Debris disk

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Image of the debris around Fomalhaut through the Hubble Space Telescope

Debris discs ( English debris disk ) are dust discs around older stars.

They were first discovered with the help of the Infrared Astronomical Satellite due to a strong infrared excess . Because of the dust disc, the stars emit more radiation in the middle and far infrared than a black body of comparable temperature. The additional radiation is the result of thermal radiation due to micrometer-sized dust particles that are heated by the electromagnetic radiation from the central star.

Debris disks are also known as second-generation dust disks because their dust was likely formed through collisions between planetesimals or the breakup of comets millions of years after star formation was complete. In contrast to protoplanetary disks (dust disks of the first generation), they are not relics from the time of star formation, since radiation pressure and stellar wind have removed the original dust from the star system in the meantime.

The asteroid belt and the Kuiper belt of the solar system can be viewed as debris disks.

Classic debris washers

The diameter of the debris is between a few tenths and up to a thousand astronomical units (AU) , with most values ​​in the range from 30 to 120 AU. The slices are very thin, usually less than 0.1 AU. The mass of the dust in a debris disk reaches a few hundredths to a few hundred times the mass of the earth and decreases with the age of the stars, cf. also below: destruction or removal of dust .

Image of the debris disk around AU Microscopii by the Hubble Space Telescope

Debris disks have been found around main sequence stars with the spectral classes  A to M, whereby debris disks around early stars can be detected with a higher probability. With the current detection technology, cold dust discs can be detected around 20 percent of the sun-like stars . There is no connection between the metallicity of the star and the probability of detection of a debris disk. The age of the stars is a few million to several billion years. The debris disk around the red dwarf GJ 581 , whose age is estimated to be two to eight billion years, has the highest known age . In older red dwarfs, the radiation or the stellar wind is not sufficient to break up the dust discs.

With the help of infrared spectroscopy , the chemical composition of the debris disk could be analyzed: the dust particles contain crystallized minerals from forsterite , enstatite , the pyroxene group and the oliving group . In terms of their composition, they roughly correspond to the undifferentiated comets of the outer solar system.

Debris disks are divided into warm and cold disks:

  • In the case of warm windows, the average temperature of the dust is between 100 and approximately 150  Kelvin . The dust reaches this temperature at a distance of a few astronomical units from the star.
  • The cold debris disks have an average temperature of only 20 Kelvin in some cases. This corresponds to the temperature of dust in the Kuiper Belt of the solar system and a distance of about 30 to a few hundred AU.

Some extremely cold disks with an infrared excess at 160 μm are interpreted as the emission of very large dust particles. These dust particles should break down into smaller pieces within a very short time due to collisions. These sources may be unresolved background galaxies .

Debris disks in binary star systems seem to be just as common as in single stars . The plane of the orbit of the disk is mostly in the plane of the double star system. For dynamic reasons, the inner part of the disk in binary star systems empties very quickly, so that these debris disks do not emit infrared radiation from warm dust.

dynamics

Submillimeter image of the debris disk around Epsilon Eridani . The intensity corresponds to the density of the dust in the pane.

Dust sources

The primary source of dust in debris disks are planetesimals, which collide with each other, releasing dust in the process. In order to achieve high collision rates and relative speeds , there must be one or more disruptive bodies in the form of protoplanets or planets that influence the orbits of the planetesimals.

In addition, comets can migrate into the interior of the star system under the gravitational influence of planets or through close encounters with other stars in star associations . The radiation from the star heats them up and vaporizes them. The dust bound in the comet is released.

Destruction or removal of dust

The dust, which emits most of the detected infrared radiation, is accelerated out of the star system by both the stellar wind and the radiation pressure.

Intense X-rays and ultraviolet radiation, such as those produced in the corona of magnetically active stars, can also reduce the life of the dust in the debris.

In addition, the Poynting-Robertson effect can also cause the dust from the inner orbits to fall onto the star and chemically enrich it with heavy elements.

Another possibility is that the dust is picked up by planets.

The processes of destruction or the removal of dust from a debris disk are also valid for protoplanetary disks , the precursors of debris disks. The lifetime of protoplanetary disks is estimated at up to ten million years, and its transition from the protoplanetary disk over a transitional disc (Engl. Transitional disk ) for debris disk is fluid. The exact age at which the dust is likely to have emerged mainly from collisions depends, among other things, on the type of star: the radiation from early stars is more energetic and more luminous , which is why they can remove the original dust from their surroundings more quickly.

Interactions

Hubble Space Telescope image of the primary and secondary debris disks around Beta Pictoris

Due to their gravitational forces, planets lead to structures in the form of empty rings and possibly spokes in the debris disks. These structures are similar in their properties to the rings of Saturn , where gaps are created in the rings by shepherd moons . However, a high gas content in the debris disks can also lead to the empty rings observed. According to this, the interaction between gas and dust in the disks 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.

If the planets do not run in the plane of the debris disk, then they create a second, inclined ring around the star, as in the case of Beta Pictoris. Conversely, the orbits of the exoplanets are also influenced by the debris disk; especially if the orbits of the planets are in resonance with one another , these initially stable orbits are disturbed to such an extent within a few hundred thousand or millions of years that the stabilizing resonance is no longer present. A mass of the debris disk of approximately one percent of the mass of Neptune is sufficient for this.

Debris disks around white dwarfs

An excess of infrared is also observed around many white dwarfs and is associated with debris disks. In addition, the presence of dust around these degenerate stars is also confirmed by spectroscopic observations.

In the case of white dwarfs, in which there is purely radiative energy transport, heavy elements have been detected in their atmospheres . Theoretically, however, due to the gravitational sedimentation , according to which heavy elements with a small effective cross-section sink into deeper layers, in the atmospheres of the white dwarfs they should only be detectable with low frequency or not at all. The observation of heavy elements in the atmospheres of these stars therefore requires a constant supply of dust from a debris disk.

In white dwarfs, collisions between planetesimals do not have to be the cause of dust formation. Instead, asteroids may be destroyed by tidal forces if they get too close to the white dwarf.

From the cooling age of the white dwarfs, the age of their debris disks can be estimated to be between 100 million and approximately one billion years; in older white dwarfs, the luminosity may be too low to heat the dust disc sufficiently. The diameter of these disks reaches a value of approximately the radius of the sun . Since the red giant from which the white dwarf developed had a considerably larger diameter, the debris disk cannot be a relic from the phase before the white dwarf was formed. The inner part of the disk reaches temperatures of up to 1500 K and is therefore considerably warmer than the debris disks of main sequence stars.

A disk of dust around a white dwarf, indistinguishable from a disk of debris, could arise in binary star systems. According to this, the components of the binary star system are two white dwarfs of different mass. If the radius of the orbit axis falls below a critical value due to the radiation of gravitational radiation , then tidal forces could tear the lower-mass white dwarf apart, and the remains would form a disk of dust around the remaining white dwarf through condensation . This hypothesis should lead to a rapidly rotating massive white dwarf surrounded by a disk of debris. However, the high rotation speeds are not observed. The cause could lie in an interaction of the magnetic field of the white dwarf with the surrounding dust disc. Correspondingly, white dwarfs with a higher mass have a stronger magnetic field of up to 10 mega- Gauss than white dwarfs with average masses.

Stars with extensively studied debris disks

Individual evidence

  1. ↑ Hannah Broekhoven-Fiene et al .: THE DEBRIS DISK AROUND GAMMA DORADUS RESOLVED WITH HERSCHEL . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1212.1450 .
  2. ^ R. Nilsson et al .: VLT imaging of the beta Pictoris gas disk . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.4427 .
  3. ↑ C. Eiroa et al .: DUst Around NEarby Stars. The survey observational results . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1305.0155v1 .
  4. ↑ J.-F. Lestrade et al .: A DEBRIS Disk Around The Planet Hosting M-star GJ581 Spatially Resolved with Herschel . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.4898 .
  5. ↑ BL de Vries et al .: Comet-like mineralogy of olivine crystals in an extrasolar proto-Kuiper belt . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.2626 .
  6. ↑ Hideaki Fujiwara et al .: AKARI / IRC 18 μm Survey of Warm Debris Disks . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.6365 .
  7. ↑ AV Krivov et al .: Herschel's "Cold Debris Disks": Background Galaxies or Quiescent Rims of Planetary Systems? In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1306.2855v1 .
  8. ^ GM Kennedy et al .: Coplanar circumbinary debris disks . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1208.1759 .
  9. ^ Andras Gaspar, George H. Rieke, Zoltan Balog: THE COLLISIONAL EVOLUTION OF DEBRIS DISKS . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.1415 .
  10. ↑ BC Johnson et al .: A SELF-CONSISTENT MODEL OF THE CIRCUMSTELLAR DEBRIS CREATED BY A GIANT HYPERVELOCITY IMPACT IN THE HD172555 SYSTEM . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.6258 .
  11. ^ W. Lyra, M. Kuchner: Formation of sharp eccentric rings in debris disks with gas but without planets . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1307.5916v1 .
  12. ↑ A.-M. Lagrange, A. Boccaletti, J. Milli, G. Chauvin, M. Bonnefoy, D. Mouillet, JC Augereau, JH Girard, S. Lacour, D. Apai: beta Pic b position relative to the Debris Disk . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1202.2578 .
  13. ↑ Alexander Moore & Alice C. Quillen: Effects of a planetesimal debris disk on stability scenarios for the extrasolar planetary system HR 8799 . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.2004 .
  14. ↑ M. Deal, S. Vauclair and G. Vauclair: Thermohaline Instabilities Induced by Heavy Element Accretion onto White Dwarfs: Consequences on the Derived Accretion Rates . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.5349 .
  15. ^ S. Hartmann, T. Nagel, T. Rauch, and K. Werner: Observations and NLTE Modeling of the Gaseous Planetary Debris Disk around Ton 345 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.4015 .
  16. ↑ Roman R. Rafikov and Jose A. Garmilla: INNER EDGES OF COMPACT DISCS DEBRIS AROUND METAL-RICH WHITE DWARFS . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.7082 .
  17. ↑ B. Kulebi, KY Eksi, P. Loren Aguilar, J. Isern and E. Garcıa-Berro: Magnetic white dwarfs with debris disks . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1209.6232 .
  18. ↑ B. Külebi et al .: Magnetic white dwarfs with debris disks . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1302.6468v1 .