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.

Contents

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">BT Monocerotis</span> Nova seen in 1939

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

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

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

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

RZ Gruis is a nova-like binary system in the constellation Grus composed of a white dwarf and an F-type main-sequence star. It is generally of apparent magnitude of 12.3 with occasional dimming to 13.4. Its components are thought to orbit each other roughly every 8.5 to 10 hours. It belongs to the UX Ursae Majoris subgroup of cataclysmic variable star systems, where material from the donor star is drawn to the white dwarf where it forms an accretion disc that remains bright and outshines the two component stars. The system is around 1,434 light-years away from Earth; or as much as 1,770 light years based on a Gaia parallax.

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

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<span class="mw-page-title-main">QS Telescopii</span> Binary star system in the constellation Telescopium

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

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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">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

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

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<span class="mw-page-title-main">GI Monocerotis</span> 1918 Nova in the constellation Monoceros

GI Monocerotis, also known as Nova Monocerotis 1918, was a nova that erupted in the constellation Monoceros during 1918. It was discovered by Max Wolf on a photographic plate taken at the Heidelberg Observatory on 4 February 1918. At the time of its discovery, it had a photographic magnitude of 8.5, and had already passed its peak brightness. A search of plates taken at the Harvard College Observatory showed that it had a photographic magnitude of 5.4 on 1 January 1918, so it would have been visible to the naked eye around that time. By March 1918 it had dropped to ninth or tenth magnitude. By November 1920 it was a little fainter than 15th magnitude.

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

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">OY Arae</span> 1910 nova in the constellation Ara

OY Arae, also known as Nova Arae 1910, is a nova in the constellation Ara. It was discovered by Williamina Fleming on a Harvard Observatory photographic plate taken on April 4, 1910. At that time it had a magnitude of 6.0, making it faintly visible to the naked eye under ideal observing conditions. Examination of earlier plates showed that before the outburst it was a magnitude 17.5 object, and by March 19, 1910, it had reached magnitude 12.

<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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