Fr.: diffusion multiple
A process of → radiative transfer in which more than one → scattering event may be of importance before → transmission, → reflection, or → absorption. In → radiation-driven winds photon scattering can take place in different → spectral lines. Each scattering occurs in a different spectral line, and successive scatterings occur at lower energies (longer wavelength). The standard theory of line driving (→ CAK model) assumes that photons can be scattered only once in the wind, which is a reasonable assumption for normal → O stars. In → Wolf-Rayet stars, where photons evolve in an atmosphere with a strong → ionization stratification, multiple scattering is important. Indeed the strength of W-R winds appears to exceed the single scattering limit.
halqehâ-ye Newton (#)
Fr.: anneaux de Newton
Fr.: diffusion incohérente
The absorption of a photon and its re-emission at a different frequency (in the observer's frame of reference) by scattering atoms.
Fr.: tore obscurcissant
A structure of dust and gas postulated to surround the central → black hole in an → active galactic nucleus (AGN). The presence of an obscuring torus allows the unification of the two main types of AGNs containing a → broad-line region (Type I) and a → narrow-line region (Type II), respectively. In this unified model, the two types represent the same sort of object, the appearance of which depends on the viewer's → line of sight. The best evidence for this model comes from spectropolarimetry observations of some type II AGNs in which broad → emission lines are seen in → polarized light, as would happen if the broad-line region truly were hidden, and the light were being reflected off the torus and into the viewer's line of sight.
Fr.: énergie de parité
In nuclear physics, the extra binding energy associated with pairs of nucleons of the same kind. This quantity expresses the fact that nuclei with odd numbers of neutrons and protons have less energy and are less stable than those with even numbers of neutrons and protons.
Fr.: anneau de PhœbÃ©
A giant ring around Saturn spanning an area of space from a distance of ~ 128 Saturn equatorial radii, RS (60,330 km) to 207 RS, that is, from about 7.7 × 106 to 12.4 × 106 km from the planet. Its vertical thickness is about 40 RS. The Phoebe ring was detected in 2009 using NASA's infrared → Spitzer Space Telescope. The Phoebe ring is made up mainly of dust particles about 10 to 20 microns in size, or about one-tenth to one-fifth the average width of a human hair. Rocks that are the size of soccer balls or larger with diameters of more than about 20 cm make up no more than about 10 percent of the ring (Verbiscer et al., 2009, Nature, 461, 1098).
photon tiring limit
hadd-e xastegi-ye foton
Fr.: limite par fatigue du photon
The maximum → mass loss rate of a star when the → wind luminosity equals the total available → stellar luminosity. The mechanical luminosity of the wind at infinity is given by: Lwind = Mdot (v∞2/2 + GM/R) = Mdot (v∞2/2 + vesc2/2). For Lwind = L*, the mass loss rate is Mdotmax = 2L*/(v∞2 + vesc2). Following Owoki & Gayly (1997), Mdottir is the maximum mass loss rate when the wind just escapes the gravitational potential, with v∞ tending toward zero. Mdottir is much larger than typical mass loss rates from → line-driven winds, where the driving lines become saturated with increasing density limiting the wind mass loss rates to about 10-4 Msun yr-1 in even the most luminous stars.
seri-ye Pikering (#)
Fr.: série de Pickering
A series of → spectral lines of → singly ionized helium, observed in very hot → O-type and → Wolf-Rayet stars associated with transitions between the → energy level with → principal quantum number n = 4 and higher levels: n = 4-5 (10124 Å), n = 4-7 (5412 Å), n = 4-9 (4541 Å), n = 4-9 (4522 Å), and n = 4-11 (4200 ˚). The 4-6 (6560 Å) and 4-8 (4859 Å) transitions were originally not included in this series because they coincided with the hydrogen → Balmer series of lines and were thus obscured.
In honor of Edward C. Pickering (1846-1919), American astronomer and physicist; → series.
planetary ring system
râžmân-e halqehâ-ye sayyâre-yi
Fr.: système d'anneaux planétaires
→ Interplanetary dust and other small particles organized into thin, flat rings encircling a planet. The most spectacular planetary rings known are those around → Saturn, but the other three → giant planets of the solar system (→ Jupiter, → Uranus, and → Neptune) have their own ring systems.
Fr.: anneaux de Priestley
Joseph Priestley (1733-1804), English chemist, author, and clergyman; → ring.
A model of radiative transfer that ignores forward scattering of photons; assuming forward-scattered light as un-scattered.
parâkaneš-e Raman (#)
Fr.: diffusion Raman
The scattering of monochromatic light (visible or ultraviolet) by molecules in which the scattered light differs in wavelength from the incident light. It is caused by the light's interaction with the vibrational or rotational energy of the medium's scattering molecules.
Fr.: diffusion Rayleigh
The scattering of light by → particles
of size small compared with the → wavelength of
light. The intensity of the light scattered by unit volume of the medium at an
angle θ to the direction of propagation of the incident light is:
Iθ = 8 π4α2 N I0
(1 + cos2θ)/(R2λ4),
where α is the → molecular polarizability,
N is the number of scattering molecules,
I0 is intensity of the incident light, λ is the wavelength, and
R is the distance from the scatterer.
The fourth power dependence on wavelength means that blue light is
much more strongly scattered than red light from a medium containing very fine particles.
The air molecules, mostly → nitrogen (78%) and
→ oxygen (21%) are some 1,000 times larger than
→ visible light wavelengths.
This accounts for the bluish appearance of smoke and of clear sky when the observation is not
along the direction of illumination. The setting Sun, seen through a considerable
thickness of atmosphere appears reddish because long wave radiation predominates in
the transmitted light.
Fr.: diffusion résonante
The absorption and prompt re-emission of photons of a particular wavelength by an atom. In this process, a photon of exactly the right wavelength (i.e. energy) excites an electron in the atom from one energy level to another. The electron then drops back down to its original energy level more or less immediately, emitting a photon of almost identical energy to the one that was absorbed in the first place, but in some random direction. Resonant scattering applies only to line radiation, unlike other forms of scattering which are of continuous radiation (Oxford Dictionary of Astronomy, 2 ed.).
niru-ye, bâzgardân, ~ bâzsâz
Fr.: force de rappel
A force that comes into play after a system is perturbed away from the equilibrium, tending to bring the system back the equilibrium position. For example, when a pendulum is displaced from its equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position. The restoring force combined with the pendulum's mass causes it to oscillate about the equilibrium position.
1) halqé (#); 2) zang (#)
Fr.: 1) anneau; 2) sonnerie
1) A circular band of something. → gossamer ring.
O.E. hring "circular band" (cf. O.N. hringr, Ger. Ring), literally "something curved," from PIE base *(s)ker- "to turn, bend."
1) Halqé, loan from Ar. Halqat "ring, hoop, circle."
Fr.: galaxie en anneau
A galaxy with a ring-like appearance around the central luminous center. The ring consists of massive, relatively young bright stars. It is believed that ring galaxies result from the head-on collision of two different galaxies.
Fr.: longitude de l'anneau
Of → Saturn, the angle measured with respect to the sub-observer point (a line connecting the observer to Saturn) in the direction of the orbital motion.
Fr.: Nébuleuse de l'Anneau
A bright → planetary nebula in the constellation → Lyra, also called M57 or NGC 6720. In small telescopes it has the appearance of a slightly elliptical luminous ring around a central hot star (15th magnitude). The radius is one-third of a → light-year, and the nebula is about 2,000 light-years away.