Cataclysmic Variables
Cataclysmic variables are
short orbital period (hours to days) binary
star systems composed of a white dwarf and a low mass main seqeunce
star (in general, sometimes the companion star is a red evolved star).
As their name implies, cataclysmic variables (CV's) are sites for
cataclysmic events. However, the events are not so cataclysmic as to
destroy the binary star systems (in general). The events lead to rapid
increases in the luminosities of the systems.
There are three main types of
cataclcysmic variables (CV's), Dwarf Novae,
Recurrent Novae, and Classical Novae. Many CV's are
strong sources of x-ray emission, and
Type I Supernovae may be also be a type of CV.
Mass Transfer in Cataclysmic Variables
The systems must have short
orbital periods (hours to a few days) or else the stars will be too far
apart to exchange significant amounts of mass. Let's define some
things.
What happens in close binary systems, depends upon the secondary (the less
massive star). There may be
detached, semi-detached, and contact systems depending
upon whether the secondary star fills its Roche lobe. CV's are
semi-detached systems. The companion fills its Roche lobe and transfers
material to the white dwarf.
Energy Sources
CV's will generate energy either through nuclear burning or through
gravity.
Nuclear Energy
- The material which flows onto the white dwarf simply piles up on the
surface of the white dwarf. The material is rich in hydrogen since it
comes from the envelope of the companion star. This is a key point,
because white dwarfs being the ashes of nuclear burning have no nuclear
fuel left. The companion replenishes its fuel supply.
- The first material accreted compresses due to the weight of the
recently added material. The compression causes the temperature
and pressure of the accreted material to increase. After around 10,000
to 100,000 years of accretion, the conditions become right for nuclear
burning.
- The ignition of the nuclear burning is not gentle. There is an
explosion
either because the material is degenerate or the ignition occurs in a
thin shell.
- The thermonuclear explosion causes the outer nuclear burning shell
to be ejected leading to a Classical Nova
outburst.
Gravitational Energy
- The material accelerates as it falls onto the white dwarf. If I
dropped some mass onto a white dwarf it would hit the surface
of the white dwarf at a speed of
around 10,000 kilometers per second. This is
a lot of kinetic energy (gained at the expense of the potential energy
of the white dwarf).
- In dwarf nova systems, the energy which powers the outbursts
is gravitational in nature -- it comes from the energy the material
gained by falling onto the white dwarf.
- Dwarf nova outbursts are smaller and occur much more often (every
several weeks to months) than nova outbursts.
- The release of the gravitational energy can also lead to the production of
x-ray emission near the surface of the white dwarf. The Classical
Nova and Dwarf Nova, GK Per is also a strong source of
x-rays.
