The accretion indicates in Astrophysique, Géologie, Médecine and Météorologie the constitution and the increase in a body, a structure or an object per contribution and/or agglomeration of matter, generally on the surface or periphery of this one.

Astronomy

The accretion indicates the capture of Matière by a Astre under the effect of the Gravitation. The accretion takes place in many astrophysical contexts in particular, when a compact object is located in a diffuse matter environment:

• the star S in formation;
• the Planet S in formation;
• the Nova E;
• the black holes, in particular in the active cores of galaxies.

The accretion transforms the mechanical energy system object-matter into thermal energy (heat) and, consequently, into Rayonnement. Two phenomena contribute to release this energy:

• the matter orbiting around star is slowed down by dissipative processes (e.g. turbulent Viscosité ) and ends up falling on the compact object; this loss of potential energy results in a heating of the matter.
• at the time of its fall on the compact object the matter loses most of kinetic sound energy of fall; this phenomenon releases this energy in thermal form.

The type of radiation emitted at the time of accretion depends of about size on the rate on accretion (mass accrétée per unit of time) and on gravity. The young stars of solar type, whose revolved field is moderate, generally release an excess Infrarouge related to the braking of the matter and an excess Ultraviolet related to the fall of the matter on the surface of star. The accretion around the black holes primarily releases from the x-rays and gamma because of the strong gravity which they generate.

Around stars and planets in formation, and black holes the matter accrète according to the equatorial Plane of the system and form a Disc of accretion. This accretion then is often accompanied by polar floods of ejection, being able to withdraw a significant proportion of matter of the disc.

Geology

The accretion in geology recovers various phenomena whose principle is the accumulation of matter.

Oceanic accretion

The oceanic accretion is the phenomenon which leads to the oceanic creation of Lithosphère on the level of the oceanic dorsal . In short, the tectonic plates trained in the Coat, on the level of the zones of Subduction to one their borders (example: the Western border of the North-American plate), deviate from the others on the level (the Eastern border of the North-American plate, in the middle of the Atlantic Ocean. This spacing involves the increase of the Asthénosphère on the level of the oceanic dorsal , the appearance of Magma and the formation of a oceanic news Lithosphère.

Accretion prism

The prisms of accretion are accumulation of marine sediments on the level of the zones of Subduction by the râclage of the sediments deposited on the plunging Tectonic plate by the overlying plaqe.

Accretion on the level of a hot spot

Accretion is called the phenomenon of increase in a continental or oceanic area per matter contribution. The volcanic islands are born from accretion, the matter is brought by the volcanic eruptions which release from the Lave. While accumulating with the wire of time this lava can reach surface water and to rise above. The island of the Meeting was born thus, just like the archipelago of Hawaii.

The formation process of the Atoll S is thus the accretion. The Corail settles on the sides immersed of an island from which it develops in coral Récif until growing at the point to level at water surface. It is thus formed a natural barrier which breaks the marine currents and limit considerably the renewal of the water trapped between the barrier and the coasts of the island. The basin thus formed is called Lagon.

Medicine

Phenomenon according to which grow certain pathological formations such as the Calcul S biliary or urinary.

Meteorology

In meteorology, the accretion indicates the enlargement of a Hydrométéore meeting a water drop in Surfusion, which freezes at the time of the impact.

See too

• Disc of accretion

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