IISc model grows a magnetized white dwarf to 2.4 solar masses, well past the Chandrasekhar limit

Theorists have predicted "super-Chandrasekhar" white dwarfs for decades, according to IISc ^[12]. This work follows one from start to finish. "The important question was not simply whether a super-Chandrasekhar white dwarf is possible, but whether a star can actually evolve into one," said Zenia Zuraiq, the study's first author and a PhD student in the institute's physics department ^[8]. "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," she said ^[9].

The question goes back further than the simulations. "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, the IISc physics professor who is corresponding author of the paper in The Astrophysical Journal Letters ^[10][11].

The twin run is the best part of the design. The magnetized and unmagnetized versions start from the same dwarf and take on matter at the same rate. Within the model, then, the gap between where they stop comes from the field physics and nothing else ^[4]. The magnetized ceiling is about 1.7 times the conventional one ^[17].

Growth is slow. At the stated feeding rate, and assuming the dwarf kept all the matter it received, the magnetized star needs about 1.4 billion years to put on the 1.38 solar masses between its starting and final mass ^[15]. Its unmagnetized twin would reach its own limit in about 380 million years ^[16].

The thing this doesn't tell you is how many real white dwarfs carry fields that strong, or what a 2.4-solar-mass dwarf does when it finally ignites. The published accounts do not state the field strength the model assumed. The ceiling is also model-dependent. The field alters the usual relationship between a white dwarf's mass and its size, and the institute says the new limit or limits vary "depending on the exact physics of magnetic fields" ^[6].

The case for distance measurement runs as a chain of steps. A carbon-oxygen dwarf nearing the limit can ignite carbon in its core and explode as a Type Ia supernova ^[18]. If the limit is fixed, the energy release and luminosity should be fixed too, and scientists have used that constancy to study the size and evolution of an expanding universe ^[19]. The institute's own account, carried by phys.org, calls these explosions "standardizable candles." It goes only as far as saying that if some come from progenitors with different masses and magnetic properties, understanding that diversity "could be important" when interpreting their brightness ^[20].

I think the result supports that much. One modelled track shows the route can exist. Any bias in cosmic distances would scale with the share of real supernovae whose progenitors took it, and estimating that share needs a population of stars as well as a single evolutionary track.

The prediction that can be checked soonest is about size. According to the institute, the same field effect could explain why some well-observed white dwarfs have larger radii than their low masses would suggest ^[14].