Fr.: paramètre d'accéleration
A measure of the departure from a constant rate of the acceleration of the Universe, expressed by: q(t) = R(t)R ..(t)/R .2(t), where R(t) represents the size of the Universe at time t. Traditionally, a negative sign is inserted in the above equation for the → deceleration parameter.
angular momentum parameter
pârâmun-e jonbâk-e zâviye-yi
Fr.: paramètre de moment angulaire
Fr.: paramètre de décéleration
A parameter designating the rate at which the expansion of the Universe would slow down owing to the braking gravitational effect of the matter content of the Universe. It is expressed by: q(t) = -R(t)R ..(t)/R .2(t), where R(t) represents the size of the Universe at time t. See also → expansion parameter; compare with → acceleration parameter.
Fr.: paramètre de densité
One of the four terms that describe an arranged version of the
→ Friedmann equations. They are all time dependent.
Fr.: paramètre d'Eddington
A → dimensionless parameter indicating the degree to which a star is close to the → Eddington limit. It is expressed as Γ = L / LEdd = κ L / (4πGMc), where L and M are the star luminosity and mass respectively, κ is the opacity, c the speed of light, and G the → gravitational constant. At the Eddington limit, Γ = 1, the star would become unbound. Because stellar luminosity generally scales with a high power of the stellar mass (L∝ M3-4), → massive stars with M larger than 10 Msun generally have electron Eddington parameters of order Γ ≅ 0.1-1.
After Arthur Stanley Eddington (1882-1944), prominent British astrophysicist; → parameter.
effective Eddington parameter
pârâmun-e Eddington-e oskarmand
Fr.: paramètre d'Eddington effectif
The effective value of the → Eddington parameter in a non-homogeneous system (porous opacity).
equation of state parameter
pârâmun-e hamugeš-e hâlat
Fr.: paramètre de l'équation d'état
In cosmology, a → dimensionless parameter introduced by the → equation of state representing the ratio of the pressure to the energy density of a fluid, such as the → dark energy: w = p/ρ. The → deceleration or → acceleration of an → expanding Universe depends on this parameter (→ accelerating Universe). A number of numerical values of this parameter are as follows: for the → cosmological constant: w = -1, for → non-relativistic matter (present-day → baryons): w = 0, and for → relativistic matter (photons, neutrinos): w = +1/3. Together with Ω(dark energy) and Ω(matter), w provides a three-parameter description of the dark energy. The simplest parametrization of the dark energy is w = constant, although w might depend on → redshift.
Fr.: paramètre d'expansion
A → scale factor that relates the size of the Universe R = R(t) at time t to the size of the Universe R0 = R(t0) at time t0 by R = aR0. The expansion parameter represents the history of expansion of the Universe.
force multiplier parameter
pârâmun-e bastâgar-e niru
Fr.: paramètre de multiplicateur de force
One of the three quantities (k, α, and δ) which are used in the → radiation-driven wind theory to express the radiation pressure due to spectral lines. These coefficients parametrize the radiation acceleration as: grad≅ k(dv/dr)αge, where ge = σeL/4πcR*2 is the radiative acceleration by electron scattering. The parameter k is dependent on the number of lines that produce the radiation pressure. The parameter α depends on the optical depth of the driving lines and varies between 0 (optically thin) and 1 (optically thick). The parameter δ describes the dependence of k on the density with k ≅ ρδ. The velocity law of radiation-driven winds depends on α and δ, but not on k. The → mass loss rate depends on k, α, and δ (Castor et al. 1975, ApJ 195, 157; Lamers et al., 1995, ApJ 455, 269 and references therein).
Fr.: paramètre de Fried
One of the parameters that characterize atmospheric → seeing. It is the diameter of the largest aperture that can be used before → turbulence starts to degrade the image quality. As the turbulence gets stronger, the Fried parameter, denoted r0, becomes smaller. The Fried parameter is wavelength dependent: r0 ∝ λ6/5. On best astronomical mountain tops it ranges between 20 and 30 cm for λ = 5000 A.
Named after David L. Fried, who defined the parameter 10 1966; → parameter.
Fr.: paramètre de Hubble
The rate pf change of the → cosmic scale factor: H(t) = (dR/dt)/R. The Hubble parameter is a time-dependent quantity and therefore is not constant. The → Hubble constant is the Hubble parameter measured today.
Fr.: paramètre d'impact
1) A measure of the distance by which a collision fails being frontal.
Fr.: paramètre d'ionisation
A ratio representing the number of ionizing photons to the number of electrons in a nebular emitting region.
mixing length parameter
pârâmun-e derâzâ-ye âmizeš
Fr.: paramètre de la longueur de mémange
In the → mixing length theory, a parameter, α, that relates the → mixing length, l, to the → pressure scale height: α = l/HP. It is usually supposed that α is of order unity. Changes in α correspond to variations in the efficiency of the → convection, hence the transfer of heat.
normalized Hubble parameter
pârâmun-e Hubble-e hanjârvaridé
Fr.: paramètre Hubble normalisé
General: Any of a set of physical properties whose values determine the
characteristics or behavior of something. → impact parameter;
→ ionization parameter.
Parâmun, from parâ-, → para-, + mun/mân "measure," as in Pers. terms pirâmun "perimeter," âzmun "test, trial," peymân "measuring, agreement," peymâné "a measure; a cup, bowl," from O.Pers./Av. mā(y)- "to measure;" cf. Skt. mati "measures," matra- "measure;" Gk. metron "measure;" L. metrum; PIE base *me- "to measure."
Fr.: paramètre physique
Fr.: paramètre de rougissement
A dimensionless quantity characterizing the → interstellar extinction, defined by the total-to-selective extinction ratio: RV = AV/E(B-V). The typical value found for the reddening parameter in the Milky Way is RV ~ 3.1, but it is known to vary from one line of sight to another, from values as 2 to as large as 6. Very large → dust grains would produce extinction with RV → ∞.
Fr.: paramètre de Rossby
The northward variation of the Coriolis parameter, arising from the sphericity of the Earth.
Fr.: paramètre de taille
A quantity that defines the type of → scattering.