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.

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.

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 = 4πMR^{2}/5T.

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.

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).

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).

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.