DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

if there exists a space-time( ˜M, ˜g), such that M
is a submanifold of ˜M with boundary I and
• ˜gab = 2 gab,>0 ∈ M
•On I,= 0 and ∇a= 0
• Any null geodetic curve in M has two
endpoints in I
• In a neighborhood of I, the space-time is
empty(orhasonlyelectromagneticfields)
asymptotic diffuse attenuation coefficient
The value of the diffuse attenuation coefficient
in the asymptotic regime; it depends on the inherent
optical properties only.
asymptotic flatness The assumption in theoretical/analytical
descriptions of gravitational
fields, that the gravitational potential goes to
zero at spatial infinity, i.e., far away from its
sources. In general relativity, the gravitational
field is reflected in curvature of spacetime, so
requiring flatness has a direct connection to requiring
vanishing gravitational effects. In situations
with a nonvanishing central mass m,
asymptotic flatness requires the metric approach
flat +O(Gm/c2r). Thus, a space-time with
Lorentzian metric g is said to be asymptotically
flat (at spatial infinity) if a set of spherical coordinates
(t,r,θ,φ) can be introduced, such that
g approaches the Minkowski tensor for larger:
lim g= diag −1, 1,r
r→+∞ 2 ,r 2 sin 2
θ .
asymptotic giant branch (AGB) star Star
of low or intermediate mass (∼ 0.8 to 5solar
masses) in the advanced evolutionary phase
where the primary energy sources are fusion of
hydrogen (by the CNO cycle) to helium and of
helium (by the triple-alpha process) to carbon in
thin shells surrounding an inert carbon-oxygen
core. The phase is important for two reasons.
First, the star develops several zones of convection
which cross back and forth so as to mix to
the surface products of the interior nuclear reactions,
including nitrogen from the CNO cycle,
carbon from the triple-alpha process, and the
products of the s process, including barium and,
© 2001 by CRC Press LLC
atmosphere
sometimes, technitium, thus confirming the occurrence
of these reactions. The longest-lived
isotope of Tchas a half life less than a million
years, showing that the reactions must be occurring
recently. Second, the star expels a wind
of up to 10 −6 to 10 −4 solar masses per year,
and this mass loss both terminates the interior
nuclear reactions and determines that the core
will become a white dwarf rather than ignitingcarbonfusion.
Thephaselastsonlyabout0.01%
of the longest, main-sequence, phase. The name
derives from the location of these stars on the
HR diagram in a diagonal strip that approaches
tangentially at high luminosity to the main red
giant branch. AGB stars are much brighter and
more extended, but cooler on the surface, than
the same stars were on the main-sequence. See
CNO cycle, convection, HR diagram, main sequence
star, red giant, s process, triple-alpha
process, white dwarf.
asymptotic regime In oceanography, depths
at which the rate of decay with depth of all radiometric
variables, given by the asymptotic diffuse
attenuation coefficient, depends only on the inherent
optical properties.
Aten asteroid A member of a class of asteroids
with Venus-crossing orbits, in contrast
to the majority of asteroids that orbit between
Mars and Jupiter. There are 30 known members
of the Aten class.
Atlas A moon of Saturn, also designated
SXV. It was discovered by R. Terrile in 1980
in Voyager photos. Its orbit has an eccentricity
of 0, an inclination of 0.3 ◦ , and a semimajor axis
of 1.38×10 5 km. Its size is roughly 20×10 km,
and its mass has not yet been determined. It appears
to be a shepherd satellite of Saturn’s A
ring and orbits Saturn once every 0.602 Earth
days. Also, magnitude 3.8 type B9 star at RA
03h49m, dec +24 ◦ 03 ′ ; “Father” of the “seven
sisters” of the Pleiades.
atmosphere The gaseous envelop surrounding
the Earth and retained in the Earth’s gravitational
field, which contains the troposphere
(up to about 10 to 17 km), stratosphere (up to
about 55 km), mesosphere (up to about 80 km),
and ionosphere (up to over 150 km). The total