Protoplanetary disk

Called a protoplanetary disk, also circumstellar disk or Proplyd (English acronym for Protoplanetary disk), an annular disk of gas and dust around a protostar at its formation from a primordial cloud.

Even a small initial angular momentum of the primordial cloud is sufficient to prevent the formation of only a single star. Instead, forms, depending on the strength of the turbulent friction, at least a double or multiple star or a star with planet system. In the latter case, one to ten percent of the star are assumed, while the vast majority of the angular momentum remains in the disk or in the planetary system for the mass of the protoplanetary disk. For the mechanism of isolation see accretion disk. A small part of the angular momentum is given via jets.

A protoplanetary disk has a flared outward structure. In the inner area, the temperature is high enough to sublimate the dust particles. The outdoor living area can be subdivided the optically thick slice vertically into several layers. The outermost layer absorbs photons from the central star and from the interstellar radiation field. Of infrared light penetrates deeper layers according to the outside so that the temperature falls to the median plane and molecules freeze. Dust particles sink down to the center plane and can coagulate there.

Development of a protoplanetary disc to planetary system

The processes that lead to the formation of planets are not yet understood in detail. Essentially, there are two models:

• Coagulation and accretion: Simulations show that interstellar dust particles can coagulate indeed, but also various processes are ( bounce, fragmentation ) that hinder grow to millimeter size. The current research attempts to break through this barrier with ever more accurate simulations and considered the possibility static electricity, lightning and magnetized particles. From a diameter of a few meters further material lumps collect a gravitationally. The larger a body is already, the faster and more widely it gathers dust, so that larger bodies grow faster than smaller ( runaway process). If mountain large planetesimals are formed, the supply of dust is largely used up, so that further growth is due to collisions. Theoretically, the larger planetesimals should grow to protoplanets up who vacate the area in their orbit. The gas planet would arise in this model through accretion of gas to the already incurred large rock bodies.

• Gravitational Instability: densities within the protoplanetary disk that satisfy the Jeans criterion, leading to the aggregation of matter and ultimately to the formation of planets. Especially for the formation of gas giant planets, this is an often assumed model. According to theoretical simulations (L. Mayer et al., Science ( 2002) ) gas planets can thus form already within 1000 years of spiral density instabilities within protoplanetary disks. It is unclear which such instabilities can be caused. Very massive disks are unstable by themselves when they cool down and thus the pressure decreases ( Alar Toomre, 1964). Maybe local instabilities can occur in less massive disks, if this area is compressed by an external disturbance, such as a nearby supernova.

Both scenarios for the formation of planets need not necessarily be mutually exclusive. Example, it is possible that gas giants are caused by gravitational instabilities, while Earth-like planets are caused by accumulation of planetesimals. The formation of Uranus and Neptune, for example, would be possible by a gravitational instability without contradiction to the limited lifetime 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 indicate that protoplanetary disks are destroyed after less than ten million years ( Haisch, Lada & Lada 2001). On the other hand, speaks of the high proportion of heavy elements, particularly at Uranus and Neptune for a direct formation of gravitational instabilities, as these would likely lead to a solar-like composition.

Protoplanetary disks are destroyed in less than 10 million years ago. The gas and the particles are smaller than about 1 micron can be driven by the wind and radiation pressure rating of the system. Medium particles to about 1 cm fall through the Poynting - Robertson effect is spiraling into the star. Only the larger particles survive.

The dust disks, which were discovered around older stars like Vega since the 80s, so there are no residues of protoplanetary disks. The dust is instead constantly replenished by the collision of asteroids. Even the dust in the solar system, which can be seen in the zodiacal light, comes from the collision of asteroids and the outgassing of comets and is not about the rest of the protoplanetary disk.

Observations of protoplanetary disks

The first proto-planetary disks were observed in 1994 by C. Robert O'Dell and employees with the Hubble Space Telescope in the Orion Nebula. In this star-forming region about 50 % of all young stars are surrounded by a protoplanetary disk. 1998 which has a disk around a massive star has been found. By infrared images crystalline silicates were 2003 for the first time demonstrated in a protoplanetary disk, by IR spectroscopy in 2008 even organic materials such as hydrogen cyanide, carbon dioxide and water (see below AA Tauri, cosmochemistry and chemical evolution ).