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

A very fast moving, → collimated beam of → ionized gas at high temperatures associated with most classes of compact objects that spin and/or accrete matter from their surroundings, such as → protostars, → X-ray binary systems, and, at a larger scale, with → active galactic nuclei, → gamma-ray bursts, and → quasars. In general, jet sources host → accretion disks and are associated with → magnetic fields. Astrophysical jets, despite their different physical scales and power, are morphologically very similar, suggesting a common physical origin. For example, in one extreme, → active galactic nuclei jets have typical sizes ≥ 10⁶ pc, velocities near that of light c, and parent sources (→ massive black holes) with masses 10^6-9  Msun and luminosities ~ 10^43-48Lsun; while in the other extreme, → young stellar objects jets have typical sizes ≤ 1 pc, velocities ≤ 10^-3 c, and emerge from low mass protostars with masses ~ 1 Msun and luminosities (0.1-2 × 10⁴) Lsun. Jets play an important → feedback role in the evolution of their host systems. See also: → jet launching.

→ astrophysical; → jet.

One of two beams of high-temperature, ionized gas ejected in two opposite directions associated with a → protostar. The collimated jets, a consequence of the → accretion process, can extend over distances of several → light-years.

→ bipolar; → jet.

A transient optical phenomenon in the → stratosphere that emerges from the tops of → thunderstorm clouds at tremendous speeds. As their name implies, blue jets are optical ejections from the top of the electrically active core regions of thunderstorms. Following their emergence, they typically propagate upward in narrow cones at vertical speeds of roughly 100 km/s, fanning out and disappearing at heights of about 40-50 km. See also → sprite; → elve.

→ blue; → jet.

A beam of particles in which every particle would have exactly the same direction of travel, thereby the jet moves without → dispersion. Collimated jets of → plasma are associated with → protostars.

→ collimated; → jet.

1) A stream of a liquid, gas, or small solid particles forcefully shooting forth from a nozzle, orifice, etc.
2) Fountain-like formations of gas and/or dust gushing out from compact regions of some astronomical objects. → astrophysical jets.
3) Meteo.: A common abbreviation for → jet stream.
4) Shortened of → jet plane.

Jet, from M.Fr. jeter "to throw," from V.L. *jectare, alter. of L. jactare, from jac- "throw" + -t- frequentative suffix + -are infinitive suffix; PIE base *ye- "to do" (cf. Gk. hienai "to send, throw;" Hittite ijami "I make").

Ešân, from ešândan, → eject; šân contraction of ešân.

An → engine that works by taking in air at the front and expelling exhaust gases at the rear so that the reaction to this exhaust propels the vehicle forward.

→ jet; → engine.

The mechanism whereby → astrophysical jets are thrown out of → accretion disks . Observed correlations between emission from the accretion disk and from the jet provide evidence that the jets are launched from the disks directly. As the energy emitted from the jets is a → synchrotron radiation, the presence of a → magnetic field is deduced for the ejection. The most promising model for such "accretion-ejection" structures is based on a scenario where a large-scale magnetic field threads an accretion disk. This model, using a → magnetohydrodynamic (MHD) approach, shows that the magnetic field can azimuthally brake the matter inside the disk (carrying off → angular momentum allowing accretion) and accelerate matter above the disk surface. The → collimation of the flow is achieved via → magnetic tension due to the presence of a → toroidal component of the magnetic field. The magnetic field provides an effective alternative to the radially outward transport of disk angular momentum by → viscosity. The interaction of the magnetic structure with the disk plasma can create a MHD → Poynting flux leaving the disk along the magnetic surface. This energy flux can then be converted into → kinetic energy of the matter within the jet. Because the → mass density in the jet is smaller than in the disk, it is thereby possible to reach high → terminal velocities for a given amount of angular momentum removed from the disk (Casse & Keppens, 2002, ApJ 581, 988, and references therein).

→ jet; → launch; → -ing.

An airplane moved by → jet propulsion.

→ jet; plane, short for airplane, from Fr. aeroplane, from aero-, → air, + plane feminine of plan "flat, level," from L. planus, perhaps by association with forme plane; apparently coined and first used by Fr. sculptor and inventor Joseph Pline in 1855.

→ jet; havâpeymâ "airplane," from havâ, → air, + peymâ "travelling; traveller," from peymudan, peymâyidan "to travel, traverse, pass over," from Mid.Pers. patmudan, paymudan "to measure (against)," from *pati-māya-. The first element *pati- "against, back" (cf. Mod.Pers. pâd- "against, contrary to;" Mid.Pers. pât-; O.Pers. paity "agaist, back, opposite to, toward, face to face, in front of;" Av. paiti; Skt. práti "toward, against, again, back, in return, opposite;" Pali pati-; Gk. proti, pros "face to face with, toward, in addition to, near;" PIE *proti). The second element from *mā- "to measure;" O.Pers./Av. mā(y)- "to measure;" cf. Skt. mati "measures," matra- "measure;" Gk. metron "measure;" L. metrum; PIE base *me- "to measure." Apart from peymâ, several other terms in Mod.Pers. are related to this second element, which occurs also as mun, mân, man, mâ, mu, and mây: pirâmun "perimeter," âzmun, âzmây- "test, trial," peymân "measuring, agreement," peymâné "a measure; a cup, bowl," man "a measure weighing forty seers)," nemudan, nemâ- "to show, display," âmâdan, âmây- "to prepare."

Powerful, forward thrust that results from the rearward expulsion of a jet of fluid, especially propulsion by jet engines.

→ jet; → propulsion.

Meteo.: An area of relatively strong winds that are concentrated in a narrow band in the upper troposphere of the middle latitudes and subtropical regions of the Northern and Southern Hemispheres.

→ jet; → stream.

An → astrophysical jet which is visible in the optical range of the electromagnetic radiation.

→ optical; → jet.

A high-velocity and highly → collimated jet associated with the earliest phase of → star formation that propagating along the polar axis of the → protostar-→ accretion disk system. Protostellar jets are usually detected in the [S II], [O I], and Hα lines and are therefore referred to as optical jets. They may have more than a parsec in length. Their formation is related to the → magnetohydrodynamics (MHD) of accretion disks. These jets are detected in protostellar sources over a wide range of masses, from the very early stages of formation (sources associated with infalling envelopes whose mass exceeds that of the growing star) all the way to the → classical T Tauri stars, whose envelopes have already dispersed. This phenomenon is thought to play a key role in regulating the star formation process by removing the excess → angular momentum of disk material and enabling matter to flow toward the center. Protostellar winds also provide an important → feedback mechanism between the forming star and the surrounding medium, to which they return mass and energy. Protostellar jets are at the origin of → bipolar outflows. The et sweeps up ambient → molecular cloud material into two thin shells, which manifest themselves as the observed bipolar lobes of → carbon monoxide (CO) emission. Once the molecular cloud material has been swept away (on a timescale of 10⁵ years), the bipolar outflow disappears, leaving the protostellar jet to erratically fire away for a further 10⁶-10⁷ years.

→ protostellar; → jet.

An → astrophysical jet appearing in the radio wavelengths of the electromagnetic spectrum.

→ radio; → jet.