Authors: Mark Hollands, Boris Gaensicke, Detlev Koester
First Author’s Institution: Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
The galaxy is littered with white dwarfs, the burnt out remnants of stars that have run out of hydrogen fuel in their cores, but were too small to explode as supernovae. But far from being lifeless orbs, around a tenth of white dwarfs have powerful magnetic fields, a million times stronger than that of the Sun. How did these magnetic white dwarfs become such strong magnets? And just how many are there? Hollands et al. set out to answer the second of these questions, in the hope that it will shed light on the first.
By far the easiest way to find magnetic fields in white dwarfs is to look at the absorption lines in their spectra. Normally, the light absorbed by an element at a certain wavelength is seen as a single, thin line. However, if the white dwarf is magnetic, then a process called the Zeeman Effect splits the line into three. The further the outer two lines are from the central, original absorption line, the stronger the field. This offers a simple way to both find magnetic white dwarfs and to measure their field strengths.
But as white dwarfs get older, they cool down, and the helium absorption lines in their spectra disappear as the atoms fall to their ground state. This hides the magnetic field, preventing us from knowing the magnetic behaviour of white dwarfs over their entire age range. Fortunately, a large fraction of these white dwarfs also have planetary systems. Asteroids and other planetesimals often fall onto the white dwarfs, producing the absorption lines needed to reveal their ancient magnetic fields. White dwarfs like that are known as DZs, Degenerate objects that have elements with a high atomic number (Z) in their atmospheres.
The authors used the huge Sloan Digital Sky Survey (SDSS), filtering down the thousands of objects observed to find 79 DZ. Of these, 10 of them were found to be magnetic, 7 of which no one had spotted before. Their magnetic fields were huge: up to 9.59 MegaGauss (nearly 1000 Tesla). For comparison, the large magnets used in MRI machines generate around 3 Tesla. The authors found that this implied that roughly 13 percent of DZ white dwarfs were magnetic. However Hollands et al. point out that there are many biases involved, such as the difficulty in seeing dim objects, or in spotting weak magnetic fields in spectra with low signal to noise, as well as the uncertainty over the geometry of the magnetic fields. Accounting for all of these biases could lead to an even higher occurrence rate.
The authors then compared this rate with that found by others looking at different types of white dwarf, or over different temperature ranges. What they found was surprising: Magnetic fields appeared to be much less common in other types of white dwarf, and much weaker.
With this in mind, Hollands et al. then tried to answer the question of where the magnetic fields came from. The simplest explanation is that they were there from the start, left over from when the white dwarfs were stars. The progenitors of the magnetic white dwarfs could have been the curious Ap/Bp stars, which have much stronger magnetic fields that most stars at around a few KiloGauss. As they shrunk down into white dwarfs, conservation forces would have ramped up the field to the required MegaGauss levels. Unfortunately there aren’t enough Ap/Bp stars to account for the high occurrence rate of magnetic DZ. Nor does this explain why they found many more magnetic DZ compared to other types of white dwarf.
The authors’ next suggestion is that the magnetic fields are the result of a binary merger, where the white dwarfs combined with companion stars into single objects. During this process a magnetic dynamo would naturally be created. This would also explain why these white dwarfs seem to have a higher average mass than other white dwarfs. The problem with this hypotheses is that the authors only know that the white dwarfs are magnetic thanks to the white dwarfs’ planetary systems, which probably wouldn’t survive such a merger. However, the authors say that it is possible that it could be a planet doing the merging, rather than a companion star. But this explanation again falls victim to the high incidence rate: models of planetary systems at white dwarfs suggest that it can’t happen enough to make all of the magnetic white dwarfs.
Thanks to these flaws in all of the potential explanations, the authors are forced to leave the question of how the magnetic white dwarfs formed unanswered. They finish by pointing out that the next set of results from the SDSS is on its way, promising to provide many more of the mysterious magnetic white dwarfs to investigate.