Dwarf nova

Dwarf novae belong to the class of cataclysmic binary star systems. They are characterized by multiple eruptions, in which the apparent brightness of the star in the short term by about 2 to 8 likes change. The outbreak mechanism is in an accretion disk around a white dwarf.

Construction of a dwarf nova

A dwarf nova is a binary star system consisting of a white dwarf around on a narrow path a companion, usually a red dwarf orbits. The red dwarf loses mass because it has exceeded its Roche limit volume. The mass flow through the inner Lagrangian point in the direction of the white dwarf. Due to conservation of angular momentum forms around the white dwarf accretion disk one that dominates the radiation of the dwarf nova in the optical spectral range. The matter orbiting the white dwarf and slowly loses due to the viscosity in the disc its kinetic energy. This matter falls after some time on the surface of the white dwarf.

Outbreak mechanism

The outbursts of dwarf novae are caused by an increase in brightness of the accretion disk. The viscosity of matter in the disk can have two values ​​. A high value at which the friction increases, and as a result releases the disc both radiation and more and more material in the white dwarf falls. During the resting phase, at which the viscosity becomes low value, more matter is stored in the accretion disk reaches the white dwarf. The cause of the bistable state of the accretion disk, the magneto rotational instability is assumed.

In eclipsing dwarf novae the development of the accretion disk can be observed. During an outbreak of the radius of the disc up to 30% increases. This is a result of the higher viscosity of the plasma in the accretion which leads to a temperature increase and expansion. Thus, the minimum becomes wider, which is produced by the companion for the coverage of the accretion disk. In the resting phase, the width of the minimum decreases continuously until a new outbreak begins. The bright spot, from the companion is at the point of impingement of the flow of matter in the accretion disk becomes brighter during outbreaks. Probably this is a feedback, after which the intense radiant accretion disk the front of the companion heated, the slightly expanded and then emits more matter.

The interval length between the outbreaks is the dwarf novae between several days to several years. The duration of an outbreak lies approximately between two and twenty days and is correlated with the interval length between the outbreaks. The dwarf novae differ from the classical novae by the eruption mechanism. In classical novae thermonuclear reaction at the surface of the white dwarf leads to an increase in brightness. However, the same cataclysmic variables both novae as well Zwergnovaeausbrüche show such as GK Persei.

Whether the mass of the white dwarfs in dwarf novae increases due to the accretion, is controversial, as is ejected at Novaeausbrüchen matter again. If the mass increases, the white dwarfs could exceed the chandrasekharsche mass limit and explode as supernovae of type Ia.

X-rays from dwarf novae

Of all the nearby dwarf novae X-rays could be detected. The source of high-energy radiation appears to be the boundary layer between the accretion disk and the white dwarf. The radiation arises from the fact that in the boundary layer, the matter must be slowed down in the accretion disk of the Keplergeschindigkeit to the much slower speed of rotation of the white dwarf. The radiation is weak in the resting phase and increases during the outbursts by a factor of 100. In this case, the optical by a few hours, the increase in X-rays lagging behind. According to the model of Akkretionsscheibeninstabilität increases somewhere in the disk viscosity and the change propagates across the disk. If the increased viscosity, and thus the increased rate of material reaches the boundary layer, the X-ray radiation increases with a time delay of the optical radiation. A small portion of the X-ray radiation can be caused by heat radiation by the accretion -heated white dwarf.

Regardless of the path inclination under which the dwarf nova from Earth is considered, many X-ray spectra show evidence of circumstellar absorption. Parallel to this observation in the field of X-ray radiation can occur in the optical P- Cygni profiles. This is interpreted as evidence of a disk wind, analogous to a stellar wind. An outflow of matter from an accretion disk has been suggested also for other objects such as X-ray binaries, T Tauri stars, etc..

At a high accretion rate may lead to a permanent hydrogen burning on the surface of the white dwarf. As only a thin atmosphere above the area with the thermonuclear reactions according to the Bethe- Weizsäcker cycle is occurs from extremely soft X-rays. Because of this low-energy X-ray radiation, these systems are also known as Super Soft X -ray source. It involves classical novae in outburst over a period of at least decades.

Subgroups

• U Geminorum - Stars: This subgroup of dwarf nova shows pronounced periods of rest in the smallest light that are almost regularly interrupted by outbursts. The increase in the maximum is faster than the descent back to rest brightness.
• Z Camelopardalis star: The shutdowns in the smallest light are very short. Time periods with changes in brightness are temporarily interrupted by intervals of nearly constant light. The shutdown begins in the brightness down from the maximum and ends at a minimum.
• SU Ursae Majoris stars: In this subgroup occur in addition to normal outbursts and Super maxima. These are about 0.7 may brighter and last 3-5 times longer. In addition, so-called Superhumps occur. These are the small peaks superimposed changes in brightness with a period that is a few percent longer than the orbital period of the binary system.
• TOAD ( Tremendous Outburst Amplitude Dwarf novae ): The difference between the SU UMa stars is the lack of normal outbursts. There are also observed in the WZ - Sagittae - stars called dwarf novae exclusively Super outbreaks.
• UX UMa stars: The Nova -Style are dwarf nova outburst and the permanent show in the spectrum of absorption lines.
• RW Tri - Star: If this nova -like binary stars are dwarf novae in outburst permanent and they show in the spectrum of emission lines.
• VY Scl stars: dwarf novae These are similar to the UX UMa stars. They sometimes show a minimum only to return to the peak after a short time. They are also called anti -Nova ..

The classification of dwarf novae is not always clear-cut. So in 1985 the prototype of the normal dwarf novae, U Geminorum, a super maximum, with an outburst duration of 39 instead of 12 days and the occurrence of Superhumps showed.

The Super outbreaks of SU Ursae Maiori -Stars and TOAD require a different mechanism than the normal maxima. In this case, all Super eruptions develop from a failed normal eruption and these systems have an orbital period of less than 2 hours. During a super outbreak is up to 80% of data stored in the accretion disk mass on the white dwarf transfers compared to a few percent in the U Gem stars. In the literature, three models are discussed:

• A normal outburst leads to heating of the companion, which then lose more mass to the accretion disk and this starts the super outbreak
• The accretion disk grows during a normal eruption to the extent that it comes at the outer edge of the disc to increased friction under the influence of a 3:1 resonance with the companion. This leads to an increased flow of matter toward the white dwarf and thus to a super eruption.
• After the third model, a super- eruption is the result of normal variation of the eruptions. The prototype SS Cyg and U Gem show a change between narrow and broad maxima. The difference between the two types is the shape of the heating front which runs from the inside at small flares outwardly and the long bursts from the outside inwards. Because the wide outbreaks are rare in SU UMa stars they show up as a super eruptions.

Ununterbrochende observations with the Kepler satellite to the SU UMa stars V1504 Cyg and V344 Lyr support model 2, which is also referred to as thermal - Tidal Instability Model.

Zwergnovaoszillation

Zwergnovaoszillation (English dwarf nova oscillation ) describe sinusoidal brightness variations of low amplitude to 0.02 % with cycle durations of 5 to 40 seconds. These oscillations have been detected in the outbreak in some dwarf novae and nova -like. Each star has its own characteristic frequency, which is, however, subjected to large amplitude variations as well as during an outbreak and between different bursts. The Zwergnovaoszillationen have been detected in the optical, ultraviolet, and in the soft X-ray radiation. Due to the high energy of X-rays, the origin of Zwergnovaoszillationen is suspected in the vicinity of the white dwarf and could be caused by a modulation of the accretion by a weak magnetic field of the white dwarf.

A similar phenomenon represent the quasi-periodic oscillations observed in parallel with the Zwergnovaoszillationen some cataclysmic variables. The difference between two intensity variations is the length of the period which is at the quasi-periodic oscillations of the order of some 100 seconds, and the low period of stability in the quasi-periodic oscillations. Possibly, the quasi-periodic oscillations correspond to the dwarf novae in which X-ray binaries.

Connection with Nova outbursts

Classical nova outbursts find as dwarf nova outbursts in cataclysmic variables held in which a white dwarf accretes matter from a companion star. While there is a flash of the accretion disk in dwarf novae, supernovae explosion sets in hydrogen burning on the surface of a white dwarf. A part of the accreted material is ejected and forms a supernova remnant. Studies of historical light curves of novae before and after their eruptions have never dwarf nova outbursts shown, although both eruptions should be held to the same binary stars. Instead, they always show a nova -like light change.

This apparent contradiction is explained by the hibernation scenario. During the thousands of years before a Nova outburst, the rate of mass transfer to the white dwarf is so high that the accretion disk is permanently positioned in their high status and balances as a nova -like variables of a dwarf nova in constant eruption. Ignite the hydrogen accumulated on the white dwarf, so it heats the companion star and the mass transfer rate remains high enough to let the binary star system appear as a nova -like variables even after the eruption. It was not until several centuries after the Nova outburst the mass transfer rate drops so much that the accretion disk can at least temporarily fall back to a resting state. This corresponds to the Z- Cam - subgroup of dwarf novae, and this star class should be the best candidate for a search for supernova remnants to dwarf novae. In fact, weak supernova remnants are so far only two Z Cam star, Z Cam and AT Cnc, has been found, let their expansion velocities close to an outbreak of more than 1000 years ago.

Related outbreaks

The model of Akkretionsscheibeninstabilität is not only used for the description of the eruptions of dwarf novae. In the X-ray novae or soft X -ray transits through an accretion disk of matter falling onto a compact star, which is probably a black hole. Since the compact companion has a smaller radius and a larger gravitational potential has as a white dwarf, the matter can orbit on closer orbits around the black hole and thereby reach higher temperatures. Therefore, it is observed in the soft X -ray transit the predominant part of the radiation in the field of X-ray radiation. The X-ray novae, dwarf novae obtained as the matter from a companion in a binary star system, which has exceeded its Roche limit.

The AM Canum - Venaticorum star correspond in many properties the dwarf novae. Only the orbital period of the erupting binary systems is shorter, as the companion of the white dwarf is a partially degenerate helium star, and is between 20 and 40 minutes. The dwarf novae- like outbreaks occur in an accretion disk around the white dwarf, which consists mostly of helium. In addition Superhumps have also been observed in short-period AM CVn system with circulation periods of between 5 and 20 minutes.

In the FU Orionis stars, the accretion disk is, however, fed by a protostellar cloud. Even with these young single stars can cause an overload of the disc that lights up at an increased mass transfer. Since the protostellar accretion disks have a diameter larger than the disks around a white dwarf in a cataclysmic binary star system to take the outbreaks even up to several decades.