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.
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).
Fr.: bulbe en forme de disque
A → galaxy bulge that is flatter than a → classical bulge. Such bulges might be difficult to see in very inclined galaxies. They may contain sub-structures such as nuclear → bars, → spiral arms, or → rings. They usually show signs of → dust obscuration, younger → stellar populations, or ongoing → star formation. These systems seem to form mostly through disk instabilities (→ disk instability), such as bars, in a relatively slow, continuous and smooth process. Essentially, such instabilities induce a redistribution of → angular momentum along the galaxy, and, as a result, mostly gas but also stars are driven to the disk center. Also called → pseudo-bulge (Kormendy & Kennicutt, 2004, ARA&A 42, 603; Fisher & Drory, 2010, ApJ 716, 942).
Fr.: disque d'excrétion
An expanding → equatorial → disk or → torus of material ejected by a star. The phenomenon may result from a stellar → merger or an increase in its rate of → rotation or → stellar winds. Contrasted with → accretion disk.
gerde-ye borun-gošâ, disk-e ~
Fr.: disque évasé
A model of → accretion disk around a → pre-main sequence star or a → protostar in which the ratio of the disk thickness to the distance from the star increases outward. Current models of the irradiation of flared disks by stellar radiation predict that a central hole is created around the young star due to the evaporation of dust by the stellar radiation. The inner rim of the disk, at 0.5 to 1 AU from the star, is irradiated by the star "frontally" (at 90° angle). The heat produced by the irradiation causes the inner rim to puff up. A part of the disk, from about 1 to 6 AU, lies in the shadow of the puffed-up inner rim. The surface layers in this region do not receive stellar photons directly. Therefore, there is no significant heating of the disk midplane by reprocessed stellar flux from the disk surface. The midplane temperatures in the shadowed part of the disk are governed by the → near infrared emission of the inner rim, scattering of stellar light by dust particles outside the disk plane, and radial diffusion which exchanges energy between adjacent slabs. As for the outer parts of the disk, the surface is irradiated by the central star thanks to the outward widening of the disk. These parts remain flared, because the absorbed stellar flux is partially emitted toward the midplane, keeping the internal temperatures high enough to push the surface layers up. The flattened-disk model explains the observed → spectral energy distribution of some objects such as HD 179218. It also accounts for the observed strong → far-infrared, → excess, strong → PAH emission, and strong [O I] emission. Compare with → self-shadowed disk. See also → protoplanetary disk.
Flared, from flare "to spread gradually outward, as the end of a trumpet, having a gradual increase in width," of unknown origin; → disk.
Gerdé, → disk; borun-gošâ "opening outward," from borun "out, the outside" (Mid.Pers. bêron, from bê "outside, out, away" + rôn "side, direction;" Av. ravan- "(course of a) river") + gošâ stem of gošâdan, gošudan "to open;" Mid.Pers. wišâdan "to open, let free;" Khotanese hiyā "bound;" O.Pers. višta "untied, loosend;" vištāspa- (personal name) "with loosened horses;" Av. višta "untied," hita- "fastened, tied on;" cf. Skt. sā- "to bind, fasten," syáti "binds."
Fr.: disque galactique
Fr.: disque de verre
Same as glass blank.
gerde-ye Kepleri, disk-e ~
Fr.: disque keplérien
A circumstellar disk (such as an → accretion disk or a → protoplanetary disk) in which the → angular velocity at each radius is equal to the angular velocity of a circular → Keplerian orbit at the same radius. The main characteristic of the Keplerian disk is that → orbital velocity varies as r-1/2. This means that an object on an orbit closer to the central mass turns more rapidly than that on a farther orbit. This velocity difference is at the origin of internal friction or kinematic viscous forces between disk particles, which heats up the material.
gerde-ye Lamberti, disk-e ~
Fr.: disque lambertien
gerde-ye Mach, disk-e ~
Fr.: disque de Mach
Same as → shock diamond.
So named because Ernst Mach (1838-1916) was the first to record its existence.
Fr.: ébauche de miroir
Same as → mirror blank.
Fr.: disque de Newton
Fr.: disque protoplanétaire
A → circumstellar disk of gas and dust surrounding a → pre-main sequence star from which planetary systems form. Protoplanetary disks are remnants of → accretion disks which bring forth stars. Typically, their sizes are ~100-500 AU, masses ~10-2 solar masses, lifetimes ~106-107 years, and accretion rates ~10-7-10-8 solar masses per year. According to the standard theory of planet formation, called core accretion, planets come into being by the growth of → dust grains which stick together and produce ever larger bodies, known as → planetesimals. The agglomeration of these planetesimals of 100 to 1000 km in size into rocky Earth-mass planets is the main outcome of this theory. Beyond the → snow line in the disk, if the masses of these cores of rock and ice grow higher than 10 times that of Earth in less than a few million years, gas can rapidly accrete and give rise to giant gaseous planets similar to → Jupiter. If core building goes on too slowly, the disk gas dissipates before the formation of → giant planets can start. Finally the left-over planetesimals that could not agglomerate into rocky planets or core of giant planets remain as a → debris disk around the central object that has become a → main sequence star. An alternative to core accretion theory is formation of planets in a massive protoplanetary disk by → gravitational instabilities. The validity of these two theories is presently debated. See also → protoplanet.
Fr.: disque protostellaire
gerde-ye Ramsden, disk-e ~ (#)
Fr.: disque de Ramsden
The small circular patch of light visible in the back focal plane of an eyepiece.
Named after Jesse Ramsden (1735-1800), English maker of astronomical instruments; → disk.
gerde-ye šekân, disk-e ~
Fr.: tache de seeing
The angular size of a stellar image for long exposures, as determined by the ratio λ/r0, where λ is the wavelength and r0 the typical size of → turbulence patches. → Fried parameter. The most common seeing measurement is the → full-width at half-maximumof the seeing disk. → Airy disk.
gerde-ye xod-pardé, disk-e ~
Fr.: disque auto-écranté
A model of → accretion disk around a → pre-main sequence star or a → protostar in which the outer parts of the disk are geometrically flat, in contrast to a → flared disk. Inward of a certain radius (0.5-1 AU from the star) the dust in the disk evaporates. Because the dust is the main source of opacity and the gas in the disk is usually optically thin, the irradiation burns a hole in the disk. Moreover, the inner rim puffs up, similarly to the case of flared disks. The difference lies in the outer parts. The inner rim casts its shadow over the disk all the way out. Since the disk thickness is almost constant, no photons can reach the surface of the disk and the outer parts of the disk remain shadowed by the inner rim and the midplane temperatures decrease accordingly. This model explains the observed → spectral energy distribution of some pre-main sequence stars, such as HD 101412. It also accounts for the observed weak → far infrared→ excess, weak or no → PAH emission, and weak or no [O I] emission.
Fr.: disque stabilisé
gerde-ye siyah-rox, disk-e ~
Fr.: disque silhouette
A dark disk seen in absorption against the bright nebular background. → proplyd.
From Fr. silhouette, in allusion to Étienne de Silhouette (1709-67), Fr. controller general of finances in 1759; perhaps from his ephemeral tenure; → disk.
Gerdé, → disk; siyah-rox, from siyah, → black, + rox, variant of ru(y) "face, surface; aspect; appearance," (Mid.Pers. rôy, rôdh "face;" Av. raoδa- "growth," in plural form "appearance," from raod- "to grow, sprout, shoot;" cf. Skt. róha- "rising, height").