White dwarfs (the dense remnants of stars) are abundant objects throughout the universe and they represent the final stage in the life cycle of most stars. Studying them helps scientists understand how stars like our sun evolve and die.
Astronomers know that some white dwarf binaries (a pair of stars orbiting each other) develop magnetic fields and some don’t yet they don’t yet understand why. Young white dwarf binaries (in this case a white dwarf and a star) rarely have strong magnetic fields, but their evolved interacting counterparts (cataclysmic variables) often do. As the objects begin to interact can they kick start the magnetic field - this is the point that is of interest to scientists.
Dr. Steven Parsons is leading a new study that focuses on the critical moment of this magnetic field emergence. This emergence briefly separates the white dwarf and its companion star, creating a short-lived detached phase. This phase offers a unique window to observe the field's formation, potentially revealing the trigger, such as the white dwarf's core solidifying (crystallising).
Until recently there were very few young magnetic white dwarf binaries known, but thanks to work done in part here at Sheffield we now know of enough of these systems that we can finally use them to understand how magnetic fields are generated in white dwarfs.
The white dwarf’s magnetic field can grab material in the wind of its companion and channel it down to the magnetic poles, making them glow. Remarkably we can measure the size and location of these roughly Wales-sized glowing regions from hundreds of light years away because the white dwarf periodically passes behind its companion. However, to do this requires special high-speed cameras, developed at the University of Sheffield. By mapping out these regions we can better understand the white dwarfs themselves and ultimately how the magnetic field was generated in the first place.
Dr. Steven Parsons
The team will use high-speed photometry (measuring brightness changes) and phase-resolved spectroscopy (analysing shifts in the wavelengths of light) on a large sample of these detached, eclipsing magnetic white dwarf binaries. They will also be developing a new method to model the light patterns from these binaries, allowing for much more accurate measurements of the stars' properties.
Understanding when and how these magnetic fields form will allow scientists to understand how these white dwarf binaries evolve over time. These pairs can explode as a type of supernova, which are used to measure huge distances across the universe. Therefore, this understanding will allow scientists to better understand the universe's scale and expansion rate. It is a fundamental piece of the complex cosmic puzzle.