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.
radiation-driven mass loss
dastraft-e jerm az râh-e bâd-e tâbeši
Fr.: perte de masse par vent radiatif
bâd-e tâbeši, ~ tâbešzâd
Fr.: vent radiatif
The loss of matter from the → photosphere due to the acceleration imparted to the outer layers of the star by photons created inside the star. The coupling between radiation and matter creates a → radiative acceleration that may exceed the → gravity. This mechanism is particularly important in → massive stars, since the luminosity is high and therefore the number of energetic ultraviolet photons important. Same as → line-driven wind.
metrik-e Robertson-Walker (#)
Fr.: métrique de Robertson-Walker
The mathematical description of the interval (→ space-time
separation) between → events ("points" in space-time)
in a → homogeneous and
→ isotropic → Universe.
It results from an exact solution of
→ Einstein's field equations
of → general relativity.
Under the assumptions, the
Robertson-Walker interval is expressed by:
Named after Howard Percy Robertson (1903-1961), American mathematician and physicist, and Arthur Geoffrey Walker (1909-2001), British mathematician and physicist, for their contributions to physics and physical cosmology; → metric.
âšubnâki-ye zâyide-ye carxeš, darhâzidé az ~
Fr.: turbulence induite par turbulence
rotation-powered pulsar (RPP)
tapâr-e carxeš-tavân, pulsâr-e ~ ~
A → neutron star that is spinning down as a result of → torques from → magnetic dipole radiation and particle emission. RPPs derive their energy primarily from the → rotation of the neutron star. The energy from their → spin-down appears as broad-band pulsations from → radio to → gamma-ray wavelengths and as a → wind of energetic particles flowing into their surrounding → pulsar wind nebulae. Since the discovery of RPPs through their radio → pulsations in 1967, more than 2000 → radio pulsars are now known with periods ranging from a few milliseconds to several seconds (A. K. Harding, 2013, Front. Phys. 8, 679).
Fr.: spectre rotation-vibration
The spectrum of a molecule resulting from the simultaneous rotation and vibration of its constituent atoms.
Fr.: expérience de Trouton-Noble
An experiment first carried out in 1903 to reveal the absolute motion of the Earth with respect to the → ether. The experiment consists of detecting a torque on a charged parallel-plate → capacitor that was suspended so that its plates were vertical. Since the Earth moves around the Sun, the moving charges were expected to produce magnetic fields, and the resulting torque should tend to turn the capacitor bringing its plates parallel to its velocity. No such effect was observed, and the absence of the torque supports the theory of → special relativity.
Named after Frederick T. Trouton (1863-1922) and Henry R. Noble; → experiment.
weak emission-line central star (wel)
setâre-ye markazi bâ xatt-e gosili-ye nezâr
Fr.: étoile centrale à faibles raies d'émission
Fr.: effet de Wilson-Bappu
The strong correlation between the equivalent width of Ca II → H and K lines of a late-type giant or supergiant star with the absolute visual magnitude of the star.
O. C. Wilson & M. K. Vainu Bappu (1957, ApJ 125, 661); → effect.