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.

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.

  Comments:

  • efficiency = 1/2 mv²/mc² = 1/2 (v/c)² ~ 0.0005

  • efficiency ~ 0.007

    ===> nuclear burning efficiency is higher

• 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.