An Etymological Dictionary of Astronomy and Astrophysics
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For any rotating planetary body, a thermal limit beyond which the → rotational velocity at the equator intersects the → Keplerian orbital velocity. Beyond this corotation limit, a hot planetary body forms a structure, called a → synestia, with a corotating inner region connected to a disk-like outer region. Beyond this limit a body cannot have a single → angular velocity. It can instead exhibit a range of morphologies with disk-like outer regions. The (CoRoL is a function that depends upon the composition, thermal state, → angular momentum and mass of a body (Simon J. Lock nd Sarah T. Stewart, 2017, arXiv:1705.07858v1).

→ co-; → rotational; → limit.

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.

A → tumbling motion of an → asteroid. See also → tumbling asteroid.

→ non-; → principal axis; → rotational; → motion.

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.

Of or pertaining to → rotation.

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

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.

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

→ rotational; → axis.

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 v_e. sini, where v_e 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.

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.

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.

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.

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.

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

→ rotational; → motion.

→ rotation period.

→ rotational; → period.

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.

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

→ rotational; → velocity.

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

→ vibrational; → rotational; → transition.