Polars - Accretion Disks Bad!

Steve Howell

I intend no slur against any of the other speakers when I say that many of the AAVSO members were probably looking forward to Steve Howell with the greatest anticipation of the session. He did not disappoint. It is clear that he still has one foot very much in the amateur camp, and his work is well-known to those observers who follow eruptive variable stars. Add to this the fact that he is an excellent communicator and you have a very entertaining and informative speaker.

Polar-type Cataclysmic Variables are not too dissimilar from their more conventional brothers; a binary system made up of a hot white dwarf of about the same size as the Earth, and a rather larger star somewhat cooler and smaller than the Sun. With polars, however, the white dwarf primary star is highly magnetic with a field strength of between 10 to 250 Megagauss. Compare this with the magnetic field of the Earth which is about 1 Gauss and you can see how this amazingly strong magnetic field is going to govern the whole system.

Polars are characterised not by quiescence and outburst, but between high and low states, where the magnitude difference can be anything between one to five magnitudes. The difference between these states lies in the amount of material grabbed from the secondary. When a high rate of transfer is in process, the system is in high state, and vice versa. Some of these secondary stars have donated so much material to the primary that they are hardly proper stars any longer and resemble those mysterious stellar wimps known as Brown Dwarfs. These are very cool, dim, under-massive objects of planetary size which are so faint they have only been seen during the past few years. An example of a Polar with a brown dwarf secondary is EF Eridani.

Another way in which Polars differ from CV's is in their lack of an Accretion Disk (hence the title of this page). Instead the material which is pulled off the faint star funnels down onto the primary's magnetic pole in a sort of 'Accretion Tower' (see the illustration at the head of the Welcome page). Conventional CV's also probably experience high and low states of material transfer, but these are 'smudged' by the already comparatively large amount of material in the existing accretion disk. Here is a light-curve by AAVSO members of the best-known Polar, AM Herculis, between 1 Jan 1998 and 1 Jan 2000.

Polars show the following types of emission:

  1. Cyclotron Emission: Beamed radiation caused by the motion of particles along the magnetic field lines
  2. Bremsstrahlung: Produced by particles donating energy as they are braked
  3. Thermal: Conventional stellar radiation

Some idea of the vast quantities involved in these processes (and these are only dwarf stars, by the way!) can be gleaned by realising that the accretion region is about the size of Arizona. On an Earth-sized star as these white dwarves are, this is about 1/1500th of the surface area. About seven billion tons of material are deposited on the primary every second producing a local shock temperature of some 100 million degrees with a luminosity equivalent to the detonation of 20,000,000,000 megatons of TNT, and forming in all probability stellar 'mountains' at the poles. Small wonder that these 'tiny' stars are visible over distances of at least 100 light-years!

There may be a wide range of behaviour exhibited by these objects, due not only to whether we view one or both poles of the white dwarf, but also to the physical parameters of each system such as distance between the stars, inclination of one member with respect to the other, orbital period, and so on. Bearing all this in mind, I think we can look forward to seeing more of these objects being observed by amateurs in the near future, both by traditional means and with CCDs.