Polars

Polars are also known as Magnetic Cataclysmic Variables or as AM Her stars, after the prototype. Polars are binary star systems where the magnetic field of the white dwarf is so strong that it dominates the transfer of mass from the companion (usually a Sun type star) to the white dwarf.

Material is pulled from the companion by the gravitational attraction of the white dwarf. However, instead of falling into an accretion disk like a regular cataclysmic variable (CV), it follows the dipole shaped magnetic field of the white dwarf in a ballistic trajectory. As it follows the magnetic field lines it is stretched and contracted into clumps of mass. Since there is no viscosity from surrounding matter (like in an accretion disk) the material moves very quickly, essentially in a state of free fall. As such it impacts the surface of the white dwarf at extremely high speeds (>12,000 km per second!). This emits a great deal of energy, especially in the X-ray spectrum.

There is a subclass known as intermediate polars (DQ Her stars). In these systems the companion is far enough away from the white dwarf to create an accretion disk. However, when material in the accretion disk gets close enough to the white dwarf it becomes directed along the magnetic field lines instead of spiraling into the white dwarf's hot spot.

Optical variability of the system is determined by the accretion rate of the matter, the impact of the clumps of mass on the surface, and the rotation and geometry of the system as seen from Earth. Scientists are interested in polars because they are complex systems to model due to the strong magnetic field and presense of quick moving plasma. The conditions are too extreme to be replicated on Earth. Also, light curves of these stars can tell us more about the star than a normal CV.

The AAVSO has many polars in our program. Their variability is chaotic. Some of them experience outbursts while some others go through periods of fading! Visual monitoring of these CVs is needed with CCD follow up during periods of activity.

The AAVSO has teamed up with the XMM Newton satellite team and Sonoma State University to observe polars over the next few years.

More Information

• AM Her: Variable Star of the Month
• Cataclysmic Variables: A NASA web page by Jesse S. Allen with good material and diagrams of magnetic CVs.
• AAVSO High Energy Network Presentation: Presented at the 92nd Spring Meeting with a section on polars, in PowerPoint or Quick Time (6MB each)
• Cataclysmic Variable Stars: How and Why They Vary by Coel Hellier.