HFML mimicks high magnetic fields of white dwarfs

The magnetic field surrounding some white dwarfs - stellar remnants - influences the energy levels of the atoms and thus the emitted radiation. Researchers at the HFML imitated this process in a lab setting, to understand more of this interaction. The results are published online on 12 February in the leading journal Nature Communications.

For astronomers it is important to know which effect high magnetic fields have on the spectra they measure from white dwarfs. This will improve their data interpretation. Therefore researchers want to mimic this interaction in a laboratory where the strength of the magnetic field can be varied and measured precisely. However, this is not an easy task: the magnetic field of a white dwarf can vary from 0.2 to 100 000 Tesla, while the HFML can ‘only' produce 33 Tesla. By working with materials that are more sensitive to magnetic fields than hydrogen, the most important component of the atmosphere of white dwarfs, the researchers translate the process into a lab setting.

The presence of a large magnetic field alters the hydrogen spectrum of a white dwarf considerably. In these fields, the magnetic energy becomes comparable to the binding energy of hydrogen atoms and the spectra can no longer be described with simple textbook quantum mechanics. Instead of hydrogen, the researchers used a low concentration of phosphorus atoms in silicon, which behave like hydrogen atoms (one electron bound to a single positive charge), but with a much lower binding energy. In this way they created a similar situation. Researcher Dr. Hans Engelkamp: "By varying the magnetic field, we could measure the effect of the field on the spectra for the first time. And with these results we can improve the theories on this topic."

Astronomer Professor Gijs Nelemans, a Nijmegen expert on white dwarfs, indicates that there are still fundamental questions on this subject: "Most white dwarfs don't have a magnetic field. Why is it then, that sometimes a magnetic field is created and why is this for example relatively common in white dwarfs in double stars? The more we know about when these magnetic fields are created and how they affect the spectra, the better we can interpret our results."

The results are not only interesting for astronomers. Engelkamp: "We performed these experiments together with researchers from the United Kingdom that see phosphorus atoms as candidates for data-bits in quantum computers. This is a nice example of how disciplines that seem to be far apart can still overlap."

Magnetic white dwarf

A white dwarf (left) forms when a star (upper right) similar in mass to our sun runs out of nuclear fuel. As the outer layers puff off into space, the core gravitationally contracts into a sphere about the size of Earth, but with roughly the mass of our sun. Some white dwarfs, including AE Aquarii depicted here, spin very rapidly and have magnetic fields millions of times stronger than Earth's. Credit: NASA / Casey Reed.