DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

cataclysmic variable [binary models of]
ble, leading to systems whose brightness varies
rather little. These are nova-like variables and
symbiotic stars. Another kind of cataclysmic
variable star is the flare star, whose prototype is
UV Ceti. Flare stars are intrinsically cool red
stars (of type M or less commonly K) on the
main sequence that will unpredictably brighten
by up to two magnitudes over the course of a
few seconds and then fade back to normal in 20
or 30 minutes. Most of these stars are known to
have close companions. Mass transfer at 10 −9
solar masses per year is unstable, with an accretion
disk building up for a while and then
dumping material onto the white dwarf rapidly.
These events are the outbursts of the dwarf novae.
In most cases, hydrogen gas builds up on
the surface of the white dwarf until it is somewhat
degenerate, at which point it burns explosively,
producing a nova. If this happens frequently
enough to have been seen twice or more
in historic times, the system is called a recurrent
nova. It is sometimes possible for similar accretion
(perhaps of helium or heavier elements) to
trigger degenerate ignition of the carbon-oxygen
core of the white dwarf. This leads to a supernova
explosion of Type Ia.
cataclysmic variable [binary models of]
Thought to be binary systems composed of a
white dwarf (primary), a main sequence star
(secondary), and characterized by accreting
mass flowing from the secondary towards the
primary. Typically the accreting mass forms a
disk or accretion disk around the primary (with
the exception of magnetic variables).
cataclysmic variable [galactic distribution]
Due to interstellar absorption, cataclysmic variables
(which have luminosities on the order
of solar luminosity) cannot be observed at a
distance greater than 1 kpc. In spite of this,
380 CVs had been detected by 1976. Thus, it
seems that CVs are relatively common astrophysical
objects.
cataclysmic variable [outbursts] Some
CVs present “outbursts” or periodical increases
in luminosity with respect to their usual quiescent
state. Classical novae are cataclysmic variables
that have been known to present only one
outburst; recurrent novae are cataclysmic vari-
© 2001 by CRC Press LLC
ables that present outbursts periodically every
10 years or more; dwarf novae are cataclysmic
variables that present outbursts every few weeks
or months.
cataclysmic variables [phenomenological
classification of] Cataclysmic variables are
classified into five groups. Classical Novae:
cataclysmic variables that have been known to
present only one outburst. Recurrent Novae:
cataclysmic variables that present outbursts every
10 years or more. Dwarf Novae: cataclysmic
variables that present outbursts every
few weeks or months. Nova-like: cataclysmic
variables that have not been known to present
outbursts but have the same spectroscopic characteristics
as other cataclysmic variables when
they are quiescent. Magnetic Variables: cataclysmic
variables that present relatively strong
magnetic fields.
cataclysmic variables [physical parameters]
The luminosities of cataclysmic variables vary
between0.001and10solarluminosities. Binary
orbital periods are typically between 0.7 hours
and 1 day. Mass accretion rates vary from 10 −10
to 10 −7 solar masses per year. The primary star
(white dwarf) typically has a mass of ∼ 0.6 solar
masses, varying between 0.4 and 0.9 solar
masses, and a radius of ∼ 0.01 solar radii. The
secondary is a main sequence star with a mass
lower than its companion star since the primary
has evolved through the main sequence faster.
catastrophic formation of solar system A
theory attributing the formation of the solar system
to a collision of another massive object (presumably
another star) with the sun, which threw
material out of the sun, or to a close encounter
with another star — a tidal encounter. Now out
of favor because it suggests solar systems are
rare, since such encounters are rare, while recent
observations provide evidence for planetary
systems around a number of local stars and even
around neutron stars.
Cauchy singularity Any region of spacetime
where violations of causality can occur
because the deterministic evolution of physical
systems from initial data is not preserved. See
naked singularity, white hole.