turbulent boundary layer
lâye-ye karâni-ye âš:ubnâk
Fr.: couche limite turbulente
The layer in which the Reynolds stresses are much larger than the viscous stresses. When the → Reynolds number is sufficiently high, there is a turbulent layer adjacent to the → laminar boundary layer.
turbulent core model
model-e maqze-ye âšubnâk
Fr.: modèle de cœur turbulent
A star formation scenario whereby → massive stars form from gravitationally bound → pre-stellar cores, which are supersonically → turbulent and in approximate pressure equilibrium with the surrounding protocluster medium. The high → accretion rates that characterize such media allow accretion to overcome the radiation pressure due to the luminosity of the star. The core is assumed to → collapse via an → accretion disk to form a single star or binary. The core density structure adopted is ρ ∝ r-k, with k = 1.5 set from observations. This choice affects the evolution of the accretion rate, which increases linearly with time. The high densities in regions of massive-star formation lead to typical time scales for the formation of a massive star of about 105 years (McKee & Tan 2003, ApJ 585, 850).
Fr.: écoulement turbulent
turbulent Jeans mass
jerm-e Jeans-e âšubnâk
Fr.: masse de Jeans turbulente
The characteristic mass for → cloud fragmentation in a → turbulent medium. While the standard → Jeans mass depends simply on the gas mean → density and → temperature, and fragmentation is purely gravitational, turbulent Jeans mass depends strongly also on the → Mach number (Chabrier et al. 2014, arXiv:1409.8466).
Fr.: plasma turbulent
A plasma characterized by a → turbulent flow regime.
1) gardidan, gaštan; gardândan; 2) gašt
Fr.: 1) tourner; faire tourner; 2) tour, tournure
1) To move, or cause to move, around, or partly around a center.
M.E. turnen; O.E. turnian "to rotate, revolve," also from O.Fr. torner "to turn," both from L. tornare "to turn on a lathe," from tornus "lathe," from Gk. tornos "lathe, tool for drawing circles."
Gardidan "to turn; turning," variant gaštan "to turn, to change;" Mid.Pers. vartitan; Av. varət- "to turn, revolve;" Skt. vrt- "to turn, roll," vartate "it turns round, rolls;" L. vertere "to turn;" O.H.G. werden "to become;" PIE base *wer- " to turn, bend."
The closest point in the path of a sound wave to the center of a star, as studied in → asteroseismology. Starting from the surface, the sound wave first moves into the star almost straight toward the center. Its path then deflects, because of the increasing → sound speed, so that it misses the center of the star. After the turning point, the wave moves out again until it reaches the surface, where it is reflected. If exactly an integer number of wavelengths fits between two reflections at the surface, the sound wave corresponds to a → standing wave with a specific pattern of → node lines on the surface.
1) A small road that branches off from a larger one, or a place where one
diverges from a former course.
Fr.: tournant final de la séquence principale
Same as → main-sequence turnoff.
Fr.: étoile du tournant final de la séquence principale
A greenish blue mineral consisting of aluminium phosphate colored by traces of copper.
From M.Fr. turquoise (M.E. turkeis), from O.Fr. turqueise "Turkish," because it was first brought to Europe from Turkestan. The gem does not occur in Turkey.
Mid.Pers. pirôzak, pirôcak, cf. Skt. peraja, peroja.
joft-e Tusi (#)
Fr.: couple de Tusi
Named for Nasireddin Tusi (1201-1274), director of Marâgha observatory who created the Ilkhani zij; → couple.
Fr.: bâton de Tusi
Named after the Iranian mathematician and astronomer Sharafeddin Tusi (c1135-1213), who invented the instrument. Not to be confused with Nasireddin Tusi (1201-1274), → Nasireddin couple.
1) A teacher who instructs students without institutional connection.
From tutor, M.E., from O.Fr. tutour "guardian, private teacher," from L. tutor "protector, watcher," from tutus, variant p.p. of tueri "to watch over," of unknown origin.
Âmuxtâr, literally "teacher," from âmuxtan, → teach, + -âr agent noun suffix (such as xaridâr).
1) A series of intensive lessons given to an individual student or
to a small group of students.
Fr.: TW Hydrae
The nearest known → classical T Tauri star, situated some 56 → parsecs away in the constellation → Hydra, far from any → molecular cloud (abbreviated TW Hya). It is a variable star with large Hα-emission → equivalent width. TW Hya is similar in mass to the Sun, but is only about 8 million years old. The star appears to be → accreting from a remarkable → face-on→ protoplanetary disk of dust and gas, resolved in images from the → Hubble Space Telescope. Stellar light scattered from the optically thick dust disk is seen from 20 to 230 AU. The → scattering profile indicates that the disk is → flared, not geometrically flat. TW Hya is accompanied by a group of other low-mass stars with similar ages and space motions, comprising the → TW Hydrae association. An → exoplanet of mass nearly 10 → Jupiters has been detected around TW Hya. It orbits the star with a period of 3.56 days at 0.04 AU, inside the inner rim of the disk.
TW Hydrae association
âhazeš-e TW Hudrâ
Fr.: Association TW Hydrae
A young (≤ 100 million years) association of stars (abbreviated TWA) with at least 27 members, from → intermediate mass stars to planetary mass objects. Five of them, including → TW Hydrae, display signatures of → T Tauri stars. TWA is the first moving group of young nearby (≤ 100 → paesecs) stars to be identified.
A cardinal number, 10 times 2.
M.E.; O.E. twentig "group of twenty," from twegen→ two; cf. O.Fris. twintich, Du. twintig, O.H.G. zweinzug, Ger. zwanzig.
Bist "twenty;" Mid.Pers. wist "twenty;" Av. vīsaiti "twenty;" cf. Skt. vimśati- "twenty;" Gk. eikosi "twenty;" L. uiginti "twenty."
Fr.: deux fois
Two times, as in succession; on two occasions.
M.E. twies, from O.E. twiga, twigea "two times," from twi-, → two; cf. L. bis, Gk. dis, Skt. dvis, Av. biš.
Dobâr, from do, → two, + bâr "time, fold," from Mid.Pers. bâr; Proto-Ir. *uara-; cf. Av. var- "to choose; to convince;" Skt. vāra- "time, turn."
The diffused light from the sky when the Sun is below the → horizon, either from daybreak to → sunrise or, more commonly, from → sunset to nightfall. There are three types of twilight: → astronomical twilight, → civil twilight, and → nautical twilight. They are divided on the basis of the → solar depression angle.
M.E., cognate with Du. tweelicht, Ger. zwielicht, from twi- a combining form meaning "two, twice," but it appears to refer to "half" light, rather than the fact that twilight occurs twice a day + → light.