IISc Study: White Dwarfs May Exceed Chandrasekhar Limit

A new study led by researchers at the Indian Institute of Science (IISc), Bengaluru, suggests that some dying stars could develop into white dwarfs far heavier than previously expected, challenging the conventional understanding of the maximum mass these stellar remnants can support.

Using advanced computer simulations, the researchers found that powerful internal magnetic fields could enable certain white dwarfs to reach approximately 2.4 times the mass of the Sun, substantially exceeding the Chandrasekhar limit of about 1.4 solar masses that applies to nonrotating, nonmagnetized white dwarfs.

The findings, published in The Astrophysical Journal Letters, offer a possible explanation for unusually bright stellar explosions known as Type Ia supernovae. They could also improve scientific understanding of how astronomers measure distances across the universe and study its expansion.

The research was conducted by scientists from IISc's Department of Physics in collaboration with researchers from other institutions. The study, titled Super-Chandrasekhar White Dwarfs by the Evolution of Magnetized Main-sequence Stars: New Mass Limits from STARS Simulation, was led by doctoral researcher Zenia Zuraiq, with IISc professor Banibrata Mukhopadhyay serving as the corresponding author.

What is the Chandrasekhar limit, and why does it matter?

A white dwarf is the dense remnant left behind when a low- or intermediate-mass star exhausts its nuclear fuel and sheds its outer layers. Although typically comparable in size to Earth, a white dwarf can contain a mass similar to that of the Sun, making it one of the densest forms of stellar matter.

Unlike ordinary stars, which generate energy through nuclear fusion, white dwarfs are supported primarily by electron degeneracy pressure, a quantum mechanical effect that prevents their matter from being compressed indefinitely.

Indian-American astrophysicist Subrahmanyan Chandrasekhar established that a white dwarf supported by this pressure has a theoretical upper mass limit of approximately 1.4 times the Sun's mass, under standard assumptions.

This threshold, known as the Chandrasekhar limit, has been fundamental to the study of stellar evolution for decades.

As a carbon-oxygen white dwarf approaches this critical mass, increasing pressure and density can create conditions that ignite carbon within its core. In certain circumstances, the resulting thermonuclear reactions can produce a Type Ia supernova, an explosion capable of briefly outshining an entire galaxy.

However, observations of exceptionally luminous Type Ia supernovae have raised questions about whether all exploding white dwarfs follow the same mass constraints.

Some observations have been interpreted as possible evidence of progenitor stars with masses approaching 2.8 solar masses, although those estimates depend on the models used to explain the explosions.

The IISc research explores whether strong magnetic fields could provide the additional support needed for such massive white dwarfs to exist.

How did IISc researchers find that white dwarfs could exceed the limit?

Rather than examining only the final structure of a white dwarf, the IISc-led team investigated whether a star could naturally evolve into a white dwarf exceeding the conventional Chandrasekhar limit.

To do this, researchers modified STARS, a stellar evolution computer program developed at the University of Cambridge.

They introduced additional physics to account for magnetic fields and the cooling of white dwarfs, allowing them to simulate stellar evolution from the main-sequence stage, when stars generate energy through hydrogen fusion, to the formation of compact stellar remnants.

The researchers then examined binary star systems in which a white dwarf draws material from a companion star.

As the white dwarf accumulates matter, its mass increases and its structure becomes more compressed. According to the simulations, this process can strengthen magnetic fields that were initially relatively weak.

Those intensified fields can contribute additional internal support against gravity, allowing the white dwarf to sustain a greater mass than it would without the magnetic field.

In one simulation, the researchers examined a carbon-oxygen white dwarf with an initial mass of 1.02 solar masses, originating from a main-sequence star approximately eight times as massive as the Sun.

The white dwarf accumulated material from a companion at a rate of one-billionth of the Sun's mass annually.

Under the model's magnetic conditions, the white dwarf reached approximately 2.4 solar masses. A corresponding simulation without magnetic effects produced a maximum mass of about 1.4 solar masses.

The results suggest that the maximum stable mass of a white dwarf can depend on the strength and configuration of its internal magnetic field, along with the way it gains matter and cools over time.

The simulations therefore do not eliminate the Chandrasekhar limit. Instead, they indicate that its familiar value may not apply universally when additional physical forces become important.

Why could magnetic fields change the fate of dying stars?

The possibility that magnetic fields might help white dwarfs exceed the conventional mass limit has been discussed by astrophysicists for years.

However, demonstrating that such a stellar configuration is mathematically possible is different from showing that it could emerge through the evolution of an actual star.

The IISc researchers sought to address this distinction by following the development of magnetized stars and modelling the subsequent accumulation of material by their white dwarf remnants.

Mukhopadhyay said the research originated in 2011, when he asked a summer student to investigate whether magnetic fields could alter the Chandrasekhar limit.

The question eventually developed into a broader examination of how stellar magnetic fields affect the evolution and stability of white dwarfs.

For Zuraiq, the central issue was establishing a plausible evolutionary pathway rather than merely calculating a higher theoretical mass.

The researchers found that, under particular magnetic and accretion conditions, a white dwarf could evolve into a massive, stable configuration exceeding the conventional limit.

The study also examined the relationship between the mass and radius of white dwarfs.

Ordinarily, a more massive white dwarf tends to have a smaller radius because stronger gravity compresses its material. But astronomical observations have identified some low-mass white dwarfs with unexpectedly large radii.

The researchers found that magnetic support, together with temperature effects, could help explain this discrepancy.

Their paper specifically discusses the white dwarf SDSS J123204.19+522548.2 as an example of an object whose observed radius may be better understood through the modified models.

How could the findings affect our understanding of supernova explosions?

One of the study's most important implications concerns Type Ia supernovae, which astronomers use to calculate distances to faraway galaxies.

These explosions are particularly valuable because their peak brightness can be standardized using relationships between their luminosity and the way their light fades over time.

By comparing their observed brightness with their estimated intrinsic brightness, astronomers can determine distances across the universe.

Such measurements have been central to research into the expansion of the universe.

However, not all Type Ia supernovae behave identically. Some are considerably brighter than expected, while others are unusually faint.

One example discussed in the research is SN 2003fg, an exceptionally luminous Type Ia supernova whose properties have been interpreted as possible evidence of a white dwarf progenitor exceeding the traditional Chandrasekhar limit.

The IISc study suggests that variations in magnetic field strength, stellar mass and evolutionary history might help explain some of these unusual explosions.

If white dwarfs can accumulate substantially more mass before undergoing thermonuclear ignition, their explosions may differ from those associated with more conventional progenitors.

Understanding these variations could help astronomers refine models of stellar explosions and identify which Type Ia supernovae are suitable for precise distance measurements.

However, the findings do not establish that existing measurements of the universe's expansion are incorrect. Type Ia supernovae are already standardized using observed light-curve properties, and the effect of the proposed stellar models on cosmological measurements would require further investigation.

What remains unproven about super-Chandrasekhar white dwarfs?

Although the IISc simulations offer a possible formation mechanism, they do not constitute direct observational evidence that a white dwarf with a mass of 2.4 solar masses exists.

The conclusions depend on the magnetic field configurations, accretion rates and other physical assumptions incorporated into the models.

The researchers examined how magnetic pressure could support additional stellar mass while accounting for stability conditions. Their models include internal magnetic fields reaching approximately \(10^{14}\) gauss, with the 2.4-solar-mass configuration having a central field of about \(8 \times 10^{13}\) gauss.

Whether nature produces these particular configurations, and whether they remain stable under more complete physical modelling, remains an important question.

Previous research has also challenged some theoretical models of exceptionally massive magnetic white dwarfs.

In a 2014 paper published in Physical Review D, researchers Rajaram Nityananda and Sushan Konar argued that certain proposed super-Chandrasekhar models neglected important magnetic forces, leaving their predicted stellar configurations physically unstable.

That criticism does not automatically invalidate the new IISc work, which examines stellar evolution and stability using a different modelling approach. It does, however, demonstrate why independent theoretical tests and astronomical observations remain essential.

The latest findings provide a new framework for examining how stars evolve under extreme magnetic conditions and how their final masses might differ from conventional expectations.

Further observational evidence will be needed to establish whether the predicted super-Chandrasekhar white dwarfs exist and how frequently they might contribute to unusual Type Ia supernovae.

For now, the research suggests that the Chandrasekhar limit remains a crucial principle in astrophysics, but the maximum mass of a white dwarf may depend on more than electron degeneracy pressure alone.