adiabatic index dišan-e bidarrow Fr.: indice adiabatique Of a gas, the ratio of its → specific heat at constant pressure to its specific heat at constant volume: γ = C_{P}/ C_{V}. |
braking index dišan-e legâmeš Fr.: indice de freinage A parameter indicating the rate at which a → pulsar slows down. Neutron stars are powered by → rotational energy and lose energy by accelerating particle → winds and by emitting → electromagnetic radiation. The → rotation frequency, Ω, thus decreases with time and this slowdown is usually described by the relation Ω^{.} = - kΩn, where k is a positive constant which depends on the → moment of inertia and the → magnetic dipole moment of the → neutron star and n is the braking index. Conventionally, the braking index is derived by differentiation of the above equation, yielding n = ΩΩ^{..} / Ω^{.2}. In a highly simplified model in which the spin-down torque arises from dipole radiation at the rotation frequency, one expects n = 3 (Johnston, S., Galloway, D., 1999, arXiv:astro-ph/9905058). |
calcium break index dišan-e gosast-e kalsiom Fr.: indice de la coupure de calcium The strength of the → calcium break, as measured from the fluxes in the intervals 3750-3950 Å and 4050-4250 Å. It is given by the expression Ca-break[%] = 100 · (f_{upper} - f_{lower})/f_{upper}, where f_{upper} and f_{lower} are the mean fluxes measured in the 3750-3950 Å and 4050-4250 Å bands, respectively, in the rest frame (Dressler & Shectman 1987, AJ 94, 899). |
color index dišan-e rang Fr.: indice de couleur The difference between the → apparent magnitude of a star measured at one standard wavelength and the apparent magnitude at another longer, standard wavelength, allowing the quantitative measure of a star's color. |
dispersive index dišan-e pâšeši Fr.: indice de dispersion The reciprocal of the → dispersive power. → dispersive; → index. |
index 1, 2, 3) dišan; 4) fehrest (#) Fr.: 1, 2) indice; 3) index 1) Math.: A number or symbol, often written as a
→ subscript or → superscript
to a mathematical expression, that indicates an operation
to be performed, an ordering relation, or a use of the associated expression. Index, from L. index "forefinger, pointer, sign," literally "anything which points out," from indicare "point out, show," from in- "in" + dicare "to proclaim," from stem of dicere "to speak, to say;" PIE base *deik- "to point out" (cf. Av. daēs- " to show;" Skt. dic- "to point out, show;" Gk. deiknynai "to prove;" O.H.G. zeigon; Ger. zeigen "to show;" O.E. teon "to accuse," tæcan "to teach"). 1) Dišan, from diš-, simple aorist of Av. daēs-
"to show," as above, + suffix -an. |
Index Catalogue (IC) kâtâlog-e fehrest Fr.: Index Catalogue Either of two catalogues of non-stellar objects, which serve as supplements to the → New General Catalogue. |
index of refraction disšan-e šekast Fr.: indice de réfraction Same as → refractive index. → index; → refraction. |
multipole index dišan-e basqotbé Fr.: indice multipolaire A variable used in → spherical harmonic expansions. Each spherical harmonic is characterized by its multipole index l: l = 0 for a → monopole, l = 1 for a → dipole, and so on. It is used in particular to describe the → cosmic microwave background anisotropy: ΔT/T_{0} (θ,φ) = Σ a_{lm}Y_{lm}(θ,φ), where θ and φ are the → spherical polar coordinates, Y_{lm} is the → spherical harmonic functions, and the sum runs over l = 1, 2, ..., ∞ and m = -l, ..., l, where the multipole index l corresponds to angular scales ≅ 180°/l. |
polytropic index dišan-e bolgašt Fr.: index polytropique A number appearing in the equation describing a → polytropic process. → polytropic; → index. |
Q index dišan-e Q Fr.: indice Q In the Johnson → UBV system, a reddening-free parameter which is related to the → effective temperature of stars and thus provides a useful, but rough, discriminant for → spectral types. It is expressed as: Q = (U - B) - 0.72 (B - V). → index. |
refractive index dišan-e šekast Fr.: indice de réfraction Of any optical medium, the ratio of the → speed of light in vacuum (c) to that in the medium (v): n = c/v. The refractive index for vacuum, by definition, is 1. The refractive index of air is 1.00029 at standard temperature (25 °C) and pressure (1 atm). The refractive index of a medium depends on the wavelength of refracted wave. With light waves, n increases as the wavelength decreases. → Snell's law can be used to derive n. Same as → index of refraction. Refractive, pertaining to → refraction; → index. |
relative refractive index dišan-e šekasti-ye bâzâni Fr.: indice de réfraction relatif The ratio of the sine of the angle of incidence to the sine of the angle of refraction for a ray passing out of one of the media into the other. → relative; → refractive; → index. |
spectral index dišan-e binâbi Fr.: indice spectral 1) The → exponent
of the → frequency on which depends
the intensity of the → continuum emission, that is:
F_{ν}∝ ν^{α}.
The exponent (α) typically
takes positive values from 0 to 2 for → thermal emission,
while → non-thermal emission, such as
→ synchrotron radiation,
leads to negative values of the spectral index ranging from about -0.5 to -1.5. |
variability index dišan-e vartandegi Fr.: indice de variabilité A measure of variability of a star (such as Welch-Stetson variability index) which combines the information from two passbands assuming that changes in the star's luminosity occur nearly simultaneously at all optical wavelengths. → variability; → index. |
volcanic explosivity index (VEI) dišan-e oskaftandegi-ye âtašfešâni Fr.: indice d'explosivité volcanique A logarithmic scale, ranging from 1 to 8, used to measure the intensity of volcano eruptions. The VEI is based on several factors: the degree of fragmentation of the volcanic products released by the eruption, the amounts of sulfur-rich gases that form stratospheric aerosols, the volume of the eruptions, their duration, and the height is reached. The largest eruptions (8) produce an amount of bulk volume of ejected → tephra of ~ 1,000 km^{3}. → volcanic; → explosivity; → index. |