Support for Magnetic Complexity in Cataclysmic Variables

Cataclysmic variables (CVs) are binary systems consisting of a white dwarf and a secondary "normal" star. Material from the secondary star gets stripped away and forms an accretion disk around the white dwarf (see Figure 1). Astronomers observe variations in brightness of the system due to activity in the accretion disk. 

Figure 1. Typical Cataclysmic Variable system. Credit NASA

One observable characteristic of a CV is its orbital period, the amount of time it takes one object to go around the other. An interesting fact to come out of studying CVs over many decades is that there is a paucity of orbital periods between roughly 2 and 3 hours (see Figure 2). This so-called period gap has been a mystery to astronomers for since the late 1970s.

Figure 2. Histogram illustrating the period gap between roughly 2 and 3 hours Credit Carraffo et al. 2018

There are some ideas about why such a period gap may exist. The prevailing hypothesis is centered on the notion of loss of angular momentum. The standard explanation is that at less than 2 hours the loss of angular momentum is dominated by gravitational radiation while at greater than 3 hours it is dominated by magnetic braking. Let's pause for a moment to consider these terms. 

Gravitational radiation (a.k.a., gravitational waves) is a byproduct of Einstein's theory of general relativity (GR). GR views gravity not as a force, but instead a result of the curvature of space created by objects with mass. When you have two massive bodies orbiting each other rapidly (i.e., a CV system), they create ripples in space. Such ripples were detected for the first time in 2015 to great fanfare. A result of gravitational radiation is that energy, and thus angular momentum, is lost from the binary system.

Magnetic braking is another way for stars to lose angular momentum.  Stars are spinning balls of plasma (ionized gas) that give rise to magnetic fields around themselves. The magnetic field lines can strip away ions in the star's atmosphere and carry them away. Again, this mass loss leads to a loss in angular momentum. This phenomena can be greatly exacerbated by stars with strong magnetic fields.

Back to CVs, remember that astronomers think that gravitational radiation dominates systems with periods less than 2 hours and magnetic braking is responsible for those above 3 hours. So why the period gap? The proposed reason for the gap is that around a period of 3 hours the magnetic breaking "turns off", the secondary star contracts and its orbit shrinks resulting in a quicker (< 2 hour) period. However, there is a dearth of evidence supporting this idea of an interruption of the magnetic field at about 3 hours. An alternative explanation offered by Taam & Spruit (1989) suggests that as the secondary star's rotation decreases, its magnetic complexity increases which leads to less effective loss of angular momentum. In other words, the magnetic field lines get gnarly and cannot shuttle the mass away in an orderly fashion.

In a recent study, Garraffo et al (2018). studied the CV period gap by attempting to model this magnetic behavior using Zeeman-Doppler imaging (ZDI). This also deserves some explanation. The Zeeman effect is the observation that individual spectral lines can be split into multiple parts in the presence of a strong magnetic field. When this effect is observed for a system over time, it can be used to essential re-create the structure of the underlying magnetic field (see Figure 3).

  

Figure 3. Zeeman-Doppler imaging reconstruction of a young star's magnetic field. Credit Wikipedia

In the models that Garraffo et al. (2018) created, they showed that surface magnetic complexity did indeed increase along with the decrease in orbital period. These results support the idea that magnetic complexity can suppress mass loss. Consequently, this reduction in the loss of angular momentum could result in the cessation of accretion to the primary. Recall that the accretion disk is how astronomers observe CVs, so when it is "off" no observations could be made. This is a very plausible explanation for the existence of the period gap. As the system continues to lose angular momentum, eventually the components are in close enough proximity for gravitational radiation to take over. As always, more studies will be needed to confirm if magnetic complexity is indeed responsible for the infamous CV period gap.