Cataclysmic Mutable

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A cataclysmic variable (abbreviation CV from English Cataclysmic Variable) is a narrow, semi-separated binary star system . It consists of an accreting white dwarf and a mass-losing red dwarf star , helium star, or subgiant . Cataclysmic variable stars exhibit a wide range of changes in brightness as a result of mass transfer between stars.

History and structure

Cataclysmic variables are known in the form of Novae from Chinese sources that are around 2500 years old. These are strong bursts of brightness of up to 20  mag , which were interpreted as new stars . The first dwarf nova U Geminorum was not discovered until the second half of the 19th century . The assumed relationship between novae and dwarf novae initially related to the shape of the light curve , the smaller burst amplitude and the shorter time between bursts.

Artist's impression of a cataclysmic binary star system

The structure of the cataclysmic variables could only be understood with the help of photoelectric photometry and spectroscopy . It is a close binary star system , consisting of a white dwarf and a companion. This crosses its Roche limit in the binary star system and therefore loses matter to the white dwarf. The companion is usually a red dwarf star or a late subgiant . The matter flows along a current towards the white dwarf and, due to the conservation of angular momentum, forms an accretion disk around the compact star in the absence of strong magnetic fields . Where the flow of matter hits the accretion disk, it is heated and forms a bright spot that leads to a hump in the light curve . The light curve of a cataclysmic variables can still a with a corresponding orientation in space covering variable portion have flicker in the range of fractions of a second (engl. Flickering ) and may vary due to a varying flow of matter in the rest brightness.

The term "cataclysmic" is derived from the ancient Greek cataclysmos for inundation and describes the fundamental property of these variables, according to which the white dwarf is inundated with matter by his companion.

classification

The classification of cataclysmic variables is based on different physical processes that occur during mass transfer and is divided into three main groups:

The parameters determining the condition of a CV are the orbital period of the system and, depending on this, the spectral type and mass of the companion or mass accretion rate as well as the magnetic field of the white decay.

The object classes often show different and characteristic shapes of the light curve on which the classification was historically based.

Disc systems

The primary emission source in disk systems (English disk CVs) comes from an accretion disk surrounding the white dwarf , in which the kinetic energy of the incident matter is converted into electromagnetic radiation. The behavior of the accretion disk depends primarily on the mass accretion rate and the mass ratio of both stars and is systematized in the following subgroups:

Dwarf novae

Dwarf novae show multiple eruptions with an increase in brightness up to 8  mag . A steep rise and a slower decrease in brightness are characteristic. The outbreaks occur at an average interval of weeks to decades. The cause of the eruptions lies in a bistable state of the accretion disc , which occurs when the mass accretion rate falls below a critical value. During the dwarf nova outbreak, when a critical density is exceeded, there is a sudden increase in viscosity, as a result of which the matter collected in the disk is increasingly transferred to the white dwarf.

Dwarf novae are further divided into:

  • U-Geminorum stars: The classic dwarf novae are mostly in their calm brightness and all outbreaks have a shape typical for the star.
  • SU-Ursae-Majoris-Stars : In this subgroup, supermaxima occur in addition to normal outbreaks. These are around 0.7 mag lighter and take three to five times longer. So-called superhumps also occur. These are small changes in brightness superimposed on the maxima with a period that is a few percent longer than the period of rotation of the binary star system.
  • WZ-Sagittae-Stars: Short-period systems with very low-mass companions (sometimes less than 0.08 solar masses) and very low accretion rates. The difference to the SU-UMa stars is the lack of normal bursts. Only super-eruptions are observed that occur in very large time intervals of up to 30 years. WZ-Sge stars are sometimes referred to by the unusual term TOADs (Tremendous Outburst Amplitude Dwarf novae).
  • Z-Camelopardalis-Stars: The Zwergnovatypical change in brightness with outbreaks from a state of rest is temporarily interrupted by intervals with almost constant light, the so-called standstills. The brightness at a standstill lies between breakout and rest brightness. The standstill begins with a decrease in brightness from the maximum and ends at a minimum. Z-Camelopardalis stars are dwarf novae whose mass accretion rate is close to the critical value above which no more eruptions occur.

The differences between the subclasses are based on different mean mass accretion rates, which decrease in a sequence of Z-Camelopardalis, U-Geminorum, SU-Ursae-Majoris and WZ-Sagittae stars.

Nova-like mutables

Disk systems in which no dwarf nova outbursts occur are grouped under the nova-like variables. The mass accretion rate is above a critical value at which the accretion disc is constantly in a stable state, similar to that of a dwarf nova in an eruption. Most nova-like ones have periods above the 3 hour period gap. As a rule, a distinction is made between the following subgroups:

  • UX-UMa stars: Classic nova-like variables with accretion disk in a stable state of high viscosity. They show hydrogen absorption lines in the spectrum and, apart from the prototype UX UMa, often have slight orbital inclinations .
  • RW-Tri-Stars: Systems with a great orbital inclination, which for this reason show spectra with emission lines and often show a change of covering light when the companion covers the accretion disk.
  • VY-Scl stars: These nova-like stars show deep minima of 3 to 5 magnitudes at irregular intervals, at which the mass transfer almost comes to a standstill. The light curves are similar to those of the polar with their high and low accretion rates depending on the activity status. The cause of the low accretion rate on the white dwarf is assumed to be an accumulation of star spots at the Lagrange point L1 . In the case of VY-Scl stars in their deep minima, a detailed examination of the white dwarf and its companion is possible, since, in contrast to other cataclysmic variables, the accretion disk is not the dominant light source. The VY-Scl stars are also misleadingly referred to as anti-dwarf novae.
  • SW-Sextantis-Stars : Spectroscopically related systems to the VY-Scl-Stars, in which due to the large inclination angle occlusions are often observed.

Magnetic CVs

Cataclysmic variables in which mass transfer is influenced by the strong magnetic field of the accreting white dwarf. One differentiates:

  • AM Herculis stars : With the AM Herculis stars or polar, the formation of an accretion disk is suppressed, since the matter coming from the companion flows along the magnetic field lines directly onto the white dwarf. Further effects of the magnetic field of up to 230  megagauss are a synchronization of the orbital movement and the rotation of the white dwarf as well as a polarization of the optical light up to 30% .
  • DQ Herculis stars : With the DQ Herculis stars or intermediate polar, the magnetic field is weaker and the accretion rate is higher than with the polar. In most cases, an accretion disk forms , from the inner boundary layer of which matter falls along the magnetic field lines onto the magnetic poles of the white dwarf. All polar and intermediate polar are sources of strong X-ray radiation .

Novae

The shape of the light curve of novae is similar to that of dwarf novae with a larger amplitude. The eruption mechanism is fundamentally different, since the eruptions of novae are the result of an explosive onset of thermonuclear reactions on the surface of the white dwarf . The radiation pressure leads to a stellar wind , which accelerates the atmosphere around the white dwarf beyond the escape speed. Novae are distinguished in:

  • classic novas with a single outbreak in historical periods and
  • repetitive novae with more than one observed outbreak in historical time periods.

AM Canum Venaticorum Stars

AM Canum Venaticorum stars are close binary systems with an orbital period of less than an hour. They consist of a white dwarf and a hydrogen-poor companion that has lost its hydrogen-rich shell. The companions are also called helium stars. AM Canum Venaticorum stars sometimes show eruptions like dwarf novae and, according to numerical calculations, also like novae. There are various signs of a flow of matter from the hydrogen-poor companion to the white dwarf.

Related object classes

Closely related to the cataclysmic variables are close binary star systems with a white dwarf as the primary component, in which the companion is not a main sequence star or there is no mass transfer yet:

Symbiotic stars

In the case of symbiotic stars , the mass transfer occurs mostly to a white dwarf from a red giant . The receiver can also be a main sequence star . Because of the size of the companion star, the spacing of the stellar components is wider than in cataclysmic variables, and the orbital periods are not hours but years or decades. An accretion disk forms around the compact star and nova eruptions or hydrogen shell burns occur, in which the source is observed as a super-soft X-ray source .

Pre-Cataclysmic Variables

The precursors of the cataclysmic variables form the class of the pre-cataclysmic variables. These are separate binary star systems consisting of a dwarf star and a white dwarf. Mass will be transferred to this within the Hubble time . Pre-cataclysmic variables are the result of a common envelope phase in which the current white dwarf transformed into a red giant and expanded so far that the companion circulated in its atmosphere. In the process, enough torque was lost due to friction to bring the orbit duration to the order of a few hours and to strip off the atmosphere of the red giant. Due to the bound rotation in a pre-cataclysmic binary star system, the dwarf star develops a strong magnetic field and magnetic activity in the form of an active corona with mass ejections and flares, such as in V471 Tauri.

While the red dwarf in a pre-cataclysmic binary star remains significantly above the level of magnetic activity of single stars due to the bound rotation , the white dwarf cools continuously over billions of years. In the case of SDSS J013851.54-001621.6, the surface temperature is only 3750  K and an age of 9.5 billion years can be derived from this. In the case of actively accreting cataclysmic variables, the temperature is at least more than twice as high. If the orbit of the pre-cataclysmic binary star is so arranged in space that there is an eclipse light change when viewed from Earth , these binary stars are a good way to calibrate the radii and the effective temperature of white dwarfs.

Origin and development

Formation of a cataclysmic binary star system in a common shell phase

The presence of a white dwarf in a short-period binary star system is initially unexpected. A white dwarf is the core of a former red giant , the diameter of which is usually larger than the distance between the stars in the cataclysmic binary star system. The emergence of a cataclysmic variable is explained today with a common shell phase. While the more massive star has formed a core of heavy elements inside, its atmosphere is expanding into a red giant. This comes into contact with the companion and its path movement is slowed down by friction. This results in an energy transfer into the atmosphere of the red giant, which then flows away, and as a result, the orbit diameter of the binary star system decreases. The common hull phase only lasts a few years and has not yet been directly observed. After the atmosphere of the former red giant has been completely thrown off, the binary star system consists of a white dwarf, the former core of the red giant and a low-mass companion. There is usually no mass transfer yet. At this stage of the pre-cataclysmic variable there is e.g. B. V471 Tauri.

In the binary star system, magnetic torque loss occurs. Plasma (ionized matter) is accelerated into space by the companion's stellar wind and follows the star's magnetic field lines . The plasma is frozen in the magnetic field lines and therefore participates in 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 reduces the distance between the components in the binary star system. After some time, the companion fills its Roche interface in the binary star system and a flow of matter begins on the white dwarf. This is the birth of the cataclysmic changeable. Due to the flow of matter, the distance between the components continues to decrease until the orbital period is around 3.18 hours.

There are hardly any cataclysmic binary star systems with orbital times between 2.15 and 3.18 hours. This phenomenon is called period gap (engl. Period CAP), respectively. If the distance between the stars leads to a value of 3.18 hours, the companion has a mass at which the energy transport in the 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 subsides. Within the period gap there is a slow loss of torque 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 first filled their Roche interface within the period gap and the matter transfer began.

With a circulation time of 2.15 hours, the companion fills its Roche interface again and the cataclysmic variability becomes detectable again as a result of the mass transfer. Due to the radiation of gravitational waves, the period of rotation decreases further to a minimum of 83 minutes. Here the hydrogen burn in the companion goes out , which transforms into a brown dwarf . This cannot react quickly enough to the loss of mass by reducing its radius and as a result, the diameter of the companion and the orbital distance in the binary star system increase. This increases the orbital period of the cataclysmic binary star system again. These double stars are known as bounce-back systems because they bounced off the lower limit of the period . In contrast to the theoretical assumptions, no high frequency of cataclysmic systems could be observed just above the lower period limit. This development model is supported by the population membership of the cataclysmic binary stars near the sun. These mainly belong to the thin disk, while the systems with periods of rotation below the period gap belong to more than 60 percent of the thick disk. A mean age for cataclysmic variables below the period gap of 13 billion years could be derived from the kinematic data, which is in agreement with the simulated population models.

In contrast to the standard model described above, there are also cataclysmic binary star systems below 83 minutes in addition to the AM-CVn systems. An example is SDSS J1507 + 52 with a rotation time of 67 minutes. This deviation can be a consequence of the population membership, since even metal-poor subdwarfs show a smaller radius compared to the main sequence stars of Population I.

Secular development

Image of an old Nova cover around the dwarf nova Z Camelopardalis

Novae and dwarf novae as well as AM Herculis stars and novae do not differ in any physical parameters of the binary star system in which they occur. So the idea arose early on that these types of cataclysmic variables are part of an evolutionary sequence. This hypothesis is confirmed after the discovery of dwarf nova-like outbreaks at the Nova Her 1960 (= V446 Her) and an old Nova shell around the dwarf nova Z Cam.

During a nova eruption, energy is transferred to the companion, which then expands and transfers more matter to the white dwarf . Therefore, the Postnova spectrum mostly resembles that of a nova-like variable. After a while the companion relaxes and the flow of matter is reduced or temporarily completely interrupted. Now, with a low mass transfer to the accretion disc , rare dwarf nova eruptions of the U Gem type occur. The flow of matter continues to increase and the dwarf nova is assigned to the Z-Cam stars, as the transfer rate is already so high that the accretion disc in the eruption stage lasts for a long time remains. With a further increase in the transfer rate, the binary star system almost always remains in the status of the eruption and is classified as a VY-Scl star. After a while, so much matter has accumulated on the surface of the white dwarf that a thermonuclear ignition occurs and a new nova eruption begins. According to theoretical considerations, cataclysmic variables are expected to go through a few thousand nova cycles.

Doppler tomography

Doppler tomography is a method for resolving the spatial structure of a binary star system with the help of the Doppler effect . The radial velocities are recorded from spectra and the structure in the accretion disk is reconstructed using tomographic methods . Usually it is assumed that the speed in the disk corresponds to a circular Kepler path . This method is used especially for cataclysmic variables, because due to the masses of the white dwarfs and the small distance between the components, a complete reconstruction can already be carried out with the data from one night and the amplitude of the Doppler shift enables a high spatial resolution. Doppler tomography results for cataclysmic variables include:

  • The formation of spiral structures in the accretion disk during the eruption and, for some stars, also during periods of rest.
  • In some cataclysmic variables, the hot spot where the companion's material meets the accretion disc is closer to the white dwarf than to the edge of the disc.
  • The unexpected lack of an accretion disk around some nova-like ones.
  • The increase in the brightness of the hot spot on some SU-UMa stars before a super-eruption.

Cataclysmic Variables as Precursors of Supernovae?

In cataclysmic binary stars , matter is transferred to a white dwarf . If the mass of the white dwarf exceeds the Chandrasekhar limit of approximately 1.4 solar masses, the degenerate matter can no longer withstand the pressure and the white dwarf collapses. This is a potential formation mechanism for a Type Ia supernova . However, all cataclysmic variables are likely to go through multiple nova outbreaks, and in the mists around novae components from the surface of the white dwarf have been detected that were detonated during a nova outbreak. Therefore, the white dwarfs tend to lose mass in cataclysmic variables and do not cross the Chandrasekhar limit. On the other hand, an investigation of the masses of white dwarfs on the basis of the light curves of cataclysmic variables did not result in the expected decrease in mass with the orbital period of the binary star system.

A second development channel to a supernova of type Ia could super-soft X-ray sources to be. These are cataclysmic or symbiotic binary star systems in which stable hydrogen burning occurs on the white dwarf . The mass of the white dwarf grows continuously due to the matter accreted by the companion and this should lead to a type Ia supernova when crossing the Chandrasekhar limit . However, the observed number of super soft X-ray sources is too low to represent a significant proportion of type Ia supernovae.

Against the formation of a type Ia supernova, it is often argued that it has not yet been possible to identify the former companion from the cataclysmic binary star system in or near a supernova remnant . Due to the release of gas masses in the supernova explosion, its orbit becomes unstable and the previous companion will move away from the scene at high speed. However, in addition to the mass, angular momentum is also transferred to the white dwarf, and with a rapid rotation centrifugal forces stabilize against the collapse even after the Chandrasekhar mass limit has been exceeded. Only after 100,000 to 1,000,000 years is the emission of gravitational radiation reduced so much angular momentum that the conditions for a supernova explosion exist. During this period, the companion cooled down greatly and, if the companion was a subgiant, even lost its extensive shell. In many cases, the companion would thus be too faint to detect it in a supernova remnant with today's instruments.

Another alternative follows from the magnetic activity of red dwarfs, as shown in UV-Ceti stars and BY-Draconis stars . In a close binary star system consisting of a red dwarf and a white dwarf, the magnetic fields of both stars should lead to a redistribution of the angular momentum, resulting in a synchronization of the period of rotation with the orbital period. As a result, the poles of the magnetic fields of both stars point towards each other and closed magnetic field lines arise over both stars of the binary system. The mass flow from the red to the white dwarf should flow along these magnetic field lines and can take on significantly higher values ​​than with a spherical accretion. The luminosity generated at the poles of the white dwarf through accretion and hydrogen burning heats the red dwarf and keeps the matter transfer going. This self-sustaining mechanism could lead to the limit mass of the white dwarf being exceeded without novae first limiting the mass of the degenerate star. The required magnetic field densities would be considerably lower than the values ​​found for polars .

According to numerical calculations, a merger of two white dwarfs can also lead to a supernova outbreak. This is a scenario for the short-period AM-Canum-Venaticorum stars , in which two (half) - degenerate stars exchange matter. Archival images of the X-ray satellite Chandra before the outbreak of the 2007on supernova in NGC 1404 revealed a weak X-ray source with a spectrum similar to that of an AM-CVn star.

X-rays from cataclysmic variables

When matter falls on a white dwarf (accretion), the matter is abruptly slowed down and reaches temperatures of up to several million Kelvin . A large part of the energy released is re-emitted in the far ultraviolet and in the X-ray range . Due to this property, X-ray surveys are used to determine the spatial density of the cataclysmic variables, since optical surveys result in selection effects that are difficult to correct due to the different burst amplitudes and relative duration of the burst phases. The spatial density from X-ray surveys has been determined to be 0.5–10 * 10 −6 per parsec .

Magnetic cataclysmic variables represent approximately 25% of the population of all cataclysmic variables. They are further subdivided into DQ Herculis stars with magnetic flux densities of up to 20,000,000  Gauss and polar with up to 230,000,000 Gauss. Polars represent about 2/3 the number of magnetic cataclysmic variables. In the DQ Herculis stars, the magnetic field penetrates the accretion disk and forces the matter to flow out of the disk towards the magnetic poles of the white dwarf. The X-rays are mainly generated at a shock front just above the surface of the white dwarf and radiate with an energy of a few 10 33  ergs per second with radiation mainly in the range of hard X-rays of 10 to 96  keV . A weaker softer component in the range of 30 to 100 eV could be thermal radiation from the surface of the white dwarf.

Polars with their orbital times mostly below the period gap have a dominant soft component of the X-ray radiation in the range of 10 to 30 eV due to thermal radiation from the white dwarf. In addition, bremsstrahlung occurs in the area of ​​the accretion current and the radiated energy in the area of ​​X-ray radiation fluctuates between 10 30  erg / s in the low state and some 10 32  erg / s in the high state. About 50% of all polars are in a state of low accretion at any point in time .

In non-magnetic or weakly magnetic white dwarfs in cataclysmic variables, hard X-rays occur in the state of low accretion rates as in dwarf novae in the resting state, since the transition layer between the accretion disk and the white dwarf is optically thin. At high accretion rates, correspondingly soft X-rays are detected, as the X-rays are absorbed and re-emitted several times in the now optically thick transition layer. During nova eruptions , a thermonuclear runaway, an explosive hydrogen burn, occurs near the surface of the white dwarf . As a result, matter is accelerated up to several thousand kilometers per second, and a pseudophotosphere leads to an increase in brightness from the ultraviolet to the infrared. When the shell is expanded to the point that it becomes transparent to X-rays, an extremely soft X-ray component can be detected. These sources are counted among the super soft X-ray sources . The very soft X-rays are interpreted as a result of the ongoing hydrogen burning on the surface of the white dwarf.

Exoplanets around cataclysmic binary stars

Exoplanets have been reported around the cataclysmic variables NN Serpentis, UZ Fornacis, DP Leonis, QS Virginis and HU Aqr, whose existence was concluded by means of the time-of-flight effect . The light transit time effect describes a change in the occurrence of a measurable point in time, e.g. B. the minimum of a cover light change due to the shift of the gravitational center of gravity by another or more bodies in the binary star system. Through continuous observations and numerical calculations, the existence and stability of these four planets can be seen as certain. The planets observed belong to the class of gas giants with masses of approx. 1–8 Jupiter's masses and orbital periods of approx. 3 to 8 years.

Asteroseismology

With the help of asteroseismology , vibrations and the associated propagation of sound waves in the atmosphere of stars are analyzed. This allows conclusions to be drawn about the structure of the stars, and asteroseismology enables the calculation of the course of temperature, density, rotation speed and chemical composition in stars below the photosphere . Pulsating white dwarfs are known as ZZ-Ceti stars and are extensively studied. In the case of cataclysmic variables with low mass transfer rates, the accretion disk does not dominate the electromagnetic radiation and light from the white dwarf can be detected. The white dwarf heats up during a mass transfer event during a dwarf nova outbreak and cools down in the meantime. This development is reflected in the changes in the frequencies and amplitudes of the pulsations of the accreting white dwarf. The mass of the white dwarfs as well as the temperature drop over the years after the eruption can be derived from the vibrations.

See also

supporting documents

  1. ^ Walter HG Lewin, Jan van Paradijs, Edward PJ van den Heuvel: X-ray Binaries . Cambridge University Press, 1997, ISBN 0-521-59934-2 .
  2. ^ A b Brian Warner: Cataclysmic Variable Stars . Cambridge University Press, 1995, ISBN 0-521-54209-X .
  3. ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne . JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
  4. ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  5. ^ Meyer F., Meyer-Hofmeister E .: A model for the standstill of the Z Camelopardalis variables . In: Astronomy & Astrophysics . 121, 1983, p. 29. bibcode : 1983A & A ... 121 ... 29M .
  6. ↑ P. Rodrıguez-Gil, L. Schmidtobreick, KS Long, T. Shahbaz, BT Gansicke and MAP Torres: The low states of CVs at the upper edge of the period gap . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1112.0902v1 .
  7. ^ SJ Kenyon: The symbiotic stars. Cambridge Univ. Press, Cambridge 2009, ISBN 978-0-521-09331-6 .
  8. ↑ N. Blind, HMJ Boffin, J.-P. Berger, J.-B. Le Bouquin, A. Mérand, B. Lazareff, G. Zins: An incisive look at the symbiotic star SS Leporis - Milli-arcsecond imaging with PIONIER / VLTI . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1112.1514v1 .
  9. ↑ MR Schreiber and BT Gänsicke: The age, life expectancy, and space density of Post Common Envelope Binaries . In: Astronomy & Astrophysics . 406, 2003, pp. 305-321. doi : 10.1051 / 0004-6361: 20030801 .
  10. ^ Iko Iben, Jr and Mario Livio: Common Envelopes in Binary Star Evolution . In: Publications of the Astronomical Society of the Pacific . 105, 1993, pp. 1373-1406.
  11. ^ Edward M. Sion et al .: Hubble Space Telescope FUV Spectra of the Post-Common-Envelope Hyades Binary V471 Tauri . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1203.4787v1 .
  12. ↑ SG Parsons, BT Gänsicke, TR Marsh, P. Bergeron, CM Copperwheat, VS Dhillon, J. Bento, SP Littlefair, MR Schreiber: An accurate mass and radius measurement for an ultracool white dwarf . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.5393v1 .
  13. ^ 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 .
  14. ↑ Krzysztof Z. Kamiński u. a .: MOST Photometry and DDO Spectroscopy of the Eclipsing (White Dwarf + Red Dwarf) Binary V471 Tau . In: The Astronomical Journal . tape 134 , no. 1 , 2007, p. 1206-1215 , doi : 10.1086 / 520923 .
  15. ↑ C. Knigge: Cataclysmic Variables: Eight Breakthroughs in Eight Years . In: Mjaa. S0c. As. 2011, arxiv : 1101.2901 .
  16. ↑ U. Kolb, AR King, H. Ritter: The CV period gap: still there . In: Astrophysics. Solar and Stellar Astrophysics . 1998, arxiv : astro-ph / 9806007 .
  17. ↑ S. Zharikov, G. Tovmassian, A. Aviles, Michel R., D. and Gonzalez-Buitrago Ma. T. Garcıa-Dıaz: The accretion disk in the post period-minimum cataclysmic variable SDSS J080434.20 + 510349.2 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.1357 .
  18. ↑ T. Ak, S. Bilir, T. Guver, H. Cakmak, S. Ak: Population types of cataclysmic variables in the solar neighborhood . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.5806 .
  19. ↑ H. Uthas et al. a .: The cataclysmic variable SDSS J1507 + 52: An eclipsing period bouncer in the Galactic halo . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1104.1180 .
  20. ^ RK Honeycutt, JW Robertson, S. Kafka: The Dwarf Nova Outbursts of Nova Her 1960 (= V446 Her) . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1102.3761 .
  21. ↑ MM Shara et al. a .: An ancient nova shell around the dwarf nova Z Camelopardalis . In: Nature . tape 446 , 2007, pp. 159–162 , doi : 10.1038 / nature05576 .
  22. ↑ SN Shore, M. Livio, EPJ van den Heuvel: Interacting Binaries: Saas-Fee Advanced Course 22nd Lecture Notes 1992. Swiss Society for Astrophysics and Astronomy (Saas-Fee Advanced Courses) . Springer Verlag, Berlin 1993, ISBN 3-540-57014-4 .
  23. ↑ J. Echevarrıa: Doppler Tomography in Cataclysmic Variables: an historical perspective . In: Mem. SA It. In press . 2012, arxiv : 1201.3075v1 .
  24. ^ Michael F. Bode, A. Evans: Classical novae. Cambridge Univ. Press, Cambridge 2008, ISBN 978-0-521-84330-0 .
  25. ↑ CDJ Savor, SP Littlefair, VS Dhillon, TR Marsh, BT Gaensicke, CM Copperwheat, P. Kerry, RDG Hickman, SG Parsons: Cataclysmic Variables below the Period Gap: Mass Determinations of 14 Eclipsing Systems . In: Monthly Notices of the Royal Astronomical Society . 2011, arxiv : 1103.2713 .
  26. ^ Joseph Patterson, John R. Thorstensen, Robert Fried, David R. Skillman, Lewis M. Cook and Lasse Jensen: Superhumps in Cataclysmic Binaries. XX. V751 Cygni . In: Publications of the Astronomical Society of the Pacific . tape 113 , 2001, p. 72-81 , doi : 10.1086 / 317973 .
  27. ↑ R. Di Stefano and Mukremin Kilic: THE ABSENCE OF EX-COMPANIONS IN TYPE IA SUPERNOVA Remnants . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.3168v1 .
  28. ↑ J. Craig Wheeler: White Dwarf / M Dwarf Binaries as Single Degenerate Progenitors of Type Ia Supernovae . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1209.1021 .
  29. ↑ Rasmus Voss & Gijs Nelemans: Discovery of the progenitor of the type Ia supernova 2007on . In: Nature . tape 451 , 2008, p. 802-804 , doi : 10.1038 / nature06602 .
  30. ↑ Magaretha L. Pretorius, Christian Knigge: The space density and X-ray luminosity function of non-magnetic cataclysmic variables . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1109.3162v1 .
  31. ↑ S¸. Balman: The X-ray Properties of Cataclysmic Variables . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1112.3893v1 .
  32. ^ Joseph Patterson: The DQ Herculis Stars . In: Publications of the Astronomical Society of the Pacific . tape 106 , 1994, pp. 209-238 .
  33. ^ Gavin Ramsay, Mark Cropper: The energy balance of polars revisited . In: Monthly Notice of the Royal Astronomical Society . tape 347 , 2004, pp. 497-507 , doi : 10.1111 / j.1365-2966.2004.07220.x .
  34. ^ Walter Lewin, Michael van der Klies: Compact Stellar X-ray Sources (Cambridge Astrophysics) . Cambridge University Press, Cambridge 2010, ISBN 978-0-521-15806-0 .
  35. ↑ K. Beuermann et al. a .: The Planets around the Post-Common Envelope Binary NN Serpentis . In: Astronomy & Astrophysics . 2011, arxiv : 1102.0508 .
  36. ↑ K. Beuermann et al. a .: The giant planet orbiting the cataclysmic binary DP Leonis . In: Astronomy & Astrophysics . 2010, arxiv : 1011.3905 .
  37. ↑ K. Beuermann et al. a .: Possible detection of two giant extrasolar planets orbiting the eclipsing polar UZ Fornacis . In: Astronomy & Astrophysics . 2011, arxiv : 1106.1404v1 .
  38. ↑ SB Qian et al. a .: A giant planet in orbit around a magnetic-braking hibernating in a Cataclysmic Variable . In: Monthly Notice of the Royal Astronomical Society . tape 401 , no. 1 , 2010, p. L34-L38 , doi : 10.1111 / j.1745-3933.2009.00780.x .
  39. ↑ R. Schwarz et al. a .: Hunting high and low: XMM monitoring of the eclipsing polar HU Aquarii . In: Astronomy & Astrophysics . 2009, arxiv : 0901.4902 .
  40. ↑ H. Uthas et al. a .: Two new accreting, pulsating white dwarfs: SDSS J1457 + 51 and BW Sculptoris . In: Astronomy & Astrophysics . 2011, arxiv : 1111.3956v1 .
  41. ↑ P. Szkody et al. a .: HST and Optical Data Reveal White Dwarf Cooling, Spin and Periodicities in GW Librae 3-4 Years after Outburst . In: Astronomy & Astrophysics . 2012, arxiv : 1205.2598v1 .