An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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<< < acc dis thi > >>

Number of Results: 43 Search : disk
accretion disk
  گرده‌ی ِ فربال   
gerde-ye farbâl

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.

accretion; → disk.

Airy disk
  گرده‌ی ِ ایری   
gerde-ye Eyri

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
  مدل ِ گرده آلفا، ~ دیسک ~   
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.

alpha, the parameter name; → disk; → model.

circumblack-hole disk
  گرده‌ی ِ پیرا-سیه‌چال   
gerde-ye pirâ-siyah câl

Fr.: disque autour de trou noir   

An → accretion disk formed around a → black hole.

circum-; → black; → hole.

circumnuclear disk
  گرده‌ی ِ پیراهسته‌ای   
gerde-ye pirâhasteyi

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.

circumnuclear; → disk.

circumstellar disk
  گرده‌ی ِ پیراستاره‌ای   
gerde-ye pirâsetâreyi

Fr.: disque circumstellaire   

Any concentration of material in the form of a disk orbiting around a star. → accretion disk; → protoplanetary disk.

circumstellar; → disk.

cold disk accretion
  گرده‌ی ِ فربال ِ سرد   
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).

cold; → disk; → accretion.

debris disk
  گرده‌ی ِ تیفال   
gerde-ye tifâl

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.

debris; → disk.

disk
  گرده، دیسک   
gerdé (#), disk

Fr.: disque   

1) General: Any thin, flat, circular plate or object.
2) Math.: The domain bounded by a circle.
3) Astro.: A thin, flattened material structure encompassing a central object, such as a star or a galactic nuclear region. → accretion disk; → circumnuclear disk; → circumstellar disk; → galactic disk.

Computers: Any of several types of material support on which data can be encoded.

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.

disk accretion
  فربال ِ گرده‌ای، ~ پت گرده، ~ ~ دیسک   
farbâl-e gerde-yi, ~ pat geredé, ~ ~ disk

Fr.: accrétion par disque   

An accretion process involving an → accretion disk.

disk; → accretion.

disk galaxy
  کهکشان ِ گرده‌دار   
kahkešân-e gerdedâr

Fr.: galaxie à disque   

A galaxy consisting of a thin disk of stars and → interstellar matter which may include → spiral arms and → bar.

disk; → galaxy.

disk instability
  ناپایداری ِ گرده، ~ دیسک   
nâpâydâri-ye gerdé, ~ disk

Fr.: instabilité de disque   

1) General: The process by which an → accretion disk cools, causing it to fragment into self-gravitating → clumps.
2) A model of → planet formation in which a region of a → circumstellar disk becomes dense and cool enough to be unstable to → gravitational collapse, resulting in the formation of a gaseous → protoplanet. Disk instability has been put forward as a mechanism for forming massive → giant planets on short time-scales (1 Ky-1 My).
3) In models of galactic structure, a process that is thought to cause the formation of → pseudo-bulges.
4) → disk instability model (DIM).

disk; → instability.

disk instability model (DIM)
  مدل ِ ناپایداری ِ گرده، ~ ~ دیسک   
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).

disk; → instability; → model.

disk locking
  گیرکرد ِ دیسک، ~ گرده   
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."

Girkard, from gir "hold, block," from gereftan "to seize, hold, take," → eclipse, + kard noun from kardan "to do," → work.

disk population
  پرینش ِ گرده، ~ ِ دیسک   
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.

disk quota
  بهرال ِ دیسک   
bahrâl-e disk

Fr.: quota de disque   

Computers: The specific amount of disk space that a user or service is allowed to use.

disk; → quota.

disk settling
  نیاشش ِ گرده، ~ دیسک   
niyâšeš-e gerdé, ~ disk

Fr.: stabilisation de disque   

The process whereby a → galaxy evolves from a disturbed to an ordered system, as it develops into a → rotation dominated → settled disk.

disk; → settling.

disk star
  ستاره‌ی ِ گرده، ~ دیسک   
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.

disk; → star.

disk truncation
  کل‌کرد ِ دیسک، ~ گرده   
kolkard-e disk, ~ gerdé

Fr.: troncature de disque   

In models of magnetized → accretion disks, the process whereby the disk is disrupted at a radius where the → magnetic pressure overcomes the → ram pressure of the accreted material. This occurs at a distance typically 3-7 stellar radii, below the → corotation radius.

disk; → truncation.

disk wind
  باد ِ گرده   
bâd-e gerdé

Fr.: vent de disque   

In → magnetocentrifugal models of → protostars, the wind arising from a significant range of radii in the → accretion disk. The contribution from innermost parts of the disk is dealt with by the → X-wind model. (Königl A. and Pudritz R. E., 2000, In Protostars and Planets IV, V. Mannings, et al. (eds.), Tucson: Univ. Arizona Press, p. 759).

disk; → wind.

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