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.: tache de diffraction, ~ d'Airy
The bright disk-like image of a point source of light, such as a star, as seen in an optical system with a circular → aperture.
Named after Sir George Biddell Airy (1801-1892), Astronomer Royal, great administrator, who much improved the equipment at Greenwich Observatory. → disk.
Gerdé, → disk; Airy, see above.
alpha disk model
model-e gerdé âlfâ, ~ disk ~
Fr.: modèle disque alpha
A simple → accretion disk model in which the → angular momentum is transported outward by action of some kind of → viscosity. In this model, first proposed by Shakura & Sunyaev (1973), the turbulent kinematic viscosity is given by ν = α cs H, where α is a parameter, cs the sound speed in the medium, and H → scale height. The α parameter controls the amount of → turbulence in the medium whose H and cs are upper limits for → mixing length and turbulent speed, respectively. Values of α = 10-3 to 10-2 yield evolution → time scales that are broadly consistent with the ages inferred for → T Tauri stars. A weak point of this model is the arbitrariness of the choice of the parameter α, which reflects the lack of a rigorous theory of turbulence.
gerde-ye pirâdorini, disk-e ~
Fr.: disque circumbinaire
gerde-ye pirâ-siyah câl
Fr.: disque autour de trou noir
Fr.: disque circumnucléaire
A thick disk of gas and dust clouds surrounding the → Galactic Center up to about 20 → light-years. The disk is very clumpy; the → clumps have densities of several 105 particles/cm3, radii of about 0.3 light-years, and gas temperatures above 100 K. The hydrogen mass of the disk is a few 104 → solar masses. Such circumnuclear disks are present also in other galaxies.
Fr.: disque circumstellaire
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).
Fr.: disque de débris
A disk developing around a star after the dissipation of the → protoplanetary disk of gas and dust whose material was used in the formation of planets during the first 10 million years. The resulting debris disk is mainly composed of residual → planetesimals analogous to → asteroids, → comets, and → Kuiper Belt Objects in the Solar System. Their mutual collisions produce observable → dust emission in a belt encompassing the planetary system.
gerdé (#), disk
1) General: Any thin, flat, circular plate or object.
From L. discus "quoit, discus, disk," from Gk. diskos, from dikein "to throw." → dish.
Gerdé, from Mid.Pers. girdag "disk, round," from gird/girt "round, all around," Proto-Iranian *gart- "to twist, to wreathe," cf. Skt krt "to twist threads, spin; to wind; to surround;" kata- "a twist of straw," Pali kata- "ring, bracelet," Gk. kartalos "a kind of basket," kyrtos "curved;" disk loanword from Fr.
farbâl-e gerde-yi, ~ pat geredé, ~ ~ disk
Fr.: accrétion par disque
An accretion process involving an → accretion disk.
Fr.: galaxie à disque
nâpâydâri-ye gerdé, ~ disk
Fr.: instabilité de disque
The process by which an → accretion disk
cools, causing it to fragment into self-gravitating
disk instability model (DIM)
model-e nâpâydâri-ye gerdé, ~ ~ disk
Fr.: modèle d'instabilité de disque
A model describing → dwarf novae and → Soft X-ray Transient (SXT)s. Accordingly, these objects are triggered by an → accretion disk instability due to an abrupt change in opacities (→ opacity) at → temperatures at which hydrogen is partially ionized. All versions of the DIM have this ingredient. They differ in assumptions about → viscosity, and about what happens at the inner and outer disk radii. Basically, during → quiescence, material accumulates in the accretion disk until a critical point is reached. The disk then becomes unstable and is dumped onto the → compact object, releasing a burst of → X-rays. However, the greater duration of SXT bursts (months) and the time interval between bursts (decades) cannot be accounted for by the standard disk instability model used for dwarf novae, and additional factors such as X-ray illumination and irradiation of the accretion disk are required for the model to match the observed properties of SXTs (J-P Lasota and J-M Hameury, 1995).
girkard-e disk, ~ gerdé
Fr.: blocage de disque
In star formation models involving magnetized accretion, a process whereby the stellar rotational → angular velocity becomes equal to the → Keplerian angular velocity of the → accretion disk. This happens at the → corotation radius. Disk locking is believed to be responsible for efficient loss of stellar → angular momentum during the → pre-main sequence contraction of → T Tauri stars. These stars are expected to spin up by a factor of about 3 due to contraction after being magnetically disconnected from the → circumstellar disk. However, observations show that a large fraction of pre-main sequence stars evolve at nearly constant angular velocity through the first 4 Myr. This process results from → magnetic braking. The idea of magnetic disk locking originated with the theory developed by Ghosh & Lamb (1979, ApJ 232, 259) for → neutron stars.
M.E., from O.E. lucan "to lock, to close," from loc "bolt, fastening, enclosure;" cf. M.L.G. lok, O.H.G. loh, O.N. lok "a cover, lid," Goth. -luk in usluk "opening," Ger. Loch "opening, hole," Du. luck "shutter."
parkeš-e disk, ~ gerde
Fr.: partition de disque
A logical division of a hard disk that is treated as a separate unit by operating systems and file systems.
porineš-e gerdé, ~ disk
Fr.: population disque
Of a spiral galaxy, those stars that lie in a flattened disk and move in nearly circular orbits around its centre. They are Population I stars of all ages up to the age of the disk, but in general are younger than stars in → halo population.
→ disk; population, from L.L. populationem "a people, multitude," from populatio, from populare "to inhabit," from populus "people," related to plebes "the common people," cf. Gk. plethos "people, multitude, great number," from PIE base *pel- "to be full;" Mod.Pers. por "full," O.Pers. paru- "much, many," Av. pouru- "much, many," pərəna- "full," par- "to fill," Skt. puru-, Gk. polus, O.E. full "completely, full," from P.Gmc. *fullaz, O.H.G. fol, Ger. voll, Goth. full.
Like Gk., Pers. uses the concepts of "multitude, many, full" to denote "people, group, herd, flock". The following examples are all terms derived from O.Pers. paru- "much, many," Av. par- "to fill," pouru- "much, many," pərəna- "full" (Mod.Pers. por "full"): literary Pers. bâré "herd, flock," parré "a rank or file of soldiers, a circular disposition of troops," Lori, Qâyeni bor "group, tribe, herd," Torbat-Heydariyeyi, Qomi borr "heap, bundle, group," Qomi borreh "group, assemblage of people," Pashtu parrak "flock, herd," Urdu para "flock, herd," Lârestâni baila "group, tribe," Tabari balik "herd, flock." With this introduction, porineš "population," verbal noun of porinidan "to populate," infinitive of porin "populous," from por "mutitude, many, full" + -in attribution suffix.
Fr.: quota de disque
Computers: The specific amount of disk space that a user or service is allowed to use.
niyâšeš-e gerdé, ~ disk
Fr.: stabilisation de disque
setâre-ye gerdé, ~ disk
Fr.: étoile de disque
A star that lies within the → galactic disk of a → spiral galaxy. Stars belonging to the → thin disk, such as the Sun or Alpha Centauri, lie at a typical distance of about 1,000 → light-years from the galactic midplane. There are also → thick disk stars, such as Lalande 21185, that lie at an average distance of about 3,500 light-years from the midplane.