White dwarf binaries that should be magnetic are not, and those that should not be are — a contradiction that has left astrophysicists puzzled for decades. This project tackles a fundamental unresolved problem in stellar astrophysics: how and when magnetic fields emerge in white dwarfs. Among close white dwarf binaries, strongly magnetic white dwarfs are absent from young detached systems but make up more than a third of their semi-detached descendants, known as cataclysmic variables. A new evolutionary scenario suggests the magnetic field emerges only after the binary comes into contact, briefly forcing the stars apart before they reconnect as a magnetic cataclysmic variable. If correct, this brief detached phase offers a unique window into magnetic field formation. The research will use high-speed photometry and phase-resolved spectroscopy on the first large sample of detached, eclipsing magnetic white dwarf binaries. A novel light-curve modelling method will yield far more accurate measurements of stellar and binary parameters, revealing the white dwarf’s state just after the field emerges. This is fundamental science with no immediate practical application. Understanding magnetic field emergence in white dwarfs is, however, crucial for explaining white dwarf binary evolution and some pathways to thermonuclear supernovae — the same kind of foundational knowledge that has historically underpinned unexpected breakthroughs in physics.
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The origin of magnetic fields in white dwarfs remains a fundamental unresolved problem in stellar astrophysics. In particular, the very different fractions of strongly magnetic white dwarfs in evolutionarily linked populations of close white dwarf binaries challenges our understanding of how these systems form and evolve. Strongly magnetic white dwarfs are absent among young detached white dwarf binaries but make up more than one third of their semi-detached descendants (cataclysmic variables). A recently developed evolutionary scenario attempts to explain these apparently contradictory facts by hypothesising that the magnetic field of the white dwarf emerges after the binary has come into contact, during the cataclysmic variable phase. The emergence of the field forces the two stars apart, briefly detaching the system, before they come back into contact and the binary becomes a magnetic cataclysmic variable. If this scenario is correct then the brief detached phase offers a unique window into the formation and emergence of magnetic fields in white dwarfs. By probing the fundamental stellar properties of the magnetic white dwarfs in these binaries during this short-lived detached phase we can for example, see if the magnetic field is generated as a result of the core starting to crystallise. This project will utilise significant amounts of high-speed photometry and phase-resolved spectroscopy of the first large sample of detached, eclipsing magnetic white dwarf binaries. The project will develop a novel method for modelling the light curves of these systems, allowing for far more accurate and precise measurements of the stellar and binary parameters, revealing the exact state of the white dwarf very shortly after the magnetic field has emerged and the effects this has had on the evolution of the binary. Knowledge of how and when magnetic fields emerge from white dwarfs is crucial for our understanding of white dwarf binary evolution as well as some channels towards the creation of thermonuclear supernovae and this project represents a pivotal step forward in this field.
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