Strong magnetic fields could allow white dwarfs to grow beyond 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 magnetized, there is a well-known upper mass limit of about 1.4 times the mass of the sun, known as the Chandrasekhar limit.

As a carbon-oxygen white dwarf approaches this limit, its core can become dense enough to ignite carbon, potentially triggering a thermonuclear explosion known as a Type Ia supernova.

If the Chandrasekhar limit is fixed, the supernova energy release rate and luminosity should be fixed. Scientists have used this constancy of luminosity to understand the evolution and size of the universe, which is presently thought to be expanding.

Magnetic fields could raise the mass ceiling

Now, simulations by researchers from the Department of Physics at the Indian Institute of Science (IISc) and collaborators show that strong internal magnetic fields could allow some white dwarfs to grow to substantially higher masses than the Chandrasekhar limit allows. In one of their simulations, the researchers found that a magnetized 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,” explains Banibrata Mukhopadhyay, a professor in the Department of Physics and corresponding author of the study published in The Astrophysical Journal Letters.

Theorists have predicted the possibility of such “super-Chandrasekhar” white dwarfs for decades. Observations of unusually overluminous Type Ia supernovae have added support, hinting at the possibility of progenitor white dwarfs with masses—and a 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,” says Zenia Zuraiq, first author of the study and a Ph.D. student in the Department of Physics. “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.”

Tracing growth and magnetic support

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 magnetized stars from the main sequence—the long phase when stars generate energy by fusing hydrogen—through their later evolution into white dwarfs. Then, they modeled 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, introducing new limit(s) on mass, depending on the exact physics of magnetic fields.

In one model, a 1.02-solar-mass carbon-oxygen white dwarf, formed from an 8-solar-mass main-sequence star, gained matter at a rate of 10⁻⁹ solar masses per year. The magnetized model reached a mass limit of about 2.4 solar masses, whereas the corresponding nonmagnetized model reached only about 1.4 solar masses.

Implications for stellar sizes and cosmic distances

The simulations also offer a possible explanation for why some well-observed white dwarfs have larger radii than expected based on their low mass: the same magnetic field effect.

The findings could also have other implications for Type Ia supernovae, which are used as standardizable candles to measure cosmic distances. If some of these explosions arise from progenitors with substantially different masses and magnetic properties, understanding this diversity could be important when interpreting their luminosity and using them to study the expansion of the universe.

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Robert Egan

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