IISc study says strong magnetic fields could allow white dwarfs to grow beyond the Chandrasekhar limit

Simulations by researchers of Indian Institute of Science (IISc) and their collaborators show that strong internal magnetic fields could allow some white dwarfs to grow to substantially higher masses than determined by the Chandrasekhar limit.

When a dying star runs out of fuel and sheds its outer layers, its remaining core can become an approximately Earth-sized, extremely dense stellar remnant called a “white dwarf”. For a white dwarf that is not strongly rotating or magnetised, there is a well-known upper mass limit of about 1.4 times the mass of the Sun, known as the “Chandrasekhar limit”.

In one of their simulations, the researchers found that a magnetised carbon-oxygen white dwarf can reach about 2.4 times the mass of the Sun – well above the limit.

“The idea started in 2011, when a summer student came to me and I gave him a problem quite casually: to check whether the Chandrasekhar limit can be violated by a magnetic field,” said Banibrata Mukhopadhyay, professor at the Department of Physics and corresponding author of the study published in The Astrophysical Journal Letters.

The IISc said that theorists have predicted the possibility of such “super-Chandrasekhar” white dwarfs for decades. Observations of unusually over-luminous Type Ia supernovae have added support, hinting at the possibility of progenitor white dwarfs having masses, and the mass limit as high as 2.8 times the solar mass.

“The important question was not simply whether a super-Chandrasekhar white dwarf is possible, but whether a star can actually evolve into one. Our simulations allowed us to follow that evolutionary pathway from the main-sequence star to the white dwarf, and show that under certain conditions, such a pathway is possible,” said Zenia Zuraiq, first author of the study and PhD student at the Department of Physics.

To track this evolution, the team modified STARS, a computer code developed at the University of Cambridge to model how stars evolve. They incorporated magnetic field effects and white dwarf cooling, which allowed them to follow magnetised stars from the main sequence – the long phase when stars generate energy by fusing hydrogen – through their later evolution into white dwarfs. Then, they modelled a binary system in which the white dwarf gains additional matter from a companion star.

The simulations showed that a magnetic field that is initially too weak can become increasingly important as the white dwarf gains mass. As matter accumulates, the white dwarf becomes denser and contracts, strengthening its magnetic field. The stronger field provides additional pressure inside the star, helping it withstand its own immense gravity and supporting more mass. This changes the usual relationship between the white dwarf’s mass and size with a new limit(s) of mass, depending on the exact physics of magnetic fields.