibcode 1999MNRAS 308 807H doi 10 1046 j 1365 8711 1999 02780 x ISSN 0035 8711 a b Warner Brian 2003 General Properties of Quiescent Novae AIP Conference Proceedings 637 3 15 arXiv astro ph 0206452 doi 10 1063 1 1518170 S2CID 43999382 a b Stockman H S Schmidt Gary D Lamb D Q 1988 09 01 V1500 Cygni Discovery of a magnetic nova The Astrophysical Journal 332 282 Bibcode 1988ApJ 332 282S doi 10 1086 166652 a b c Littlefield Colin Mukai Koji Mumme Raymond Cain Ryan Magno Katrina C Corpuz Taylor Sandefur Davis Boyd David Cook Michael 2015 05 21 Periodic eclipse variations in asynchronous polar V1432 Aql evidence of a shifting threading region Monthly Notices of the Royal Astronomical Society 449 3 3107 3120 Bibcode 2015MNRAS 449 3107L doi 10 1093 mnras stv462 ISSN 0035 8711 Further reading editCoel Hellier 2001 Cataclysmic Variable Stars How and Why They Vary Springer Praxis ISBN 978 1 85233 211 2 Retrieved from https en wikipedia org w index php title Polar star amp oldid 1192309221, wikipedia, wiki, book, books, library,

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