1) The process by which an object increases its mass under the influence of its
→ gravitational attraction.
Accretion plays a key role in a wide range of astrophysical phenomena.
In particular stars result from the accretion of material by a
→ protostar from a surrounding
→ molecular cloud.
The accumulation of mass on the protostar involves
the formation of an → accretion disk.
Theoretical and observational investigations of protostars and newborn stars
indicate the important role of → magnetic fields
in this process. They favor the magnetospheric accretion model for mass transfer
from the circumstellar disk onto the newborn star. In this model, the stellar magnetosphere
→ truncates the disk at a few stellar radii. Gas from the disk
accretes onto the star along the magnetic field lines and hits the stellar surface at
approximately the → free fall velocity, causing a strong accretion
shock. Various → emission lines, such as the hydrogen
→ Balmer series, He I 5876 Å, Brγ 2.17 μm,
and so forth are formed in the infalling magnetospheric flow. Moreover, optical/ultraviolet
excess continuum emission is produced in the → accretion shocks.
The accretion is accompanied
by mass ejection through collimated → bipolar jets.
L. accretionem (nom. accretio, gen. accretionis) "a growing larger," from stem of accrescere, from ad- "to" + crescere "to grow".
Farbâl from prefix → far- which conveys "increase, abundance" + bâl, from bâlidan "to grow, to wax great," Mid.Pers. vâlitan, Av. varəd-, varədait- "to increase, augment, strengthen, cause to prosper," Skt. vrdh-, vardhati.
Fr.: colonne d'accrétion
The channel through which matter is accreted onto a body such as a → protostar, → white dwarf, → neutron star, or → black hole. The accreting body possesses magnetic fields strong enough to disrupt the → accretion flow and carry the material through column-shaped channels directly on to a small fraction of the stellar surface near the magnetic poles.
Fr.: disque d'accrétion
A rotating disk of gas and dust formed around a center of strong gravity that pulls material off a surrounding or near-by gaseous object. Accretion disks are associated with several astrophysical objects such as → binary stars, → protostars, → white dwarfs, → neutron stars, and → black holes. Accretion disk forms because the infalling gas does not directly crash the accreting object due to its too high → angular momentum. The individual particles go into a circular orbit around the accretor because the circular orbit has the lowest energy for a given angular momentum. A spread in angular momentum values will give a population of particles moving on different orbits, so that a rotating disk of matter forms around the object. The matter in the disk becomes very hot due to internal friction and → viscosity as well as the tug of the accreting object. Since this hot gas is being accelerated it radiates energy and loses angular momentum and falls onto the accretor. Theoretical and observational pieces of evidence point to the importance of → magnetic fields in the accretion process. According to current models, the stellar magnetosphere → truncates the disk at a few stellar radii. Gas from the disk accretes onto the star along the magnetic field lines and hits the stellar surface at approximately the → free fall velocity, causing a strong accretion shock. See also → flared disk, → self-shadowed disk, → protoplanetary disk, → alpha disk model.
Fr.: flot d'accrétion
1) Flow of matter during an accretion process.
Fr.: taux d'accrétion
The amount of mass → accreted during unit time. The accretion rate for the → collapse of a singular → isothermal sphere is expressed by: dM/dt = 0.975 cs3/G, where cs is the isothermal → sound speed (Shu 1977, ApJ 214, 488). This relation can be written as: dM/dt = 4.36 x 10-6 (T / 20 K)3/2 in units of solar masses per year, where T is the temperature. Observed temperatures of 10-20 K in regions of → low-mass star formation imply accretion rates of about 10-6 to 10-5 solar masses per year. Accretion rates for → massive stars amount to values of 10-4 to 10-3 solar masses per year.
Fr.: choc d'accrétion
A → shock wave occurring at the surface of a compact object or dense region that is accreting matter with a → supersonic velocity from its environment. In the case of → young stellar objects the process is believed to take place by funneled streams in the form of → accretion columns that originate in the surrounding → accretion disk and flow along the → field lines of the → protostar → magnetosphere. The gas falls supersonically onto the surface of the central body and its impact produces strong shocks of a few million → kelvin, a phenomenon that is observable in → X-rays.
zamân-e farbâl, direš-e ~
Fr.: temps d'accrétion
Fr.: accrétion de Bondi-Hoyle
The → accretion of mass by a star (assumed as point particle) moving at a steady speed through an infinite, uniform gas cloud. It is directly proportional to the star mass (M) and the medium density (ρ) and inversely proportional to the relative star/gas velocity (v). In its classical expression: 4πρ(G M)2 / v3, where G is the → gravitational constant. See Bondi & Hoyle (1944, MNRAS 104, 273) and Bondi (1952, MNRAS 112, 195). For a recent treatment of accretion in a turbulent medium see Krumholtz et al. 2006 (ApJ 638, 369).
Named after Hermann Bondi (1919-2005), an Anglo-Austrian mathematician and cosmologist and Fred Hoyle (1915-2001), British mathematician and astronomer best known as the foremost proponent and defender of the steady-state theory of the universe; → accretion.
Bondi-Hoyle accretion radius
šo'â'-e farbâl-e Bondi-Hoyle
Fr.: rayon de l'accrétion de Bondi-Hoyle
In the → Bondi-Hoyle accretion process, the radius where the gravitational energy owing to star is larger than the kinetic energy and, therefore, at which material is bound to star. The Bondi-Hoyle accretion radius is given by RBH = 2 GM / (v2 + cs2) where G is the gravitational constant, M is the stellar mass, v the gas/star relative velocity, and cs is the sound speed.
cold accretion flow
tacân-e farbâl-e sard
Fr.: écoulement d'accrétion froid
1) A type of → accretion flow by a
→ compact object
such as a → black hole
that consists of cool → optically thick
gas and has a relatively high mass → accretion rate,
in contrast to → hot accretion flows.
cold disk accretion
gerde-ye farbâl-e sard
Fr.: disque d'accrétion froid
An accretion process whereby material coming from an → accretion disk settles onto the → protostellar surface through a geometrically thin layer or thin accretion columns. Heat brought into the protostar in the accretion flow radiates freely into space until the temperature attains the photospheric value. Most of the stellar surface is unaffected by the accretion flow (see, e.g., Hosokawa et al. 2010, ApJ 721, 478).
competitive accretion model
model-e farbâl-e hâjuyeši
Fr.: modèle d'accrétion compétitive
A scenario for → massive star formation whereby developing → protostars in their natal → molecular clouds compete with each other to gather mass. The protostars → accrete mass with a rate which depends on their location within the protocluster. They use the same reservoir of gas to grow. Therefore those protostars nearest the center, where the potential well is deep, and gas densities are higher, have the highest → accretion rates. The competitive accretion model explains the observational fact that the most massive stars are generally found in cluster cores. It accounts also for the distribution of stellar masses. In this model the accretion process depends on the content of the cluster. In clusters where gas dominates the potential (e.g. at initial stages of cluster formation), the accretion process is better modeled by using the → tidal radius as the accretion radius. In contrast, when the stars dominate the cluster potential and are virialized, the accretion is better modeled by → Bondi-Hoyle accretion (Bonnell et al. 1997, MNRAS 285, 201; 2001, MNRAS 323, 785).
farbâl-e gerde-yi, ~ pat geredé, ~ ~ disk
Fr.: accrétion par disque
An accretion process involving an → accretion disk.
hot accretion flow
tacân-e farbâl-e dâq
Fr.: écoulement d'accrétion chaud
A type of → accretion flow by a → compact object such as a → black hole which has a high → virial temperature, is → optically thick, and occurs at lower mass → accretion rates compared with → cold accretion flows. In a hot accretion flow with a very low mass accretion rate, the electron mean free path is very large, and so the accreting → plasma is nearly collisionless. In this type of accretion flow, thermal conduction transports the energy from the inner to the outer regions. As the gas temperature in the outer regions can be increased above the → virial temperature , the gas in the outer regions can escape from the gravitational potential of the central black hole and form outflows, significantly decreasing the mass accretion rate.
Fr.: accrétion de glace
Meteo.: The process by which a layer of ice builds up on solid objects that are exposed to freezing precipitation or to supercooled fog or cloud droplets.
farbâl-e bâdi, ~ pat bâd
Fr.: accrétion par vent
A quasi-spherical accretion that is likely to occur in a → high-mass X-ray binary (HMXB) when the optical star of → early spectral class (O-B) does not fill its → Roche lobe, but has a significant → mass loss via → stellar wind. In → close binary systems another accretion regime, → disk accretion, occurs when the optical star overfills its Roche lobe.