An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



Number of Results: 15 Search : rotational
magnetorotational instability (MRI)
  ناپایداری ِ مغنات-و-چرخشی   
nâpâydâri-ye meqnâtocarxeši

Fr.: instabilité magnétorotationnelle   

An instability that arises from the action of a weak → poloidal magnetic field in a → differentially rotating system, such as a → Keplerian disk. The MRI provides a mechanism to account for the additional outward → angular momentum transport. To put it simply, the → frozen magnetic field line acts as a spring connecting two radially neighboring fluid parcels. In a Keplerian disk the inner fluid parcel orbits more rapidly than the outer, causing the spring to stretch. The magnetic tension forces the inner parcel to slow down reducing its angular momentum by moving it to a lower orbit. The outer fluid parcel is forced by the spring to speed up, increase its angular momentum, and therefore move to a higher orbit. The spring tension increases as the two fluid parcels grow further apart, and eventually the process runs away. The MRI was first noted in a non-astrophysical context by E. Velikhov in 1959 when considering the stability of → Couette flow of an ideal hydromagnetic fluid. His result was later generalized by S. Chandrasekhar in 1960. The MRI was rediscovered by Balbus and Hawley 1991 (ApJ 376, 214) who demonstrated that this instability does indeed manifest itself in → accretion disks, and could account for the turbulent mixing needed to explain the observed mass → accretion rates.

magneto-; → rotational; → instability.

projected rotational velocity
  تندای چرخشی ِ فراشانده   
tondâ-ye carxeši-ye farâšândé

Fr.: vitesse rotationnelle projetée   

The → angular velocity of a star deduced from the → rotational broadening of its → spectral lines. It is expressed as v sini, where i is the → inclination of the rotational axis with respect to the normal to the → plane of the sky. The real equatorial rotational velocity can be determined only if the inclination of the rotational axis is known.

Projected, p.p. of → project; → rotational; → velocity.

carxeši (#)

Fr.: rotationnel   

Of or pertaining to → rotation.

Rotational, adj., from → rotation + → -al.

rotational angular momentum
  جنباک ِ زاویه‌ای ِ چرخشی   
jonbâk-e zâviyeyi-ye carxeši

Fr.: moment angulaire rotationnel, moment cinétique ~   

The → angular momentum of a body rotating about an axis. The rotational angular momentum of a solid homogeneous sphere of mass M and radius R rotating about an axis passing through its center with a period of T is given by: L = 4πMR2/5T.

rotational; → angular; → momentum.

rotational axis
  آسه‌ی ِ چرخش   
âse-ye carxeš

Fr.: axe de rotation   

An imaginary line about which a solid object rotates. Same as → rotation axis and → axis of rotation.

rotational; → axis.

rotational broadening
  پهنش ِ چرخشی   
pahneš-e carxeši

Fr.: élargissement rotationnel   

The spectral line broadening caused by stellar rotation. Light from two rims of the star will be Doppler shifted in opposite directions, resulting in a line broadening effect. The line broadening depends on the inclination of the star's pole to the line of sight. The derived value is a function of ve. sini, where ve is the rotational velocity at the equator and i is the inclination, which is not always known. The fractional width (Δλ/λ) is of the order of 10-3 for B stars.

rotational; → broadening.

rotational Eddington limit
  حد ِ ادینگتون ِ چرخشی   
hadd-e Eddington-e carxeši

Fr.: limite d'Eddington avec rotation   

The → Eddington limit of luminosity for a → rotating star in which both the effects of → radiative acceleration and rotation are important. Such objects mainly include → OB stars, → LBV, → supergiants, and → Wolf-Rayet stars. It turns out that the maximum permitted luminosity of a star is reduced by rotation, with respect to the usual Eddington limit (Maeder & Meynet, 2000, A&A, 361, 159).

rotational; → Eddington limit.

rotational energy
  کاروژ ِ چرخشی   
kâruš-e carxeši

Fr.: énergie rotationnelle   

The → kinetic energy due to the → rotation of and object. Rotational energy is part of the total kinetic energy of the body. It is given by: (1/2)Iω2, where I is the → moment of inertia and ω is the → angular velocity. Same as → angular kinetic energy.

rotational; → energy.

rotational mixing
  آمیزش ِ چرخشی   
âmizeš-e carxeši

Fr.: mélange rotationnel   

A consequence of → stellar rotation that deforms the star, triggers instabilities (→ shear turbulence and → meridional currents) leading to → transport of chemical species in the star. The efficiency of rotational mixing (measured for instance by the degree of surface → enrichments at a given → evolutionary stage) increases when the initial mass and rotation increase. This efficiency increases also when the initial → metallicity decreases. This is due to the fact that when the metallicity is lower, the stars are more compact. This makes the → gradients of the → angular velocity steeper in the stellar interiors. Steeper gradients produce stronger shear turbulence and thus more mixing. Rotational mixing can bring to the surface heavy elements newly synthesized in the stellar core. Rotation thus produces an increase of the → opacity of the outer layers and activates strong → mass loss through → radiatively driven winds. This effect may be responsible for the loss of large fractions of the initial mass of the star (Meynet et al. 2007, arXiv:0709.2275).

rotational; → mixing.

rotational modulation (ROT)
  دگر‌آهنگش ِ چرخشی   
degarâhangeš-e carxeši

Fr.: modulation rotationnelle   

A very small variation in the surface brightness of a single star due to its rotation. Several types of stars are known to have photospheric spots. Brightness variation occurs as rotation carries star spots or other localized activity across the line of sight.

rotational; → modulation.

rotational motion
  جنبش ِ چرخشی   
jonbeš-e charkheshi

Fr.: mouvement de rotation   

Of a → rigid body, a motion in which there are always two points of the body which remain motionless.

rotational; → motion.

rotational period
  دوره‌ی ِ چرخش   
dowre-ye carxeš

Fr.: période de rotation   

rotation period.

rotational; → period.

rotational transition
  گذرش ِ چرخشی   
gozareš-e carxeši

Fr.: transition rotationnelle   

A slight change in the energy level of a molecule due to the rotation of its constituent atoms about their center of mass.

rotational; → transition.

rotational velocity
  تندای ِ چرخشی   
tondâ-ye catxeši

Fr.: vitesse de rotation   

The velocity of a → rotational motion; same as → angular velocity.

rotational; → velocity.

vibrational-rotational transition
  گذرش ِ چرخشی-شیوشی   
gozareš-e carxeši-šiveši

Fr.: transition vibrationnelle-rotationnelle   

A slight change in the → energy level of a → molecule due to → vibrational transition and/or → rotational transition.

vibrational; → rotational; → transition.