Researchers from the Indian Institute of Science and other global institutions used mathematical simulations to study white dwarf stars. They found that magnetic fields allow these stars to exceed the 1.44 solar mass Chandrasekhar limit. These massive objects, called B-WDs, show how magnetic effects change star evolution and stability.
The Chandrasekhar limit is defined as the maximum mass a white dwarf star can have, which is 1.44 times the mass of the Sun (M☉). According to theory, any star that exceeds this limit must collapse into a neutron star or a black hole. However, some overluminous Type Ia supernovae, which are explosions that occur when a white dwarf devours a companion star, such as SN 2003fg and SN 2009dc, have been measured to have masses as high as 2.8 M☉. Researchers at the Indian Institute of Science, Bengaluru, the University of Florida, the University of Cambridge, the Czech Academy of Sciences and IIT Palakkad have now investigated this cosmological phenomenon.
The research team used magnetic fields to explain the existence of white dwarfs with masses beyond Chandrashekar’s limit. They call these magnetised white dwarfs that exceed the limit "B-WDs". The team used computer simulations using the Cambridge stellar evolution code STARS, modified to include magnetic effects and cooling. They simulated a spherical, one-dimensional, and non-rotating star with density and magnetic dependence.
Most stars with masses 3–8 M☉ evolve through a group called the asymptotic giant branch, which consists of bloated Red giant stars, to become white dwarfs of 0.45–1.02 M☉. When the magnetic field is dormant, the white dwarf's mass does not change. However, when the central density passes a critical value, the dormant field becomes strong enough to support more mass, resulting in more massive white dwarfs.
Further, they also observed that the composition of the white dwarf matters in defining its mass. A helium–carbon–oxygen hybrid white dwarf, formed from a 3 M☉ star, becomes destabilised by unstable helium burning. This happens with or without a magnetic field, so it stops short of its maximum mass. Switching off helium burning leads to its mass–radius relation to the limit. For carbon–oxygen white dwarfs, core carbon ignition occurs close to the maximum mass. Even so, the authors find B-WDs can reach significantly super-Chandrasekhar masses even before burning the chemical composition plays a vital role.
The models also address white dwarfs that are larger than expected at sub-Chandrasekhar mass. They observed that finite temperature alone can push radii above the Chandrasekhar limit, and a magnetic field increases the deviation further.
The Chandrashekhar limit for white dwarfs is one of the most celebrated discoveries of the 20th century. The idea of super dwarfs has been proposed before, but they have never been modelled to this precision using magnetic fields. Nevertheless, to confirm or refute their existence, one needs to study a full stellar evolution, with stars passing through complicated phases, including nuclear and (magneto)hydrodynamical instabilities, mass loss, and cooling. While the study aims at discovering the reason, their model needs to be further refined. The authors conclude that the Chandrasekhar limit may remain sacrosanct, but its value depends greatly on the underlying physics, in this case, the magnetic field.