Debris disk

Debris disks (English debris disk) are dust disks around older stars. They were first using the Infrared Astronomical Satellite discovered due to a strong infrared excess. Because of the dust disk the stars emit more radiation in the mid and far infrared as a black body with a comparable temperature. The additional radiation is the result of thermal radiation by micrometer-sized dust particles which are heated by the electromagnetic radiation of the central star. In contrast to protoplanetary disks are debris disks not relics from the time of star formation, since the radiation pressure and stellar wind has removed the original dust from the star system. The debris discs are also known as dust disks of the second generation because the dust has likely formed by collisions between planetesimals or by the dissolution of comets million years after the completion of star formation. The asteroid belt and the Kuiper belt in the solar system can also be referred to as debris disks.

Properties of classical debris discs

The diameter of rubble situated discs in a wide range of a few tenths up to a thousand units astronomical (AE), with most values ​​in the range of 30 to 120 AU. The slices are very thin, its thickness usually exceeds values ​​of 0.1 AE not. The mass of dust in the debris disks reaches values ​​of a few hundredths to a few hundred Earth masses and decreases with the age of the stars from.

Debris disks have been found around main sequence stars with spectral types A to M, where by early star debris disks can be detected with a higher probability. With the current detection technique by about 20 percent of sun-like stars can be detected cold dust disks. There is no relationship between the metallicity of the star and the detection probability of a debris disk. The age of the star is several million to several billion years ago. The highest known age, the debris disk around the red dwarf GJ 581, whose age is estimated between two and eight billion years. In older red dwarfs the radiation and the stellar wind is not sufficient to resolve the dust disks.

By means of infrared spectroscopy, the chemical composition of the washer debris could be analyzed. The dust particles containing crystallized minerals from forsterite, enstatite, pyroxene and olivine of. They are therefore consistent in their composition roughly the undifferentiated comets in the outer solar system.

The debris discs are divided into hot and cold slices. In warm disks, the average temperature of the dust has values ​​from 100 to about 150 Kelvin. This temperature reaches the dust at a distance of several astronomical units from the star. The cold debris disks have an average temperature of 20 Kelvin only partially. This corresponds to the temperature of dust in the Kuiper Belt of our solar system and a distance of about 30 to a few hundred AU. Some extremely cold slices with an infrared excess at 160 microns is interpreted as the emission of very large dust particles. These dust particles but should decay by collisions within a very short time into smaller parts. They may concern in these sources to unresolved background galaxies.

Debris discs in binary systems seem to occur as frequently as in single stars. The orbital plane of the disk is usually in the orbital plane of the binary system. Dynamic reasons, the inner part of the disc empties in binary systems fairly quickly, so that these debris disks do not emit infrared radiation from warm dust.

Dynamics

The primary source of dust in debris disks are planetesimals that collide with each other, releasing dust. To achieve high collision rates and relative velocities, there must be one or more bluff body in the form of proto-planet or planet that affect the orbits of the planetesimals. In addition, comets can migrate to the interior of the star system under the gravitational influence of planets or by close encounters with other stars in stellar associations. By the radiation of the star they are heated and vaporized. In this case, the bound in the comet dust is released. The dust which emits the largest part of the detected infrared radiation is accelerated out through both the star and the wind pressure from the radiation star system. Intense X-rays and ultraviolet radiation as occurs in the corona of magnetically active stars can also reduce the lifetime of dust in debris disks. In addition, the Poynting - Robertson effect can also cause the dust falls out of the inner webs on the star and chemically enriched with heavy elements. A further possibility is that the dust is collected by the planet.

The processes of destruction or removal of dust from a debris disk are also valid for Protoplanetary discs, the precursors of the debris disks. The lifetime of protoplanetary disks is estimated at up to ten million years, and the transition from the protoplanetary disk over a transitional disk ( engl. transitional disk) to the debris disk is fluid. The exact age at which the dust is likely to be mainly resulted from collisions is, among other things, depending on the type of star. The radiation earlier star is more energetic and they are more luminous, why can remove the original dust from their surroundings this faster.

Planets through their gravitational forces on structures in the form of empty rings and any spokes in the debris disks. These structures have properties that are similar to the rings of Saturn, where generated by shepherd moons gaps in the rings. Running the planet not in the orbital plane of the debris disk around, then generate the planet a second inclined ring around the star as in the case of beta Pictoris. Conversely, the orbits of the exoplanets are influenced by the debris disk. In particular, if the orbits of the planets, the stable orbits are in resonance with each other to the extent disturbed within a few hundred thousand or million years that the stabilizing resonance is no longer available. For this purpose, it is sufficient that the mass of the debris disc reaches about one percent of the mass of Neptune. However, even a high gas content in the debris disks lead to the observed empty rings. Accordingly, the interaction between gas and dust in the discs can lead to clumping, which bundles the dust in narrow eccentric orbits and leads to the formation of planets, but is not caused by planets.

Debris disks around white dwarfs

Also, to many white dwarfs an infrared excess is observed and associated with debris disks. In addition, the presence of dust around these degenerate stars is also confirmed by spectroscopic observations. In white dwarfs, for which there is purely radiative energy transport, heavy elements have been detected in their atmospheres. This should be demonstrated by the effect of gravitational sedimentation, after which heavy elements with a small cross -section through the gravitational fall into deeper layers in the atmospheres of white dwarfs with only low frequency or not at all. The observation of these elements in the atmospheres of these stars therefore requires a steady supply of dust from a debris disk. In white dwarfs collisions between planetesimals must not be the cause of the formation of dust. It could also be the destruction of asteroids by tidal forces act at a close approach to the white dwarf.

From the cooling age of the white dwarfs the age of the debris disks to 100 million can be estimated to about one billion years, which in older white dwarfs the luminosity may be too low to the dust disk to heat nor sufficient. The diameter of these discs has reached a value of about one sun radius. Because the red giant, the predecessor of the white dwarf, a considerably larger diameter had the debris disk can not be a relic from the period before the emergence of the white dwarf. The inner part of the disk reaches a temperature of up to 1500 K and is considerably warmer than the main sequence stars.

A dust disk around a white dwarf, which is indistinguishable from a debris disk could arise in binary systems. Then the contents of the binary system are two white dwarfs with different masses. Decreases the radius of the track axis due to the emission of gravitational radiation below a critical value could tidal forces tear the less massive white dwarf and the remains would form by condensation of a dust disk around the remaining white dwarf. This hypothesis should lead to a rapidly rotating massive white dwarf, which is surrounded by a debris disk. The high rotational speeds are, however, not observed. The cause may lie in an interaction of the magnetic field of the white dwarf with the dust disk surrounding it. Matching white dwarfs have a higher mass rather a strong magnetic field of up to 10 mega Gauss as white dwarfs with average masses.

Stars with intensively studied debris disks

• Gamma Doradus
• Wega
• Fomalhaut
• Beta Pictoris
• AU Microscopii