Protoplanetary disk

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Protoplanetary disk around HL Tauri

A protoplanetary disk , also circumstellar disk or Proplyd ( engl. Acronym for Protoplanetary disk ), an annular disk of gas and dust to a proto-star or a similar object (young star, brown dwarf , planetary object mass ). As a result, it is assumed that they arise from a collapsing molecular cloud core.

description

Even a small initial angular momentum of the primordial cloud is enough to prevent the formation of a single star. Instead, depending on the strength of the turbulent friction , at least a double or multiple star or a star with a planetary system is formed .

In the latter case, the mass of the protoplanetary disk is assumed to be one to ten percent of the star, with the vast majority of the angular momentum remaining in the disk or in the planetary system. For the mechanism of separation see accretion disk . A small part of the angular momentum is also given off via jets .

A protoplanetary disk has an outwardly expanded structure. In the inner area, the temperature is high enough for dust particles to sublime . In the outer areas, the optically thick pane can be divided vertically into several layers:

  • the outermost layer absorbs photons from the central star and from the interstellar radiation field.
  • Infrared light penetrates outwards from deeper layers , so that the temperature drops towards the central plane and molecules freeze out. Dust particles sink to the central plane and can coagulate there .

Development to the planetary system

The processes that lead from the protoplanetary disk to the formation of planets are not yet understood in detail. There are essentially two models:

  • Coagulation and accretion: Simulations show that interstellar dust particles can coagulate , but that there are also various processes (ricochet, splintering) that prevent them from growing to millimeter size. Current research tries to break this barrier with ever more accurate simulations and also considers static electricity , lightning and magnetized particles. From a diameter of a few meters, the lumps gravitationally collect further material. The larger a body is, the faster and more spaciously it collects dust, so that larger bodies grow faster than smaller ones ( runaway process). When mountain-sized planetesimals have formed, the supply of dust has largely been used up, so that further growth is based on collisions . In theory, the larger planetesimals should grow into protoplanets that clear the area around their orbit . The gas planets would in this model by accretion arising from gas to the already incurred large rock bodies.
  • Gravitational instability: Compressions within the protoplanetary disk, which meet the Jeans criterion , lead to the agglomeration of matter and ultimately to the formation of planets. This is an often adopted model, especially for the formation of gas planets. According to theoretical simulations, gas planets can form from spiral density instabilities within protoplanetary disks within 1000 years. It is unclear what causes such instabilities. Very massive disks become unstable on their own when they cool down and the pressure decreases. Local instabilities may also occur in low-mass disks if this area is condensed by an external disturbance, for example a nearby supernova .

Both scenarios for the formation of planets do not necessarily have to be mutually exclusive. For example, it is possible that gas giants are created by gravitational instabilities, while Earth-like planets are created by the accumulation of planetesimals. The formation of Uranus and Neptune, for example, would be possible through a gravitational instability without contradicting the limited lifespan of protoplanetary disks; In the conventional coagulation model, the formation of the outer gas giants would take up to several hundred million years, while observations suggest that protoplanetary disks are destroyed after less than ten million years. On the other hand, the high proportion of heavier elements, especially in Uranus and Neptune, speaks against a direct formation from gravitational instabilities, as these would rather lead to a sun-like composition.

Protoplanetary disks around stars are destroyed within less than 10 million years: the gas and particles smaller than about 1 µm are driven out of the system by the stellar wind and radiation pressure , medium-sized particles up to about 1 cm fall on spiral orbits due to the Poynting-Robertson effect in the star; only the larger particles survive. The dust disks that have been discovered around older stars such as Vega since the 1980s are therefore not remnants of protoplanetary disks; instead, the dust is continually replenished by the collision of asteroids . The dust in the solar system, which can be seen in the zodiacal light , comes from the collision of planetoids and the outgassing of comets and is not the rest of the protoplanetary disk.

Observations

The first protoplanetary disks were observed in 1994 by C. Robert O'Dell and co-workers with the Hubble Space Telescope in the Orion Nebula ; In this star formation region around 50% of all young stars are surrounded by a protoplanetary disk. In 1998 a disk around a massive star was found for the first time. In 2003, infrared images were used for the first time to detect crystalline silicates in a protoplanetary disk, and in 2008 even organic materials such as hydrocyanic acid , carbon dioxide and water were detected by IR spectroscopy (see under AA Tauri , Cosmochemistry and chemical evolution ).

See also

literature

  • A. Natta, V. Grinin, V. Mannings: Properties and Evolution of Disks around Pre-Main-Sequence Stars of Intermediate Mass . In: Protostars and Planets , IV, 2000, ISBN 0-8165-2059-3 , p. 559.
  • Antonella Natta: Circumstellar Disks in Pre-Main Sequence Stars . 2003, arxiv : astro-ph / 0304184

Web links

Commons : Protoplanetary Disk  - collection of images, videos and audio files

Individual evidence

  1. ↑ Lucio Mayer, Thomas Quinn, James Wadsley, Joachim Stadel: Formation of Giant Planets by Fragmentation of Protoplanetary Disks . In: Science , 298, 2002, pp. 1756-1759, arxiv : astro-ph / 0311048
  2. ^ Alar Toomre : On the gravitational stability of a disk of stars . In: The Astrophysical Journal , 193, 1964, pp. 1217–1238 (deals with galactic disks, but is often cited in connection with protoplanetary disks)
  3. ^ Karl E. Haisch, Elizabeth A. Lada, Charles J. Lada: Disk Frequencies and Lifetimes in Young Clusters . In: The Astrophysical Journal , Volume 553, pp. L153-L156, arxiv : astro-ph / 0104347 .