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."