SW Sextantis variable

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SW Sextantis variable stars are a kind of cataclysmic variable star; they are double-star systems in which there is mass transfer from a red dwarf to a white dwarf forming a stable accretion disc around the latter. Unlike other non-magnetic cataclysmic variables, the emission lines from hydrogen and helium are not doubled, except briefly near phase 0.5.

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Characteristics

SW Sextantis stars have an orbital period between 2.8 and 4 hours; most systems were discovered by surveys of eclipsing variables, so the orbit is nearly edge-on with respect to the Earth. Their spectra resemble those of a dwarf nova in outburst, with signs of a permanently ionised accretion disc. Material is constantly flowing into the disc from the companion star, and friction within the disc causes it to emit optical light. It is more difficult to find SW Sextantis systems with low inclination, since it is necessary to examine many stellar spectra without being able to restrict to eclipsing variables; however, surveys have been performed, and suggest that some of the observed properties of SW Sextantis stars are accidental results of a sample restricted to high inclination systems [1]

Emission lines of hydrogen (the Balmer series) and helium are observed, and are not doubled (as one would expect by Doppler shift of light emitted from the edges of a fast-rotating disc), but the wings are broadened to the point that the spread of source velocities can be as much as 4000 km/s. For a brief period near phase 0.5 of their orbits, SW Sextantis stars do show doubling of their emission lines and this is a defining character of the class. [2] In eclipsing systems, the emission lines are scarcely detected at minimum light because the white dwarf and the central part of the accretion disc are hidden behind the red dwarf. [3]

In the ultraviolet we observe emission lines from the white dwarf, which indicate an unusually high temperature and imply a high accretion rate. [4] Furthermore, the radial velocity of an SW Sextantis star determined from the disc emission lines is not the same as that determined from the white dwarf.

The orbital period of SW Sextantis systems is always just above the period gap, suggesting a joint-development phase for these cataclysmic variables.

Interpretation

Models of SW Sextantis stars must explain the high mass transfer rate and the period distribution just above the period-gap. The standard theory of cataclysmic variables suggests that the rate of mass transfer is determined by loss of angular momentum due to magnetic fields. The stellar wind of the red dwarf sends ionised plasma into space, which travels along magnetic field lines; indeed, it is trapped in the magnetic field lines and follows the rotation of the star. Since the magnetic field accelerates the escaping plasma, the rotation of the star is braked. This in turn reduces the total angular momentum of the double-star system, which along with the rearrangement of the matter in the system leads to the orbital radius getting smaller, which keeps the mass transfer rate steady. [5]

Under this model, the core of the red dwarf is rotating faster than the orbital period. As mass transfer causes the radius of the star to shrink, conservation of angular momentum means that it spins faster, and this means the dynamo effect generates a stronger magnetic field. This increases the magnetic braking effect and accordingly the mass transfer rate. [6]

Another interpretation of SW Sextantis stars is that the high mass transfer rate is only temporary. Some cataclysmic variables (e.g. the classical novae RR Pictoris, XX Tauri and V728 Scorpii) have periods just above the period gap, and this is interpreted as part of the hibernation model, where, after a nova eruption, the white dwarf is unusually hot; it heats the red dwarf, causing a higher mass transfer rate until the white dwarf has cooled down again. As it cools, the red dwarf shrinks and the mass transfer rate drops to quite low levels; eventually loss of orbital angular momentum causes the stars to get closer together again, and mass transfer resumes. In this model, SW Sextantis stars represent a stage in the life of a cataclysmic variable either shortly before or shortly after a nova eruption. [7]

Examples

Donald W. Hoard at the Max Planck Institute for Astronomy in Heidelberg maintains a list [8] of SW Sextantis stars mentioned in the literature, and a description [9] of the characteristics used to identify them.

Related Research Articles

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<span class="mw-page-title-main">DQ Herculis</span> Nova in the constellation Hercules

DQ Herculis, or Nova Herculis 1934, was a slow, bright nova occurring in the northern constellation of Hercules in December 1934. This cataclysmic variable star was discovered on 13 December 1934 by J. P. M. Prentice from Stowmarket, Suffolk. It reached peak brightness on 22 December 1934 with an apparent magnitude of 1.5. The nova remained visible to the naked eye for several months.

<span class="mw-page-title-main">BT Monocerotis</span> Nova seen in 1939

BT Monocerotis was a nova, which lit up in the constellation Monoceros in 1939. It was discovered on a spectral plate by Fred L. Whipple on December 23, 1939. BT Monocerotis is believed to have reached mag 4.5, which would have made it visible to the naked eye, but that value is an extrapolation; the nova was not observed at peak brightness Its brightness decreased after the outbreak by 3 magnitudes in 182 days, making it a "slow nova". The light curve for the eruption had a long plateau period.

<span class="mw-page-title-main">V1494 Aquilae</span> Nova seen in 1999 in the constellation of Aquila

V1494 Aquilae or Nova Aquilae 1999 b was a nova which occurred during 1999 in the constellation Aquila and reached a brightness of magnitude 3.9 on 2 December 1999. making it easily visible to the naked eye. The nova was discovered with 14×100 binoculars by Alfredo Pereira of Cabo da Roca, Portugal at 18:50 UT on 1 December 1999, when it had a visual magnitude of 6.0.

<span class="mw-page-title-main">Polar (star)</span> Highly magnetic type of cataclysmic variable binary star system

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An AM Canum Venaticorum star, is a rare type of cataclysmic variable star named after their type star, AM Canum Venaticorum. In these hot blue binary variables, a white dwarf accretes hydrogen-poor matter from a compact companion star.

<span class="mw-page-title-main">EX Hydrae</span> Cataclysmic binary star system in the constellation Hydra

EX Hydrae is a variable star classified as an eclipsing intermediate polar-type cataclysmic variable, specifically of the DQ Herculis type. The system varies in apparent magnitude from 9.6 to 14. The system consists of a white dwarf primary and an M-type secondary, of masses of 0.4–0.7 M☉ and 0.07–0.10 M☉ respectively. The orbital period is 98.25696 minutes (0.068233846 days). The system is 65±11 parsecs distant, making EX Hya one of the closest cataclysmic variable stars. The cataclysmic outbursts appear to be caused by accretion of material from the M-star to the white dwarf.

<span class="mw-page-title-main">RZ Gruis</span> Star in the constellation of Grus

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<span class="mw-page-title-main">CE Gruis</span> Binary star system in the constellation Grus

CE Gruis is a faint binary star system in the constellation Grus. It is a variable star, with a B-band brightness that ranges from a peak magnitude of 17.4 down to a minimum of 19.5 over a period of 108.6 minutes. The system is composed of a white dwarf and donor star, locked into a close, synchronous orbit. In such systems, known as polars, material from the donor star does not form an accretion disc around the white dwarf because of its intense magnetic field, but rather streams directly onto it along columns.

<span class="mw-page-title-main">Circumstellar disc</span> Accumulation of matter around a star

A circumstellar disc is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form. Around mature stars, they indicate that planetesimal formation has taken place, and around white dwarfs, they indicate that planetary material survived the whole of stellar evolution. Such a disc can manifest itself in various ways.

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

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

PX Andromedae is an eclipsing cataclysmic variable star in the constellation Andromeda. It has been classified as a SW Sextantis variable, and its apparent visual magnitude varies between 14.04 and 17.

<span class="mw-page-title-main">V455 Andromedae</span> Dwarf nova star in the constellation Andromeda

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<span class="mw-page-title-main">V1315 Aquilae</span> Variable star in the constellation Aquila

V1315 Aquilae is a cataclysmic variable star in the north of the equatorial constellation of Aquila. It is in the sub-set of nova-like (NL) variables, specifically a SW Sextantis star. These were characterized as having non-magnetic white dwarfs – thus that do not undergo dwarf-nova bright luminations ("eruptions"). There is countering evidence for some magnetism. Being a SW Sextantis star, V1315 Aquilae has a high rate of mass transfer, so it is in steady-state accretion and in a constant state of outburst. It emits most of its light in the visible range, and this comes from the accretion disk. The eclipse depth is 1.8 mag. No description of the donor star is made.

<span class="mw-page-title-main">Post common envelope binary</span> Binary system consisting of a white dwarf and a main sequence star or a brown dwarf

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<span class="mw-page-title-main">UZ Fornacis</span> Binary star system in the constellation Fornax

UZ Fornacis is a binary star in the constellation of Fornax. It appears exceedingly faint with a maximum apparent magnitude 17.0. Its distance, as measured by Gaia using the parallax method, is about 780 light-years.

<span class="mw-page-title-main">QZ Aurigae</span> Nova seen in 1964

QZ Aurigae, also known as Nova Aurigae 1964, was a nova which occurred in the constellation Auriga during 1964. It was discovered by Nicholas Sanduleak on an objective prism photographic plate taken at the Warner and Swasey Observatory on 4 November 1964. Examination of pre-discovery plates from Sonneberg Observatory showed that the eruption occurred in early February 1964, and it had a photographic magnitude of 6.0 on 14 February 1964. Its brightness declined in images taken after the 14th, suggesting that its peak brightness was above 6.0. It was probably visible to the naked eye for a short time.

<span class="mw-page-title-main">UX Ursae Majoris</span>

UX Ursae Majoris is an Algol type binary star system in the northern circumpolar constellation of Ursa Major. It is classified as a nova-like variable star similar to DQ Herculis, although no eruptions have been reported. Since its discovery in 1933, this system has been the subject of numerous studies attempting to determine its properties. The combined apparent visual magnitude of UX UMa ranges from 12.57 down to 14.15. The system is located at a distance of approximately 952 light years from the Sun based on parallax, and is drifting further away with a radial velocity of 112 km/s.

<span class="mw-page-title-main">BZ Ursae Majoris</span> Dwarf Nova in the constellation Ursa Major

BZ Ursae Majoris is a dwarf nova star system in the northern circumpolar constellation of Ursa Major. It consists of a white dwarf primary in a close orbit with a red dwarf. The latter star is donating mass, which is accumulating in an accretion disk orbiting the white dwarf. The system is located at a distance of approximately 505 light years from the Sun based on parallax measurements.

<span class="mw-page-title-main">DW Ursae Majoris</span> Variable star in the constellation Ursa Major

DW Ursae Majoris is an eclipsing binary star system in the northern circumpolar constellation of Ursa Major, abbreviated DW UMa. It is a cataclysmic variable of the SX Sextanis type, consisting of a compact white dwarf that is accreting matter from an orbiting companion star. The brightness of this source ranges from an apparent visual magnitude of 13.6 down to magnitude 18, which is too faint to be viewed with the naked eye. The distance to this system is approximately 1,920 light years based on parallax measurements.

References

  1. ↑ V. S. Dhillon; D. A. Smith; T. R. Marsh (2013). "The SW Sex enigma". Monthly Notices of the Royal Astronomical Society. 428 (4): 3559–3568. arXiv: 1210.7145 . Bibcode:2013MNRAS.428.3559D. doi:10.1093/mnras/sts294. S2CID   36011209.
  2. ↑ Knigge, Christian; Araujo-Betancor, Sofia; Gänsicke, Boris T.; Long, Knox S.; Szkody, Paula; Hoard, D. W.; Hynes, R. I.; Dhillon, V. S. (2004). "Time-resolved Ultraviolet Spectroscopy of the SW Sex Star DW UMa: Confirmation of a Hidden White Dwarf and the Ultraviolet Counterpart to Phase 0.5 Absorption Events". The Astrophysical Journal. 615 (2): L129. arXiv: astro-ph/0410292 . Bibcode:2004ApJ...615L.129K. doi:10.1086/426118. S2CID   118988616.
  3. ↑ V. S. Dhillon, T. R. Marsh and D. H. P. Jones (1997). "On the nature of SW Sex". Monthly Notices of the Royal Astronomical Society. 291 (4): 694–708. arXiv: astro-ph/9709171 . Bibcode:1997MNRAS.291..694D. doi:10.1093/mnras/291.4.694. S2CID   14772712.
  4. ↑ Linda Schmidtobreick, Pablo Rodríguez-Gil & Boris T. Gänsicke (2012). "The Search for SW Sex Type Stars". Memorie della Societa Astronomica Italiana. 83: 610. arXiv: 1111.6678 . Bibcode:2012MmSAI..83..610S.
  5. ↑ Christian Knigge (2011). "Cataclysmic Variables: Eight Breakthroughs in Eight Years". arXiv: 1101.2901 [astro-ph.SR].
  6. ↑ Linda Schmidtobreick (2013). "The SW Sex Phenomenon as an Evolutionary Stage of Cataclysmic Variables". Central European Astrophysical Bulletin. 37: 361–368. arXiv: 1211.2171 . Bibcode:2013CEAB...37..361S.
  7. ↑ Claus Tappert; et al. (2013). "Life after eruption – II. The eclipsing old nova V728 Scorpii". Monthly Notices of the Royal Astronomical Society. 431 (1): 92–101. arXiv: 1302.5570 . Bibcode:2013MNRAS.431...92T. doi:10.1093/mnras/stt139. S2CID   46958131.
  8. ↑ "The Big List of SW Sextantis Stars". October 2016. Archived from the original on 2018-08-17. Retrieved 2016-04-21.
  9. ↑ "Observational Characteristics of the SW Sextantis Stars". October 2016. Archived from the original on 2018-08-17. Retrieved 2016-04-21.
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