Magnetic white dwarfs grew to 2.4 Suns in IISc simulations, past the Chandrasekhar limit

A white dwarf with a strong internal magnetic field can keep gaining mass well past the Chandrasekhar limit of about 1.4 Suns, according to simulations by researchers at the Indian Institute of Science published October 8 in The Astrophysical Journal Letters. In one model, the star reached about 2.4 solar masses before running out of road.

That matters because Type Ia supernovae, the exploding white dwarfs astronomers use to measure cosmic distances, are trusted partly because they are thought to blow up at roughly the same mass.

Following a star from start to finish

The team, led by PhD student Zenia Zuraiq and physicist Banibrata Mukhopadhyay, adapted STARS, a Cambridge-built program for modelling how stars evolve, to include magnetic fields and white dwarf cooling. They followed magnetized stars from the main sequence until they became white dwarfs, then let those white dwarfs pull gas from a companion.

In the headline case, a white dwarf of 1.02 solar masses, born from a star of 8 solar masses, accreted matter at a billionth of a solar mass per year. With a magnetic field, it grew to about 2.4 solar masses. The same star without a field stopped near 1.4.

The mechanism is gradual. A weak field barely matters at first, but as the white dwarf gains mass it shrinks, and the field inside it strengthens, from about 10^12 to roughly 10^14 gauss in the model. The extra magnetic pressure then holds up more mass.

A limit that is not one number

The paper's conclusion is that the Chandrasekhar limit "greatly depends on the underlying physics, here the magnetic field." The new mass limits shift with the shape of the field, the feeding rate and how the star cools.

Some cases fail. A hybrid white dwarf that kept about 0.1 solar masses of helium destabilized through helium burning before it reached the Chandrasekhar limit at all. The authors also note that their model accretes slowly and evenly, and that faster, more realistic rates could ignite carbon at the surface and end the growth early.

The model is one-dimensional, non-rotating and spherical, and the field follows a chosen profile rather than being solved from Maxwell's equations. Observational comparisons in the paper are described as illustrative.

An idea from a summer project

Mukhopadhyay traces the work to 2011, when he gave a visiting summer student a problem "quite casually." In 2013 his group proposed a mass limit of about 2.58 solar masses for magnetized white dwarfs. What the new paper adds is an evolutionary path showing how such a star could form and grow, rather than assuming one exists.

The release points to over-luminous Type Ia supernovae, whose light hints at progenitors of up to about 2.8 solar masses, as possible support. If some Type Ia explosions do come from heavier, magnetized stars, the authors say, that diversity could affect how the explosions are used as distance markers. Testing that will need observations, not more simulations.