SW sextantis star

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SW sextantis stars are a subset of the cataclysmic variables . These binary star systems consist of a red dwarf that transfers matter to a white dwarf via an accretion disk . In contrast to other non-magnetic cataclysmic variables, in SW sextantis stars the emission lines of the Balmer series and helium are only observed as single lines and hardly weaken during the cover light change when the white dwarf and the central part of the accretion disk are removed from the Earth from being obscured by the red dwarf.

properties

All SW sextantis stars have orbital periods between 2.8 and four hours and originally have a high orbital inclination , which means that there is a high probability that the light will change. Their spectral properties correspond to those of dwarf novae in permanent eruption, the nova-like ones with a permanently ionized accretion disk. With these cataclysmic variables, so much matter flows from the companion star into the accretion disk that the disk lights up due to its viscosity and dominates the optical spectrum . In the case of SW sextantis stars, the emission lines of the Balmer series and of helium are observed as individual lines. Since these emission lines arise in the disk, two lines would be expected that are shifted both blue and red due to the Doppler shift . Furthermore, the intensity of these emission lines hardly decreases during the cover light change. The wings of the emission lines are widened up to speeds of up to 4000 km / s. In the UV, emission lines of the white dwarf are observed, which point to unusually high temperatures for cataclysmic variables and support the assumption of a high accretion rate.

Furthermore, the emission lines of the SW sextantis stars do not follow the movement of the white dwarf. The period of rotation of the SW sextantis systems are all just above the period gap and indicate a common development phase of these cataclysmic variables. B / W sextantis systems with a low angle of inclination are more difficult to discover, since they do not have an easily discovered cover light change. These binary star systems now make up almost half of the SW sextantis stars and are in contrast to older hypotheses, according to which SW sextantis systems have no unusual physical properties, but are the result of a selection effect of systems with a high orbital inclination.

interpretation

Every interpretation of the SW sextantis stars must relate the high mass transfer rate to the period of orbit just above the period gap. The standard theory of cataclysmic variables assumes that the mass transfer rate is controlled by magnetic torque loss. Plasma , ionized matter, is accelerated into space by the stellar wind of the red dwarf and follows the star's magnetic field lines . The plasma is frozen in the magnetic field lines and follows the rotation of the star. Since the star has to drag the outflowing plasma with it, the rotation of the star is slowed down. This in turn reduces the total angular momentum of the binary star and, together with the redistribution of matter in the binary star system, leads to a reduction in the orbital axis , whereby the mass transfer is maintained.

In this context, it is assumed that the core of the red dwarf rotates faster than the orbit period. Since the radius of the core decreases due to the mass transfer, the core has to rotate faster in order to maintain the torque and therefore generate a stronger magnetic field via the alpha-omega dynamo . This in turn leads to a greater loss of magnetic torque and consequently to a higher mass transfer rate.

Another interpretation of the SW sextantis phenomenon is that it is only a temporary high mass transfer. There are also known cataclysmic variables with a period length just above the period gap such as RR Pictoris , XX Tauri and V728 Scorpii . This is interpreted in the context of the hibernation model, according to which, after a nova outbreak , the heating of the red dwarf leads to a high mass transfer rate until the white dwarf has cooled down again. The radius of the red dwarf shrinks and the mass transfer rate drops to quite low values. Due to the loss of torque, the two components of the binary star system come into contact again and the mass transfer rates increase again. Accordingly, the SW sextantis stars would be cataclysmic variables shortly before or shortly after a nova eruption.

Examples

Individual evidence

  1. ^ VS Dhillon, TR Marsh and DHP Jones: On the nature of SW Sex . In: Astrophysics. Solar and Stellar Astrophysics . 1997, arxiv : astro-ph / 9709171 .
  2. ↑ Linda Schmidtobreick, Pablo Rodrıguez-Gil and Boris T. Gänsicke: The Search for SW Sex Type Stars . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1111.6678 .
  3. ^ VS Dhillon, DA Smith, TR Marsh: The SW Sex enigma . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.7145 .
  4. ↑ C. Knigge: Cataclysmic Variables: Eight Breakthroughs in Eight Years . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1101.2901 .
  5. ↑ Linda Schmidtobreick: THE SW SEX PHENOMENON AS AN EVOLUTIONARY STAGE OF CATACLYSMIC VARIABLES . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.2171 .
  6. ↑ C. Tappert et al .: Life after eruption - II. The eclipsing old nova V728 Scorpii . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1302.5570v1 .