Period gap

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The period gap ( English period gap ) describes in astronomy that in the non-magnetic cataclysmic variables just with circulation periods are observed between two and three hours.

The cataclysmic variables consist of a red dwarf and a white dwarf . The red dwarf fills its Roche limit volume and transfers matter to its companion, the white dwarf. Due to the preservation of the angular momentum, an accretion disk forms around the white dwarf , through which matter flows onto the white dwarf. The period gap is caused by a shrinking of the diameter of the red dwarf due to a change in the internal structure when the energy transport in the star becomes completely convective .

Non-Magnetic Cataclysmic Variables

All of these non-magnetic cataclysmic variables show a high underfrequency for orbit periods between 2.15 and 3.18 hours. The number of observed cataclysmic variables within the period gap is a factor of 20 lower than above and below the period gap. If the distance between the stars leads to a value of 3.18 hours, the red dwarf has a mass of about 0.4 solar masses, at which the energy transport in the entire star takes place exclusively by means of convection . As a result, due to its changed structure, the companion shrinks below the Roche interface, whereupon the flow of matter stops and the cataclysmic activity dies down. Within the period gap, there is a slow loss of angular momentum due to the radiation of gravitational waves , whereby this mechanism takes up to a billion years to bring the binary star system back into contact with an orbital period of 2.15 hours. There are some active cataclysmic variables within the period gap, and these probably filled the Roche interface for the first time and matter transfer started within the period gap. In addition, binary stars from pre-cataclysmic variables such as GD 448 and SDSS 1355 + 0856 have also been observed, which have migrated directly from the common envelope phase into the period gap. These double stars are still separate and difficult to detect due to the lack of mass transfers,

The period gap can be demonstrated for the non-magnetic cataclysmic variables, with the subdivision based on their light curve :

  • as novae , when the hydrogen-rich matter on the surface of the white dwarf undergoes thermonuclear reactions ,
  • as dwarf novae , if an instability increases the flow rate in the accretion disc and this lights up due to increased friction,
  • as nova-like, which correspond to a dwarf nova in permanent eruption.

In the case of magnetic cataclysmic variables, the mass transfer to the white dwarf does not take place via an accretion disk, but the flow of matter happens along the magnetic field lines of the compact star. Since there are hardly any AM Herculis stars and DQ Herculis stars with orbital periods of less than three hours, the magnetic cataclysmic variables are not referred to as a period gap.

Magnetic activity

The model described above would suggest a large number of old magnetically active red dwarfs with rotation periods between two and three hours in the form of BY-Draconis stars through star spots or through flares in UV-Ceti stars . In young red dwarfs, who also rotate rapidly, there are a large number of these variables with a spectral type between M4 and M6, corresponding to the spectral type of the cataclysmic variables within the period gap. But these kinematically old magnetically active red dwarfs in cataclysmic variables have never been observed. It is possible that the red dwarfs in inactive cataclysmic variables do not rotate differentially and are therefore not magnetically active.

Individual evidence

  1. ^ Brian Warner: Cataclysmic Variable Stars . Cambridge University Press, 1995, ISBN 0-521-54209-X .
  2. ↑ U. Kolb, AR King, H. Ritter: The CV period gap: still there . In: Astrophysics. Solar and Stellar Astrophysics . 1998. The CV period gap: still there
  3. ^ C. Knigge: The donor stars of cataclysmic variables . In: Monthly Notice of the Royal Astronomical Society . tape 373 , no. 2 , 2006, p. 484-502 , doi : 10.1111 / j.1365-2966.2006.11096.x .
  4. ^ PFL Maxted, TR Marsh, C. Moran, VS Dhillon, RW Hilditch: The mass and radius of the M dwarf companion to GD 448 . In: Astrophysics. Solar and Stellar Astrophysics . 1998. The mass and radius of the M dwarf companion to GD 448
  5. ↑ Carles Badenes, Marten H. van Kerkwijk, Mukremin Kilic, Steven J. Bickerton, Tsevi Mazeh, Fergal Mullally, Lev Tal-Or, Susan E. Thompson: SDSS 1355 + 0856: A detached white dwarf + M star binary in the period gap discovered by the SWARMS survey . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.6532 .
  6. ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne . JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
  7. ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  8. ↑ Gaitee AJ Hussain: Magnetic braking in convective stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1202.5075 .