Fr.: modèle de pointage
A mathematical model that reproduces the diurnal rotation of the Earth and is used to direct a telescope toward a precise position on the sky.
model-e estândé, tarzâl-e ~
Fr.: modèle standard
The accepted but possibly incomplete theoretical framework which usually describes a set of phenomena. For example, the model that describes the origin of the Universe, or the model concerned with the processes in the interior of the Sun.
standard model of particle physics
model-e estânde-ye fizik-e zarre-yi
Fr.: modèle standard de la physique des particules
The theory developed since the 1970s, which is based on the theories and discoveries since the 1930s, and aims at explaining the fundamental structure of matter. According to the standard model, everything in the universe is made from a few basic building blocks called fundamental particles, governed by four fundamental forces. The particles occur in two basic types, called quarks and leptons. Three of the four fundamental forces (except gravity) and their carrier particles are included in the Standard Model. The Standard Model has successfully explained almost all experimental results and precisely predicted a wide variety of phenomena. Over time and through many experiments, the Standard Model has become established as a well-tested physics theory.
stellar atmosphere model
model-e javv-e setâré
Fr.: modèle d'atmosphère stellaire
A model that computes the radiation field crossing the boundary layers of a star at all frequencies. The parameters used for the characterization of a stellar atmosphere model are: → effective temperature, → surface gravity, and → metallicity.
→ stellar; → atmosphere; → model.
A simplified model that succeeds in capturing and furthering our understanding of one particular aspect of a physical situation, but which does not manage to describe all important aspects of that situation (Carl H. Brans).
turbulent core model
model-e maqze-ye âšubnâk
Fr.: modèle de cœur turbulent
A star formation scenario whereby → massive stars form from gravitationally bound → pre-stellar cores, which are supersonically → turbulent and in approximate pressure equilibrium with the surrounding protocluster medium. The high → accretion rates that characterize such media allow accretion to overcome the radiation pressure due to the luminosity of the star. The core is assumed to → collapse via an → accretion disk to form a single star or binary. The core density structure adopted is ρ ∝ r-k, with k = 1.5 set from observations. This choice affects the evolution of the accretion rate, which increases linearly with time. The high densities in regions of massive-star formation lead to typical time scales for the formation of a massive star of about 105 years (McKee & Tan 2003, ApJ 585, 850).
Fr.: modèle tychonique
A hybrid of → geocentric and → heliocentric systems in which the Earth remains at the center, with the → Sun orbiting the Earth, but with all the other planets revolving about the Sun. Moreover, the fixed stars were located in a crystalline shell centered on the Earth.
Related to the Danish astronomer Tycho Brahe (1546-1601), who imagined the model; → model.
Fr.: modèle non blanketé
A stellar atmosphere model which ignores metals and their physical effects. → line-blanketed model; → line blanketing.
→ un-; → blanketed model.
model-e bâd-e X
Fr.: modèle de vent X
A → magnetocentrifugal model for → accretion and → outflow in → protostars, which considers the interaction between the → magnetosphere of a → young stellar object and an & rarr; accretion disk. The model assumes that the → magnetic field originates from the protostar, and the outflow is driven from a small region near the inner edge of the disk, called X, where the inner disk corotates with the star (→ corotation radius). The accretion disk is → truncated in its central region, that is the disk does not extend to the protostar. The matter spiraling toward the protostar is either funneled by the magnetic field connecting the star to the disk, or blown outward under the effect of → centrifugal force. The X-wind model is able to account for many observations in one fairly self-consistent model. The observations include time variable accretion/wind phenomena in → T Tauri stars, the slow rotation rates of T Tauri stars, protostellar X-ray activity, and a number of the properties of → bipolar jets and → molecular outflows (F. H. Shu et al., 2000, in Protostars and Planets IV, V. Mannings et al. (eds.), Tucson: Univ. Arizona Press, 789).
X, referring to a configuration of the magnetic field where the lines of force intersect with an "X" shape; → wind; → model.