DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

accretion, Eddington
presenceispartofthe“standardmodel”ofactive
galactic nuclei, and whose observational status
is becoming secure. Active galactic nuclei are
thought to be powered by the release of potential
gravitational energy by accretion of matter
onto a supermassive black hole. The accretion
disk dissipates part of the gravitational potential
energy, and removes the angular momentum
of the infalling gas. The gas drifts slowly
toward the central black hole. During this process,
the innermost annuli of the disk are heated
to high temperature by viscous forces, and emit
a “stretched thermal continuum”, i.e., the sum
of thermal continua emitted by annuli at different
temperatures. This view is probably valid
only in active galactic nuclei radiating below the
Eddington luminosity, i.e., low luminosity active
galactic nuclei like Seyfert galaxies. If the
accretion rate exceeds the Eddington limit, the
disk may puff up and become a thick torus supported
by radiation pressure. The observational
proof of the presence of accretion disks in active
galactic nuclei rests mainly on the detection
of a thermal feature in the continuum spectrum
(the big blue bump), roughly in agreement with
the predictions of accretion disk models. Since
the disk size is probably less than 1 pc, the disk
emitting region cannot be resolved with presentday
instruments. See accretion, active galactic
nuclei, big blue bump, black hole, Eddington
limit.
accretion, Eddington As material accretes
onto a compact object (neutron star, black hole,
etc.), potential energy is released. The Eddington
rate is the critical accretion rate where the
rate of energy released is equal to the Eddington
luminosity: G˙MEddingtonMaccretor/Raccretor =
LEddington ⇒ ˙Maccretion = 4πcRaccreting object
κ
where κ is the opacity of the material in units
of area per unit mass. For spherically symmetric
accretion where all of the potential energy
is converted into photons, this rate is the
maximum accretion rate allowed onto the compact
object (see Eddington luminosity). For
ionized hydrogen accreting onto a neutron star
(R NS = 10 km M NS = 1.4M⊙), this rate is:
1.5 ×10 −8 M⊙ yr −1 . See also accretion, Super-
Eddington.
© 2001 by CRC Press LLC
accretion, hypercritical See accretion,
Super-Eddington.
accretion, Super-Eddington Mass accretion
at a rate above the Eddington accretion limit.
These rates can occur in a variety of accretion
conditions such as: (a) in black hole accretion
where the accretion energy is carried into the
black hole, (b) in disk accretion where luminosity
along the disk axis does not affect the accretion,
and (c) for high accretion rates that create
sufficiently high densities and temperatures that
the potential energy is converted into neutrinos
rather than photons. In this latter case, due to
the low neutrino cross-section, the neutrinos radiate
the energy without imparting momentum
onto the accreting material. (Syn. hypercritical
accretion).
Achilles A Trojan asteroid orbiting at the L4
point in Jupiter’s orbit (60 ◦ ahead of Jupiter).
achondrite A form of igneous stony meteorite
characterized by thermal processing and
the absence of chondrules. Achondrites are generally
of basaltic composition and are further
classified on the basis of abundance variations.
Diogenites contain mostly pyroxene, while eucrites
are composed of plagioclase-pyroxene
basalts. Ureilites have small diamond inclusions.
Howardites appear to be a mixture of eucrites
and diogenites. Evidence from micrometeorite
craters, high energy particle tracks, and
gas content indicates that they were formed on
the surface of a meteorite parent body.
achromatic objective The compound objective
lens (front lens) of a telescope or other optical
instrument which is specially designed to
minimize chromatic aberation. This objective
consists of two lenses, one converging and the
other diverging; either glued together with transparent
glue (cemented doublet), or air-spaced.
The two lenses have different indices of refraction,
one high (Flint glass), and the other low
(Crown glass). The chromatic aberrations of
the two lenses act in opposite senses, and tend
to cancel each other out in the final image.