Variable Star Of The Month

June, 2001: AM Herculis
(1813+49)

Artistic impression of an AM Her system. The blue haze represents the white dwarf's magnetosphere. Image copyright Russell Kightley Media

In May 1975, Berg and Duthie made the first photoelectric observations of AM Herculis. They found that the light from AM Her "flickered incessantly". This rapid light variability had been seen in two other stars which were associated with X-ray sources, so the team was optimistic that AM Her would turn out to be the optical counterpart of 3U 1809+50.

By May 1976, word had spread throughout the astronomical community that AM Herculis was an important object to observe in as much detail as possible. The Chilean astronomer S. Tapia, at the University of Arizona, had access to a polarimeter and used it to observe this star. The results were startling. He discovered in August 1976 that AM Her is both linearly and circularly polarized (these concepts are discussed later in the article) at optical wavelengths (Tapia 1977a). The detection of variable circular polarization was surprising since it was only known to exist in 9 other stars, all magnetic white dwarfs. The circular polarization in AM Her signified the presence of an immense magnetic field. This confirmed the suspicion that AM Her was the optical counterpart to the X-ray source. Consequently, a new class of magnetic cataclysmic variables called "AM Her stars" or "polars" was born.

Magnetic Cataclysmic Variables

1. Intermediate Polars (DQ Her stars)
   The intermediate polars or DQ Her stars (named after the prototype DQ Her) show magnetic field strengths around the white dwarf star on the order of 1-10 Mega Gauss. An accretion disk forms, but is disrupted close to the white dwarf (primary) star due to the magnetic field. The magnetosphere is not strong enough to synchronize the orbits of the rotating white dwarf with the orbital period of the system (as seen in AM Her stars).
2. Polars (AM Her stars)
   The polars or AM Her stars (named after the prototype AM Her) display magnetic field strengths on the order of 10-100 Mega Gauss. This magnetic field is so powerful that it prevents the formation of an accretion disk around the white dwarf and locks the two stars together so they always present the same face to each other. Thus the white dwarf star spins at the same rate as the two orbit each other - a synchronous rotation that is the defining characteristic of an AM Her star. (About 10% of AM Her stars are asynchronous, where the rotations of the white dwarf and the orbit are off by ~ 1%) (Hellier 2001)

Model of an AM Her system

Southern Lights and Shuttle Glow Credit: STS-39 Crew, NASA Bands of the Aurora Australis shine at a height of 50-80 miles with the glow from the shuttle Discovery engines at left. Aurora are caused by charged particles from the sun channeling into Earth's atmosphere at the magnetic poles, a path similar to the acretion flow in polars.

The material in this funnel, or accretion column, is channeled by the magnetic field towards the white dwarf in virtual free-fall. The potential energy is converted to kinetic energy and the stream slams into the white dwarf at roughly the ~ 3000 km/s escape velocity. In the resulting accretion shock the kinetic energy is converted into X-rays and radiated away. Magnetic cataclysmic variables emit most of their energy as X-rays and extreme-ultraviolet photons (Hellier 2001).

It is found that in an effort to obtain the lowest-energy configuration for this system, the magnetic field of the white dwarf often tilts over, so that one magnetic pole "points" towards the direction from which the stream comes. As a result, material flows preferentially onto that pole; material can still flow to the other pole, but only by going the long way around and only a fraction of the material succeeds in doing this. Eclipses in AM Her systems provide a graphic illustration of this stream geometry. Lightcurves reveal that the tiny and thus rapidly eclipsed, accretion spot at the magnetic pole emits roughly half of the total light; the other half comes from the extended stream, which enters and leaves eclipse more gradually (Hellier 2001).

A Look at the Light Curve

Visual light curve of AM Herculis from the AAVSO International Database; August 1976 through March 2001.

Some of the short-term phenomena in the light curve of AM Her can be explained by the orbital motion of a binary system with a period of 3.1 hours. The 3.1 orbital period was discovered in AM Her from the eclipsing light-changes, the strongly variable linear and circular polarization, and the periodic radial velocity changes in the H and He lines (Hoffmeister et al 1985). Liller (1977) explains two kinds of optical variations associated with the orbital motion that are taking place in AM Her.

First, the red dwarf star is distorted into an egg-shaped figure by the attraction of its companion, toward which the long axis of the egg points. When we see the normal star broadside, it appears slightly brighter than when end on. Hence, as the entire system rotates, there are two long weak brightness maxima and two long shallow minima per period. Second, there can sometimes be observed brightness fluctuations due to the heating of the surface of the red secondary star by X-rays emitted by the collapsed star. This "hot spot" is periodically lost from view on the far side of the rotating normal star. Moreover, the short term light variations described earlier as "incessant flickering", occur due to the turbulent nature of the transfer of mass from the secondary to the white dwarf star (Hellier 2001).

Polarized Light

Fig. 1  An electron in an AM Her system spirals around the magnetic field line producing cyclotron emission. Credit: Hellier 2001

Fig. 2   When the field line is seen side on the apparent motion of the electrons are up and down, producing photons that are linearly polarised in this direction. Credit: Hellier 2001

Fig. 3  When we look directly at a field line, the apparent motion is circular, producing circularly polarised photons. Credit: Hellier 2001

Thus, movement along the field lines beams linearly polarized light perpendicular to the field line and circularly polarized light along the field line. This type of emission is called cyclotron emission because spiraling electrons emit the photons that we see. Since the cyclotron emission can be as much as half the total light of AM Her stars, they are the most polarized objects in the sky (Hellier 2001).