Fr.: mécanique non-relativiste
Mechanics in which the masses under consideration move at speeds much slower than the speed of light.
Fr.: non thermique
The nature of a → non-thermal radiation.
gosil- nâgarmâyi (#)
Fr.: émission non thermique
non-thermal filament (NTF)
Fr.: filament non thermique
Any of many long and slender structures visible in → radio continuum images of the inner hundred parsecs of the → Galactic Center. NTFs are typically tens of parsecs long and only a fraction of parsec wide. They may occur in isolation or in bundles, such as those comprising the linear portion of the prominent → radio Arc. Their → non-thermal spectrum and strong → linear polarization indicate → synchrotron radiation. The magnetic fields in the NTFs have been estimated from various means. Early estimates centered on the radio Arc, and focused on a comparison between the → magnetic pressure and the estimated → ram pressure from nearby → molecular cloud interactions, indicated magnetic field strengths as high as 1 mG (Morris and Yusef-Zadeh 1985). More recent observations, however, have pointed to significantly weaker magnetic fields among the population of NTFs. Synchrotron models of the radio spectrum imply equipartition magnetic fields between 50-200 μG. Theoretically, it has been challenging to understand the nature of these filaments that resemble extragalactic → radio jets but are not accompanied with any obvious source of acceleration of charged particles to high energy → relativistic energies. Although a number of detailed models have been considered, there is no consensus as to the origin of the NTFs. These models suggest that molecular and ionized gas clouds, mass-losing stars, → Galactic winds, magnetic activity of the → supermassive black hole at the Galactic center, and → lepton production due to → dark matter annihilation play a role in the processes that lead to the production of the NTFs (Linden et al. 2011, ApJ 741,95, and references therein). In most models, the magnetic field is strong and its global geometry in the central region of the Galaxy is considered to be → poloidal and static. However, some recent models have argued that the magnetic field is local and dynamic.
tâbeš-e nâgarmâyi (#)
Fr.: rayonnement non thermique
The electromagnetic radiation whose characteristics do not depend on the temperature of the emitting source. In contrast to → thermal radiation, it has a different spectrum from that of → blackbody radiation. The three common types of non-thermal radiation in astronomy are: → synchrotron radiation, → bremsstrahlung radiation, and → maser → stimulated emission.
Fr.: spectre non thermique
A radio emission with a negative → spectral index. In this type of emission the intensity of the emitted radiation increases with wavelength.
Fr.: non zéro, non nul
Not equal to zero.
Fr.: polynôme non nul
bar- (#), dar- (#)
Adverb, used as prefix denoting "in, into, onto; toward; with continuous activity," etc.
O.E. on, variant of an "in, on, into," from P.Gmc. (cf. Du. aan, Ger. an, Goth. ana "on, upon"), from PIE base *ano "on" (cf. Av. ana "on," Gk. ana "on, upon," L. an-, O.C.S. na, Lith. nuo "down from").
Bar "on; up; upon; in; into; at; forth; with; near; before;
(Mid.Pers. abar; O.Pers.
upariy "above; over, upon, according to;" Av. upairi "above, over,"
upairi.zəma- "located above the earth;" cf. Gk. hyper- "over, above;"
L. super-; O.H.G. ubir "over;" PIE base *uper "over").
Fr.: en ligne
The state when two or more devices are directly connected and are communicating efficiently.
Fr.: réduction enligne
Preliminary reduction of observational data at a telescope simultaneously with their acquisition.
Fr.: observation sur la source
In comparison with → off-source observation, an observation which is concerned with the source itself.
on-the-spot (OTS) approximation
An approximation in the treatment of photoionized → H II regions, whereby secondary ionizing photons are absorbed immediately very close to their site of emission. The secondary photons, produced by → radiative recombinations directly to the → ground states, are thus ignored with respect to the ionizing photons emitted by the → exciting star. The OTS approximation requires that the ionized gas be sufficiently dense so that secondary ionizing photons are very likely absorbed within the H II region.
Fr.: bras Orion-Cygne
Same as → Orion Arm.
Fr.: plasma photon-baryon
principle of non-contradiction
Fr.: principe de non-contradiction
zanjire-ye proton-proton (#)
Fr.: chaîne proton-proton
A series of → thermonuclear reactions, taking place mainly in → low-mass stars, such as the Sun, which transforms four hydrogen nuclei (protons) into one helium (4He) nucleus and thereby generates energy in the stellar core. First, two protons (1H) combine to form a → deuterium nucleus (2H) with the emission of a → positron (e+) and a → neutrino (ν): 1H + 1H → 2H + e+ + ν. The deuterium nucleus then rapidly captures another proton to form a helium-3 nucleus (3He), while emitting a → gamma ray (γ): 2H + 1H → 3He + γ. There are three alternatives for the next step. In the PP I chain, occurring in 86% of the cases, two 3He nuclei fuse to a final 4He nucleus while two protons are released: 3He + 3He → 4He + 1H + 1H. The mass of the resulting 4He nucleus is less than the total mass of the four original protons used to produce 4He (→ mass defect). The difference, ~ 0.7% of the total mass of the protons, is converted into energy and radiated by the Sun. In this process, the Sun loses some 4 million tons of its mass each second. See also → CNO cycle.
Fr.: réaction proton-proton
Fr.: Univers dominé par le rayonnement
An early epoch in the history of the → Universe when the radiation → density parameter was Ωr≈ 1, while other density parameters had negligible contributions. A radiation-dominated Universe is characterized by R/R0 ∝ t1/2, where R is the → cosmic scale factor and t is time. According to the → Big Bang model, the radiation-dominated phase was followed by the → matter-dominated phase.
radiation-driven implosion (RDI)
forukaft az râh-e tâbeš
Fr.: implosion induit par rayonnement
A hydrodynamic process occurring in star forming regions where a neutral cloud (→ clump) is subjected to the intense ultraviolet radiation of a newly-born → massive star. The gas within the layer exposed to the radiation is ionized and forms an → ionization front at the front surface. The increased pressure due to temperature rise at the top layer drives an → isothermal → shock front into the clump, which compresses the neutral gas ahead of it, below the surface. A density → gradient builds up leading rapidly to the formation of a condensed core. With further concentration of the gas, the hydrogen density in the center of the core increases drastically, reaching 108 cm-3 about 4 x 105 years after the first impact of the ionizing radiation on the clump, according to current models (e.g. Bertoldi 1989, ApJ 346, 735; Miao et al. 2006, MNRAS 369, 143, and references therein). The core can develop further to form a → cometary globule or → collapse under its self-gravity, eventually giving rise to new → low-mass stars (→ triggered star formation). In the process, the whole clump accelerates away from the initial ionizing star. Indeed, the gas evaporated off the side of the clump facing the ionizing star can create a rocket effect accelerating the clump away from the star (with a velocity of up to 5 km s-1), while losing part of its initial mass.