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Keplerian rotation curve xam-e carxeš-e Kepleri (#) Fr.: courbe de rotation keplérienne A → rotation curve in which the speed of the orbiting body is inversely proportional to the → square root of its distance from the mass concentrated at the center of the system. |
kilonova kilonovâ Fr.: kilonova A fast-evolving → supernova-like phenomenon resulting from the → merger of compact, binary objects such as two → neutron stars or a neutron star and a → black hole. A kilonova represents an → electromagnetic counterpart to → gravitational waves. Also called → macronova. A simple model of the phenomenon was put forward by Li and Paczynski (1998, ApJL 507, L59). The kilonova phenomenon can last between days and weeks following the merger. Within the small volume of space where a merger occurs, the combination of a huge amount of energy, and a large number of neutrons, is the instigator for the → r-process. The high density favors this rapid → neutron capture by nuclei, leading to the formation of new → chemical elements with high → atomic numbers and high → atomic weights. Many elements heavier than → iron form in these environments, including many rare elements, most notably → platinum (atomic number 78) and → gold (atomic number 79). The decay of heavy atomic nuclei leads to the radioactive heating and a release of electromagnetic radiation. The heat cannot easily escape as radiation, because of the high opacity of the ejected material. The heat is radiated thermally, heating up the nearby matter, which can be then seen in the → near-infrared. It was long thought that the r-process could also occur during core-collapse supernovae, but the density of neutrons within supernovae appears to be too low. The first indication of a kilonova following a short GRB came from the extensive follow-up of GRB 130603B, which was one of the nearest and brightest short GRBs ever detected, and also the first short GRB with an optical afterglow spectrum. The first kilonova found to be associated with a gravitational waves was detected in the study of → GW170817. The term kilonova was introduced by Metzger et al. (2010, MNRAS 406, 2650), who argued that the peak luminosities of neutron star merger transients are typically ~ few × 10^{41} erg s^{-1}, or a factor of ~ 10^{3} larger than the → Eddington luminosity for a solar mass object. They therefore dubbed these events kilonovae; from → kilo-; → nova. |
klystron klistron (#) Fr.: klystron An electron tube for converting direct-current energy into radio frequency energy by alternately speeding up and slowing down the electrons. It is used as a microwave amplifier or oscillator in radar and high-frequency radio work. From Gk. kluzein, klus- "to wash, break over" + -tron. |
Kolmogorov constant pâyâ-ye Kolmogorov (#) Fr.: constante de Kolmogorov The proportionality constant C in Kolmogorov's mathematical analysis of → turbulence which states that the spectral energy E(k) in the range of turbulent scales is E(k) =C ε^{2/3} k^{-5/3}, where k represents the → wave number (inversely proportional to the wavelength or → eddy size), and ε is the average energy dissipation per unit mass in the fluid. Experimental measurements give C close to 1.5. Andrei Nikolaevich Kolmogorov (1903-1987), a prominent Soviet mathematician, who made major advances in different scientific fields, mainly probability theory, topology, turbulence, classical mechanics, and computational complexity; → constant. |
Kronecker delta deltâ-ye Kronecker (#) Fr.: delta de Kronecker The function δ^{i}_{k} of two variables i and j defined by δ^{i}_{k} = 1 if i = j, and δ^{i}_{k} = 0 if i ≠ j. Leopold Kronecker (1823-1891), a German mathematician; delta, Gk. letter of alphabet. |
krypton kripton (#) Fr.: krypton A colorless, odorless, highly un-reactive gaseous chemical element and a member of the inert gas family. Symbol Kr; atomic number 36; atomic weight 83.80; melting point -156.6°C; boiling point -152.3°C. Krypton, from Gk. kryptos "concealed, hidden". It was discovered in liquefied atmospheric air by the Scottish chemist William Ramsay and the English chemist Morris William Travers in 1898. |
lagoon mordâb (#) Fr.: lagune 1) A body of seawater that is almost completely cut off from the ocean by a barrier beach. Lagoon, from Fr. lagune, from It. laguna "pond, lake," from L. lacuna "pond, hole," from lacus "pond;" → nebula. Mordâb "lagoon," literally "dead water," from mord, mordé "dead"
+ âb "water." |
Lagoon Nebula (M8, NGC 6523) miq-e mordâb (#) Fr.: nébuleuse de la lagune A giant → H II region lying in the direction of → Sagittarius about 5,000 → light-years away. It represents a giant cloud of interstellar matter which is currently undergoing star formation, and has already formed a considerable cluster of young stars (NGC 6530). |
Lagrange's equations hamugešhâ-ye Lagrange Fr.: équation de Lagrange A set of second order → differential equations for a system of particles which relate the kinetic energy of the system to the → generalized coordinates, the generalized forces, and the time. If the motion of a → holonomic system is described by the generalized coordinates q_{1}, q_{2}, ..., q_{n} and the → generalized velocities q^{.}_{1}, q^{.}_{2}, ..., q^{.}_{n}, the equations of the motion are of the form: d/dt (∂T/∂q^{.}_{i}) - ∂T/∂q^{.}_{i} = Q_{i} (i = 1, 2, ..., n), where T is the kinetic energy of the system and Q_{i} the generalized force. → Lagrangian; → equation. |
Lagrangian function karyâ-ye lâgrânž (#) Fr.: Lagrangien, fonction de Lagrange A physical quantity (denoted L), defined as the difference between the → kinetic energy (T) and the → potential energy (V) of a system: L = T - V. It is a function of → generalized coordinates, → generalized velocities, and time. Same as → Lagrangian, → kinetic potential. → Lagrangian; → function. |
Lambda Orionis Lâmbdâ-Šekârgar, ~-Oryon Fr.: Lambda (λ) Orionis Same as → Meissa. Lambda (λ), a Greek letter used in the → Bayer designation of star names. |
Landau resonance bâzâvâyi-ye Landau Fr.: résonance de Landau For parallel propagating → electrostatic waves in a → plasma, the → resonance which occurs when the particle velocity equals the parallel phase velocity of the wave. → Landau damping; → damping. |
Lane-Emden equation hamugeš-e Lane-Emden Fr.: équation de Lane-Emden A second-order nonlinear → differential equation that gives the structure of a → polytrope of index n. Named after the American astrophysicist Jonathan Homer Lane (1819-1880) and the Swiss astrophysicist Robert Emden (1862-1940); → equation |
Langevin equation hamugeš-e Langevin Fr.: équation de Langevin Equation of motion for a weakly ionized cold plasma. Paul Langevin (1872-1946), French physicist, who developed the theory of magnetic susceptibility of a paramagnetic gas; → equation. |
language paleontology pârinšenâsi-ye zabâni Fr.: paléontologie linguistique An approach in which terms reconstructed in the → proto-language are used to make inferences about its speakers' culture and environment. → language;→ paleontology. |
Laplace resonance bâzâvâyi-ye Laplace Fr.: résonance de Laplace An → orbital resonance that makes a 4:2:1 period ratio among three bodies in orbit. The → Galilean satellites → Io, → Europa, → Ganymede are in the Laplace resonance that keeps their orbits elliptical. This interaction prevents the orbits of the satellites from becoming perfectly circular (due to tidal interactions with Jupiter), and therefore permits → tidal heating of Io and Europa. For every four orbits of Io, Europa orbits twice and Ganymede orbits once. Io cannot keep one side exactly facing Jupiter and with the varying strengths of the tides because of its elliptical orbit, Io is stretched and twisted over short time periods. This commensurability was first pointed out by Pierre-Simon Laplace, → Laplace; → resonance. |
Laplace's demon pari-ye Laplace Fr.: démon de Laplace An imaginary super-intelligent being who knows all the laws of nature and all the parameters describing the state of the Universe at a given moment can predict all subsequent events by virtue of using physical laws. In the introduction to his 1814 Essai philosophique sur les probabilités, Pierre-Simon Laplace puts forward this concept to uphold → determinism, namely the belief that the past completely determines the future. The relevance of this statement, however, has been called into question by quantum physics laws and the discovery of → chaotic systems. |
Laplace's equation hamugeš-e Laplace Fr.: équation de Laplace A → linear differential equation of the second order the solutions of which are important in many fields of science, mainly in electromagnetism, fluid dynamics, and is often used in astronomy. It is expressed by: ∂^{2}V/ ∂x^{2} + ∂^{2}V/ ∂y^{2} + ∂^{2}V/ ∂z^{2} = 0. Laplace's equation can more concisely expressed by: ∇^{2}V = 0. The function V may, for example, be the potential at any point in the electric field where there is no free charge. The general theory of solutions to Laplace's equation is known as potential theory. |
Larson relation bâzâneš-e Larson Fr.: relation de Larson An → empirical relationship between the internal → velocity dispersion of → molecular clouds and their size. The velocity dispersions are derived from molecular → linewidths, in particular those of → carbon monoxide. It was first established on star forming regions and found to be: σ (km s^{-1}) = 1.10 L (pc)^{0.38}, where σ is the velocity dispersion and L the size. The relation holds for 0.1 ≤ L ≤ 100 pc. More recent set of cloud data yield: σ (km s^{-1}) = L (pc)^{0.5}. This relation indicates that larger molecular clouds have larger internal velocity dispersions. It is usually interpreted as evidence for → turbulence in molecular clouds. Possible sources of interstellar turbulence include the following processes operating at various scales: galactic-scale (→ differential rotation, → infall of extragalactic gas on the galaxy), intermediate-scale (expansion of → supernova remnants, → shocks, → stellar winds from → massive stars), and smaller-scale processes (→ outflows from → young stellar objects). First derived by Richard B. Larson, American astrophysicist working at Yale University (Larson, 1981, MNRAS 194, 809). See Falgarone et al. (2009, A&A 507, 355) for a recent study; → relation. |
Larson-Penston solution luyeš-e Larson-Penston Fr.: solution de Larson-Penston The analytical solution to the → hydrodynamic equations describing the → collapse of an → isothermal sphere. The Larson-Penston solution is → self-similar for a purely dynamical isothermal collapse with spherical symmetry. It corresponds to the collapse prior to the formation of a → protostar, and thus is suitable for the study of → pre-stellar cores. The Larson-Penston solution was extended by Shu (1977) to obtain a whole family of solutions for this problem. Named after R. B. Larson (1969, MNRAS 145, 271) and M. V. Penston (1969, MNRAS 144, 425), who simultaneously, but independently, did this study. |
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