AM CVn star

AM Canum - Venaticorum star or AM CVn - stars are compact close binary systems consisting of an accreting white dwarf and another degenerate companion. The orbital period of the components is between 5 and 65 minutes. The difference to the cataclysmic variable stars is the lack of hydrogen in the atmosphere of the companion and the accreted matter. This class of variable stars named after the prototype AM Canum Venaticorum.

Construction

The AM Canum - Venaticorum star consist of a white dwarf in a binary star system with a companion, who also is either a white dwarf, a helium star or an evolved main sequence star. The companion fills its Roche volume and transfers matter to the white dwarf. The matter flows along a stream on to the white dwarf and forms the basis of the conservation of angular momentum accretion disk around a compact star. At the point at which the matter stream strikes the accretion disk, the matter is decelerated; it forms a luminous hot spot. This leads to a modulation of the light curve of the AM CVn system with the period of rotation time. Another indication of the accretion of matter is the Flickering, a small irregular brightness variation in the seconds range. The accreted matter is lost in the disk around the white dwarf angular momentum, and finally falls on this. Upon impact, the resulting thermal radiation is emitted mainly in the field of X-ray radiation. In ES Ceti, the matter could fall directly onto the white dwarf due to the small distance between the two degenerate stars, without passing through an accretion disk.

Classification

The AM Canum - Venaticorum star are classified mainly according to the orbit period:

• In the long-period systems with an orbital period of more than 40 minutes, only a small mass exchange takes place. The accretion disks are optically thin and in the spectrum of the emission lines of helium dominate. The variability is often not pronounced, and this AM CVn stars are difficult to discover.
• The short-period systems with an orbital period of less than 20 minutes are always in a state of high mass transfer with an optically thick accretion disk. Their spectrum shows prominent broad absorption lines of helium. This AM CVn stars show always or partially Superhumps. This is a sinusoidal variation of the light curve with a period that is a few percent longer than the orbital period of the binary system and is probably caused by a rotating elliptical accretion disk. These systems comply with the nova -like cataclysmic variables, dwarf novae, which are in a state of permanent eruption.
• The erupting systems with an orbital period of 20 to 40 minutes. They show outbursts with amplitudes of 3-5 like that match those of dwarf novae in the cataclysmic variable stars. Even with them Superhumps may occur. The eruptions take to a period of a few weeks and are repeated irregularly over the period of months. Some of these group AM CVN -star show in front of the outbreaks a brightness burglary (English dip) of unknown cause.

The dwarf nova -like outbursts can be explained according to the model for hydrogen-rich cataclysmic variables with a disk instability model. The crucial difference is the strong influence of a variable mass transfer rate, which dominates the development of super eruptions, dips and shutdowns. Probably the mass transfer rate varies due to the different heating of the mass donor in previous outbreaks, which in turn is attributed to a precessing accretion disk bent. The similarity to the hydrogen-rich cataclysmic variables can also be seen in the light curve in X-rays. The X-ray radiation in the rest light at low accretion rates at the boundary layer between the white dwarf and the accretion disk. The temperature of the boundary layer, in which the plasma is decelerated by the Kepler velocity in the accretion disk to the rotation speed of the white dwarf, reaches values ​​of a few kilo - electron volts and only the stellar wind from the white dwarf absorbs some of the X-rays. The outbreak at higher accretion while the temperature in the boundary layer continues to rise, but the boundary layer also absorbs X-rays almost entirely due to an increased opacity. This behavior corresponds to the hydrogen-rich dwarf novae.

Thermonuclear bursts

The normal outbreaks of AM CVn - stars are similar to those of dwarf novae. Here, the accretion disk oscillates between two stable states. In the active state, increasing the viscosity of the material, and due to the increased friction, the disc heats up. If the accretion disk has partially emptied, ending the outbreak, and it goes to the low state. Here, less matter is transferred to the white dwarf, to flow as the accretion disk, which leads after some time to a renewed outbreak.

In addition, there could be the equivalent of classical novae in AM Canum - Venaticorum stars. While it comes to an explosive hydrogen burning in novae, the result for the AM CVn systems unstable helium burning on the surface of the white dwarf. These kinds of eruptions are expected in the short-period AM CVn systems. At low mass transfer rate from the companion to the white dwarf, it could even lead to an unstable helium flash with a participating mass of up to 0.1 solar masses. Due to the high pressure of helium near the surface of the white dwarf thermonuclear reactions can produce heavy elements up to 56Ni. These radioactive isotopes are also the source of energy for the afterglow of supernovae, and a corresponding helium flash would as a faint supernova of type Ia perceived that reaches only one-tenth of the maximum brightness in its class. In archival footage of the X-ray satellite Chandra before the outbreak of the supernova 2007on in NGC 1404, a faint X-ray source was found whose spectrum is similar to an AM CVn star.

However, recent studies raise doubts about whether there is a supernova of type Ia in the merging of two degenerate white dwarfs. First, the total mass of a merging binary system of two white dwarfs scattered from 1.4 to 2 solar masses and can hardly explain the uniformity in the energy release of these stellar explosions. Secondly, simulations show that it directly to the formation of a neutron star leads in most cases to either an accretion -induced collapse than a thermonuclear explosion or to convert into a massive O -Ne -Mg white dwarf, which by electron also be a neutron star turns. Therefore Supernova Type Ia probably very rarely the product of a merger of two white dwarfs from an AM Canum - Venaticorum star. With a very thin helium-rich layer with a mass less than 0.1 solar masses, it may at a firing of the helium burning come with massive white dwarf in an AM CVn system for the propagation of a shock front, which runs at the speed of sound through the zone with convective energy transport. The result could instead of a nova outburst, the luminosity does not exceed the Eddington limit, be an unstable ignition of carbon burning in the core of the white dwarf. This type of supernova Type Ia should be detected by a specific chemical composition of the expanding shell with little 52Fe and 56Ni as well as an increased proportion of 40Ca, which is accompanied by a deviation from spherical symmetry.

It is only possible to take a deflagration of detonation when the interface between the core and a CO helium atmosphere unstable helium flash occurs. This subspecies of thermonuclear supernovae is called a type. Ia, since the luminosity reaches only one-tenth the value of a normal Ia supernova. The faint supernova SN 2010X is counted among the. Ia supernovae.

Development

There are several development channels for the formation of AM CVn systems known to get two degenerate stars in a tight orbit:

• In the so-called white - dwarf - channel a pair of white dwarfs arises as a consequence of a common - envelope phase. The first resulting white dwarf is immersed in the atmosphere of his companion developed, and the friction leads both to a reduction of the railway axis and a dropping of the atmosphere of the companion. The result is a separate binary star system consisting of two white dwarfs, which are due to the emission of gravitational waves in contact and thus develop into an AM Canum - Venaticorum star.
• In the helium star channel accreted a white dwarf from a first non-degenerate helium star. Here is transferred over time enough matter from the companion to bring the helium burning to extinction. in consequence, the binary star system developed to shorter circulation times to a minimum of 10 minutes due to the emission of gravitational waves. At this point, the internal structure of the companion changes in such a way that it expands, and as a result also increases the path axis of the binary system again. The AM CVn star ends its active phase and remains a separate pair of white dwarfs. The helium star channel is also called a double common- envelope channel because develop in this development model, both stars in a red giant, dragging their companion at times in their stretched atmosphere orbit.
• In the developed - cataclysmic - variable channel is normal cataclysmic variables, in which the mass transfer occurs only when the companion of the white dwarf evolved from the main sequence away and has the hydrogen supply already used in its core. The hydrogen-rich envelope of the companion is lost in the course of development of the cataclysmic variable by mass transfer. What remains is a partially degenerate helium star as a companion of the accreting white dwarf, the atmosphere still contains a few percent of hydrogen in contrast to the other two development scenarios.

In all scenarios, the development of an AM CVn star with the emission of gravitational waves is controlled. The gravitational waves transport from the angular momentum of the binary system, and thus the double star is always half separate contact system. The gravitational radiation is so strong that they can be detected using LISA due to the small distance between the two stars. With the merger of two white dwarfs can arise depending on the nature of the mass transfer, which can be stable or unstable, hot sub- dwarfs, massive white dwarfs, extreme helium stars, R Coronae Borealis stars and supernovae of type Ia or. Ia.

In the area of ​​circulation periods of AM CVN -star, there are also separate double star, consisting of two white dwarfs. These are called double stars from white dwarfs with extremely low mass, the mass of the star is at values ​​below 0.2 solar masses. After the English word these binary star systems are called ELM (extremely low mass ) helium white dwarfs respectively. They only come in contact and the mass transfer starts with circulation periods of values ​​below 10 minutes. The separate ELM binary stars are more suitable for verification of general relativity and the derived gravitational waves as the AM CVn star, since the interaction between the components makes it difficult to determine their physical properties. J0651 2844 is the closest known eclipsing binary star system consisting of white dwarfs without mass exchange. The orbital period is only 765 seconds and increases by 0.31 ms per year starting in accordance with the general theory of relativity.