<< < -ph pal par par par par pas Pau pen per per per per Per per pha Pho pho pho pho Pia Pis Pla pla pla pla Ple Pog pol pol pol pol pop pos pot Poy pre pre pre pre pri pri pro pro pro pro pro pro pro pub pul pyr > >>
parity hamâli (#) Fr.: parité 1) General: Equality, as in amount, status, or character;
equivalence; correspondence; similarity; analogy. Opposite of disparity. From M.Fr. parité, from L.L. paritas "equality," from L. adj. par "equal." Hamâli, quality noun of hamâl, → pair (equivalent 2). |
parity conservation pâyeš-e hamâli, pâyandegi-ye ~ Fr.: conservation de parité In quantum mechanics, the condition of parity in strong and electrodynamic interactions, where it remains constant and does not change with time. In other words, parity conservation implies that Nature is symmetrical and makes no distinction between right- and left-handed rotations or between opposite sides of a subatomic particle. Thus, for example, two similar radioactive particles spinning in opposite directions about a vertical axis should emit their decay products with the same intensity upward and downward. Same as → parity symmetry. → parity; → conservation. |
parity symmetry hamâmuni-ye hamâli Fr.: symétrie de parité The invariance of physical laws under a transformation that changes the sign of the space coordinates. Parity symmetry is sometimes called mirror symmetry. It is known that the parity symmetry is violated in some weak interactions, while it is well preserved in all other three interactions (gravitational, electromagnetic, strong). Same as → P-symmetry and → parity conservation. |
parity violation enâheš-e hamâli Fr.: violation de la parité In quantum mechanics, the condition of → parity in the → weak interaction. For example, the emitted → beta particles in → radioactive decay of → cobalt-60 nuclei are not equally distributed between the two poles of cobalt-60. More beta particles emerge from one pole than the other, and it would be possible to distinguish the mirror image nuclei from their counterparts. |
Parker instability nâpâydâri-ye Parker Fr.: instabilité de Parker A type of instability found in some astrophysical phenomena involving → magnetic fields; it arises if a gas layer is supported by the horizontal magnetic fields against → gravity. Also called → magnetic buoyancy. Briefly, this instability works as follows. Consider a uniform disk of gas which is coupled to a magnetici field that is parallel to the disk. Suppose that the disk is gravitationally stratified in the vertical direction, and is in dynamical equilibrium under the balance of gravity and pressure (thermal and magnetic). Now consider a small perturbation which causes the field lines to rise in certain parts of the disk and sink in others. Because of gravity, the gas loaded onto the field lines tends to slide off the peaks and and sink into the valleys. The increase of mass loads in the valleys makes them sink further, while the magnetic pressure causes the peaks to rise as their mass load decreases. Consequently, the initial perturbation is amplified, causing the production of density fluctuations in an initially uniform disk. The characteristic scale for the Parker instability is ~4πH, where H is the scale height of the diffuse component of the disk. For the Milky Way, where H ~ 150 pc, this scale is about 1-2 kpc. Numerical simulations show that the density contrast generated by the Parker instability is generally of order unity before the instability saturates. This implies that the Parker instability on its own may not be able to drive collapse on large scales. Nevertheless, it may trigger gravitational instability in a marginally unstable disk and/or induce strong motions in the medium, thereby acting as a source of turbulence on large scales (see, e.g., Houjun Mo, Frank van den Bosch, Simon White, 2010, Galaxy Formation and Evolution, The University Press, Cambridge, UK). First studied by E. N. Parker, 1966, ApJ 145, 811; → instability. |
Parkes Selected Region (PSR) nâhiye-ye gozide-ye Parkes Fr.: Région sélectionnée de Parkes A catalog of 397 radio sources between declinations +20° and +27° which were compiled from a finding survey made at 635 MHz with the 64m radio telescope at the Australian National Radio Astronomy Observatory, Parkes, N.S.W. and published in 1968. Originally abbreviated PSR, this catalog, which is also called PKS, replaces and improves on four earlier lists (1964 to 1966). |
parsec (pc) pârsek (#) Fr.: parsec A basic unit of astronomical distances, corresponding to a → trigonometric parallax of one second of arc. In other words, it is the distance at which one → astronomical unit (the mean radius of the Earth's orbit) subtends an angle of 1 arcsecond. 1 pc = 3.2616 → light-years = 206 265 astronomical units = 30.857 x 10^{12} km. From parallax + second. |
parselene pârâmâh Fr.: parasélène An optical phenomenon resulting from the refraction and reflection of moonlight within ice crystals in cirrus cloud; also known as paraselene, mock Moon or moondog. It is the lunar counterpart of the → parhelion. From Gk. para- "beside," → para-, + selene "moon," from Gk. selene "moon," related to selas "light, brightness, flame." |
Parseval's theorem farbin-e Parseval Fr.: théorème de Parseval A theorem relating the → Fourier coefficients to the function that they describe. It states that: (1/L) ∫ |f(x)|^{2}dx (integrated from x_{0} to x_{0} + L) = (a_{0}/2)^{2} + (1/2) Σ (a_{r}^{2} + b_{r}^{2}) (summed from r = 1 to ∞). In other words, the sum of the moduli squared of the complex Fourier coefficients is equal to the average value of |f(x)|^{2} over one period. Named after Marc-Antoine Parseval (1755-1836), French mathematician; → theorem. |
part pâr Fr.: partie 1) A portion or division of a whole that is separate or distinct; piece, fragment, fraction,
or section; constituent. M.E., from O.Fr. part "share, portion; character; dominion; side, path," from L. partem (nominative pars) "a part, piece, a share, a division; a party or faction," related to portio "share, portion," from PIE root *per- "to assign, allot;" cf. Pers. pâr, pâré "piece, part, portion, fragment;" as below. Pâr, variant pâré "piece, part, portion," parré "portion, segment (of an orange)," pargâlé, "piece, portion; patch;" (dialects Kermâni pariké "portion, half;" Tabari perik "minute quantity, particle;" Lârestâni pakva "patch;" Borujerdi parru "patch"); Mid.Pers. pârag "piece, part, portion; gift, offering, bribe;" Av. pāra- "debt," from par- "to remunerate, equalize; to condemn;" PIE *per- "to sell, hand over, distribute; to assign;" Gk. peprotai "it has been granted;" L. pars, as above; Skt. purti- "reward;" Hitt. pars-, parsiya- "to break, crumble." |
partial pâri (#), pârâl Fr.: partiel Being such in part only; not total or general; incomplete. M.E. parcial, from O.Fr. parcial, from M.L. partialis "pertaining to a part," from L. pars, → part; → -al. |
partial derivative vâxane-ye pâri Fr.: dérivée partielle The derivative of a function of two or more variables, e.g., z = f(x,y), with respect to one of the variables, the others being considered constants (denoted ∂z / ∂x). → partial; → derivative. |
partial differential equation hamugeš-e degarsâne-yi bâ vâxane-ye pâri Fr.: équation différentielle aux dérivées partielles A type of differential equation involving an unknown function (or functions) of several independent variables and its (or their) partial derivatives with respect to those variables. → partial; → differential; → equation. |
partial eclipse gereft-e pâri Fr.: éclipse partielle An eclipse that is not total. → partial lunar eclipse, → partial solar eclipse. |
partial ionization zone zonâr-e yoneš-e pâri Fr.: zone d'ionisation partielle One of several zones of the stellar interior where increased → opacity can provide the → kappa mechanism to drive → pulsations. See also → Kramers' law. In these zones where the gases are partially ionized, part of the energy released during a layer's compression can be used for further ionization, rather than raising the temperature of the gas. Partial ionization zones modulate the flow of energy through the layers of the star and are the direct cause of → stellar pulsation. The partial ionization zones were first identified by the Russian astronomer Sergei A. Zhevakin (1916-2001) in the 1950s. In most stars there are two main ionization zones. The hydrogen partial ionization zone where both the ionization of neutral hydrogen (H ↔ H^{+} + e^{-}) and the first ionization of helium (He ↔ He^{+} + e^{-}) occurs in layers with a characteristic temperature of 1.5 x 10^{4} K. The second, deeper zone is called the He^{+} partial ionization zone, and involves the second ionization of helium (He^{+}↔ He^{++} + e^{-}), which occurs deeper at a characteristic temperature of 4 x 10^{4} K. The location of these ionization zones within the star determines its pulsational properties. In fact if the → effective temperature of the star is ≥ 7500 K, the pulsation is not active, because the ionization zones will be located very near to the surface. In this region the density is quite low and there is not enough mass available to drive the oscillations. This explains the blue (hot) edge of the instability strip on the → H-R diagram. Otherwise if a star's surface temperature is too low, ≤ 5500 K, the onset of efficient convection in its outer layers may dampen the oscillations. The red (cool) edge of the instability strip is believed to be the result of the damping effect of convection. He^{+} ionization is the driving agent in → Cepheids. See also → gamma mechanism. → partial; → ionization; → zone. |
partial lunar eclipse mâhgereft-e pâri Fr.: éclipse partielle de lune A → lunar eclipse when the Earth's → umbra passes over only part of the Moon, causing only moderate darkening of the full Moon. See also → penumbral lunar eclipse. |
partial solar eclipse xorgereft-e pâri Fr.: éclipse partielle de soleil A → solar eclipse when only the → penumbra of the Moon touches the Earth. The → umbra passes either just above the North Pole or just below the South Pole, missing the Earth. |
partial truth râstini-ye pâri, ~ pârâl Fr.: vérité partielle A → truth value in → fuzzy logic where it can range between "completely true" and "completely false." |
participate pârgertidan Fr.: participer To take part, be or become actively involved. From L. paticipatus p.p. of partcipare "to share," from particeps "partaking, sharing," from part-, pars "part," → partial, + capere "to take," → concept. Pârgertidan, from pâr "part," → partial, + gertidan "to take," → concept. |
participation pârgert Fr.: participation An act or instance of participating. The fact of taking part. Verbal noun of → participate. |
<< < -ph pal par par par par pas Pau pen per per per per Per per pha Pho pho pho pho Pia Pis Pla pla pla pla Ple Pog pol pol pol pol pop pos pot Poy pre pre pre pre pri pri pro pro pro pro pro pro pro pub pul pyr > >>