astrophysics, the term accretion is used for at least two distinct processes.
The first and most common is the growth of a massive object by gravitationally attracting more matter, typically
gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants in a close binary, or black holes in the centers of spiral galaxies. Some dynamics in the disc are necessary to allow orbiting gas to lose angular momentumand fall onto the central massive object.
The second process is somewhat analogous to the one in atmospheric science. In the
nebular theory, accretion refers to the collision and sticking of cooled microscopic dust and ice particles electrostatically, in protoplanetary discs and Gas giant protoplanetsystems, eventually leading to planetesimalswhich gravitationally accrete more small particles and other planetesimals.
Use of the term accretion disc for the protoplanetary disc thus leads to confusion over the planetary accretion process, although in many cases it may well be that both accretion processes are happening simultaneously (e.g. T Tauri).
The Jovian protoplanets probably have discs of their own, in close analogy to the
solar systemas a whole. The Jovian protoplanet may be accreting gas from its surrounding disc in the first process, at the same time that dust and ice particles in the disc are accreting into moonlets and ring systems, in the second process.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Accretion — may refer to:*Accretion (finance), predictable changes in the price of certain securitiesAccretion in scienceIn science, accretion is a process in which the size of something gradually increases by steady addition of smaller parts. This term is… … Wikipedia
Accretion disc — An accretion disc (or accretion disk) is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a central body. The central body is typically either a young star, a protostar, a white dwarf, a neutron star,… … Wikipedia
Disque d'accrétion — Une vue d artiste d un système stellaire binaire composé d un trou noir et une étoile de la séquence principale. Le disque d accrétion et les jets de radiations électromagnétiques sont représentés en bleu. Un disque d accrétion est une structure… … Wikipédia en Français
Disque D'accrétion — Disques vus en absorption autour d étoiles jeunes dans la constellation d Orion. Un disque d accrétion est une structure astrophysique formée par de la matière en orbite autour d un corps central et qui chute sur celui ci en raison de la… … Wikipédia en Français
Disque d’accrétion — Disque d accrétion Disques vus en absorption autour d étoiles jeunes dans la constellation d Orion. Un disque d accrétion est une structure astrophysique formée par de la matière en orbite autour d un corps central et qui chute sur celui ci en… … Wikipédia en Français
Frost line (astrophysics) — In astronomy or planetary science, the frost line, also known as the snow line, refers to a particular distance in the solar nebula from the central protosun where it is cool enough for hydrogen compounds such as water, ammonia, and methane to… … Wikipedia
Mass transfer — Part of Chemical engineering History Concepts Unit operations Unit processes Chemical engineer Chemical process Process integration Unit operation … Wikipedia
White dwarf — For other uses, see White dwarf (disambiguation). Image of Sirius A and Sirius B taken by the Hubble Space Telescope. Sirius B, which is a white dwarf, can be seen as a faint pinprick of light to the lower left of the much brighter Sirius A … Wikipedia
Planet — This article is about the astronomical object. For other uses, see Planet (disambiguation) … Wikipedia
Cygnus X-1 — For other uses, see Cygnus X 1 (disambiguation). HDE 226868 HDE 226868 (not shown) is located next to the star η (lower center) in this map of constellation Cygnus … Wikipedia