Authors: Jordi Isern, Enrique García-Berro, Baybars Külebi, Pablo Lorén-Aguilar
First Author’s Institution: Institut de Ciències de l’Espai (CSIC), Spain
Status: Published in ApJ [open access]
If you have ever used a compass, you know the Earth has a magnetic field. That’s lucky for us, because this field protects us from highly energetic particles that could make life on Earth quite difficult, like it is on Mars (although NASA might have a solution). The Sun has a magnetic field too, which is beautifully pictured on the featured image. The explanation for these fields is a dynamo effect: in short, ionised matter circling inside the Sun and the Earth generates the field. This explanation holds for most stars in which we detect a magnetic field. White dwarf stars, the most common end-point of stellar evolution (which makes them extremely useful in understanding the history of the Galaxy and even of the Universe), seemed to be an exception. They can present unusually high magnetic fields, up to a 100 million times the field of the Sun! The explanation for such colossal fields is still an open question. The authors of today’s paper present a possible solution by cleverly making use of already well-known astrophysical mechanisms.
Magnetism in white dwarfs: how common is it?
The fraction of magnetic white dwarfs is also open to discussion. It may be as high as 20%, but as low fields can be difficult to spot, we cannot be sure. Observations also indicate that this fraction is larger for cool white dwarfs, suggesting that the field is somehow amplified during the white dwarf’s evolution, which is basically a cooling process. Another observational fact is that the average mass of magnetic white dwarfs is higher than that estimated for non-magnetic white dwarfs. A good explanation for the origin of white dwarf magnetism should be able to explain these facts as well. As you will see, today’s paper fits the bill!
One, two, three possible scenarios
There are three proposed explanations in the literature for the observed magnetism in white dwarfs. The first hypothesis is that the observed fields are simply the remnants of those of their progenitors. Specifically, white dwarf magnetic fields could be the left-over “fossil fields” of main sequence Ap/Bp stars (which have stronger magnetic fields than classical A/B type stars). As the magnetic flux must be conserved throughout the star’s evolution (assuming mass loss doesn’t carry away a significant portion of the flux), the amplification of the magnetic field can be accounted for by the contraction of the star into a white dwarf. However, the fraction of observed Ap/Bp stars is not enough to explain the number of observed white dwarfs with high magnetic fields.
In the second scenario, magnetic white dwarfs occur as the result of the evolution of binary systems. In this case, the magnetic field is amplified by a dynamo either in the common envelope phase or in the hot corona produced by the merger of two white dwarfs. Again, population synthesis models suggest that the number of white dwarfs produced by this channel cannot explain what we find observationally.
Last but not least, in the third scenario, the magnetic field is generated inside the convective envelope formed as a white dwarf cools down. The problem with this explanation is that it cannot account for the strength of the observed fields. Therefore, we need a more efficient mechanism!
The missing ingredient
Another characteristic of most white dwarfs is that, as they cool, their nucleus will eventually undergo a phase transition and crystallise, releasing energy without changing the star’s temperature significantly. This process provides extra energy that can boost the dynamo effect and lead to higher fields. A similar effect occurs for the Earth and Jupiter (where the convective dynamos are powered by cooling and chemical segregation in their interiors) as well as for T Tauri stars and rapidly rotating M dwarfs. The authors estimated the dynamo energy density for a number of white dwarfs with known fields taking that into account, as shown on Figure 1. They noted that this boost in energy is sufficient to explain the observed fields in most white dwarfs with hydrogen-dominated atmospheres. This implies that the magnetic fields observed in planets, non-evolved stars and white dwarfs all share a common origin!
As the crystallisation happens when the white dwarfs are relatively cool, this could also explain why magnetic white dwarfs usually show low temperature. Moreover, the amount of energy released during crystallisation is larger for more massive white dwarfs, so this mechanism naturally explains why magnetic white dwarfs are more massive than average.
Another cool thing about this mechanism is that it doesn’t exclude other possibilities. On the contrary, it alleviates one of the major drawbacks of the other two hypotheses, which didn’t predict a sufficient number of magnetic white dwarfs. As a bonus, the authors offer an explanation for the fact that most white dwarfs that do not fit into their mechanism are hydrogen-deficient: they are indeed formed by the merger of two white dwarfs, as suggested by scenario two. During their coalescence, the temperatures reached are so high that the hydrogen in the outer layers is burned. So, using already known mechanisms, the authors may have finally solved the mystery of the highly magnetic white dwarfs: no need to reinvent the wheel!