discrete absorption component (DAC)
hamne-ye daršami-ye gosasté
Fr.: composante d'absorption discrète
The rapid, systematic changes in the absorption parts of the → P Cygni profiles of the ultraviolet → resonance lines (Si IV, C IV, and N V) observed in a majority of massive → hot stars. DACs are typically seen to accelerate to the → blue wing of the profile over a few days, becoming narrower as they approach the → terminal velocity.
dâdehâ-ye gosasté (#)
Fr.: données discrètes
Data that can only take a small set of particular values, usually whole numbers. For instance, number of children in a family cannot be 2.4, since parents cannot actually have 2.4 children. → continuous data.
discrete radio source
xan-e râdioyi-ye jodâ
Fr.: radiosource discrète
A localized source on the celestial sphere that can be observationally separated at radio wavelengths from its background emission.
Fr.: spectre discret
A spectrum in which the component wavelengths (and wave numbers and frequencies) constitute a discrete sequence of values (finite or infinite in number) rather than a continuum of values.
Fr.: transition discrète
A transition between two quantum-mechanical energy levels. See also → discrete spectrum.
Fr.: valeur discrète
A quantity that has certain magnitudes and does not represent a → continuous variable.
Fr.: variable discret
A variable which has only → discrete values and has no in-between values.
discrete-time quantum walk
puyeš-e kuântomi bâ zamân-e gosasté
Fr.: marche quantique à temps discret
A → quantum walk involving a probabilistic → operator that changes the direction while leaving the position fixed, and a shift operator that changes the position. Discrete-time quantum walk was introduced by J. Watrous (2001, Journal of Computer and System Sciences 62, 376)
bahsidan, bahs kardan
M.E., from Anglo-Fr. discusser, from L. discussus "struck asunder, shaken, scattered," p.p. of discutere to break up, "strike asunder," from → dis- "apart," + quatere "to shake, strike."
Bahsidan, infinitive from bahs, from Ar. baHS (
Verbal noun from → discuss.
Fr.: antenne parabolique
A large parabolic structure that collects radio waves and focuses them on a detector by means of a secondary reflector. A similar device used as antenna for radar transmitting.
O.E. disc "plate, bowl, platter," from L. discus "dish, platter, quoit," from Gk. diskos "disk, platter."
Jâm "cup, chalice, goblet, bowl," Mid.Pers. jâm "vessel, goblet; glass," Av. yama- "glass, glass vessel," yāmô.pacika- "baked glass;" related to Skt. camasa- "a vessel used at sacrifices for drinking Soma, kind of flat dish or cup?"
vâpâši, forupâši (#)
The breaking up of a body into fragments. For example, that of an unstable nuclei either spontaneously or as a result of bombardment by fast-moving particles, or the breaking up of a comet.
Any of the components in a logical → disjunction.
A → proposition of the form "A or B" (A ∨ B), where A and B are themselves propositions. A disjunction is → true when one or the other of its components (called → disjuncts) is true, and false otherwise.
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 → clumps.
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."