Computer simulations suggest that strong magnetic fields inside white dwarfs could allow some of these stellar remnants to exceed the long-standing Chandrasekhar limit of about 1.4 solar masses. In one model, a carbon-and-oxygen white dwarf grew to roughly 2.4 times the mass of the Sun, offering a possible explanation for unusually bright Type Ia supernovae and for some white dwarfs with unexpectedly large radii.
Magnetic fields and the Chandrasekhar limit
White dwarfs are the dense cores left behind when certain stars exhaust their nuclear fuel and shed their outer layers. They are about the size of Earth but can contain as much mass as, or more mass than, the Sun.
For white dwarfs without strong magnetic fields or rapid rotation, physicists have long recognized an upper mass limit of approximately 1.4 solar masses. Known as the Chandrasekhar limit, it is central to understanding how some stars end their lives in spectacular explosions.
When a white dwarf composed mainly of carbon and oxygen approaches this limit, the increasing pressure and density in its core can ignite carbon. The resulting runaway nuclear reaction can produce a powerful thermonuclear explosion called a Type Ia supernova.
New simulations by researchers in the Department of Physics at the Indian Institute of Science (IISc) and their collaborators suggest that this traditional limit may not apply to white dwarfs containing sufficiently strong internal magnetic fields.
In one simulation, a magnetized white dwarf reached a mass of about 2.4 times that of the Sun—well above the conventional limit of 1.4 solar masses.
“The idea started in 2011, when a summer student came to me and I very casually gave him a problem: to see if Chandrasekhar’s limit is violated by a magnetic field,” says Vanibrata Mukhopadhyay, a professor in the Department of Physics and the study’s corresponding author. The study was published in the June 2006 issue of Astrophysical Journal Letters.
The possibility that white dwarfs could exceed the Chandrasekhar limit is not entirely new. Theoretical physicists have studied these objects, sometimes called “super-Chandrasekhar” white dwarfs, for decades.
Astronomers have also observed a Type Ia supernova that was much brighter than expected. Such unusually luminous explosions raise the possibility that some white dwarfs could reach as much as 2.8 solar masses before exploding.
However, showing that extremely massive white dwarfs can exist in theory is only part of the challenge. Scientists must also explain how ordinary stars could naturally evolve into them.
Tracing stellar evolution
“The key question was not just whether super-Chandrasekhar white dwarfs are possible, but whether stars can actually evolve into white dwarfs,” says Xenia Zureik, the study’s first author and a doctoral student in the Department of Physics. “Our simulations allow us to trace an evolutionary path from a main-sequence star to a white dwarf and show that such a path is possible under certain conditions.”
To investigate this process, the researchers modified STARS, a stellar-evolution computer program originally developed at the University of Cambridge.
They expanded the software to include magnetic fields and the cooling of white dwarfs. These additions allowed the researchers to simulate how a magnetized star changes over time, beginning with the main-sequence stage, when it fuses hydrogen into helium and generates energy.
The simulation followed the star through later stages of evolution until it became a white dwarf. The researchers then modeled a binary system in which the white dwarf gradually drew material from a companion star.
This process, known as accretion, increases the white dwarf’s mass over time. The researchers examined whether a growing magnetic field could change how much additional material the star could support.
How magnetic fields support more mass
The simulations produced several notable effects. Even when the magnetic field was initially relatively weak, its influence grew as the white dwarf accumulated material.
As matter gathered, the star became denser and contracted under its own gravity. This compression strengthened the internal magnetic field, creating additional pressure that helped counteract the star’s gravitational pull.
Stronger magnetic support allowed the white dwarfs to hold more mass than would otherwise be possible. The familiar relationship between a white dwarf’s mass and size could also change, producing different maximum masses depending on how the magnetic field behaves inside the star.
One simulation illustrates the scale of the effect. The researchers modeled a carbon-and-oxygen white dwarf with an initial mass of 1.02 solar masses. The white dwarf had originally formed from a main-sequence star with a mass of eight Suns.
In the model, the white dwarf gradually accreted material from its companion at a rate of 10−9 solar masses per year, equivalent to one-billionth of the Sun’s mass annually.
When magnetic effects were included, the white dwarf reached a mass of about 2.4 solar masses. Without those effects, comparable models reached the traditional limit of approximately 1.4 solar masses.
The results suggest that magnetic fields can fundamentally alter the maximum mass of a white dwarf.
The same mechanism may help explain other astronomical observations. Some well-studied white dwarfs have unexpectedly large radii despite their relatively low masses. Magnetic pressure could help explain why these stars appear larger than conventional models predict.
Implications for Type Ia supernovae
The possible effects extend beyond the evolution of dying stars. Type Ia supernovae are among astronomy’s most important tools for measuring cosmic distances. Because their peak brightness can be standardized, astronomers use them as reference points, and they are sometimes called standardizable candles.
By comparing an explosion’s intrinsic brightness with how bright it appears from Earth, researchers can estimate its distance. These measurements have helped scientists study the size, history and continuing expansion of the universe.
The Chandrasekhar limit has traditionally provided an important part of the physical framework for understanding Type Ia supernovae. If a white dwarf reaches a similar mass before exploding, its energy output and brightness are expected to follow a broadly predictable pattern.
However, the new simulations suggest that the masses and internal magnetic fields of some exploding white dwarfs may vary significantly.
Those differences could affect the properties and brightness of the resulting supernovae, potentially helping explain why some Type Ia explosions are unusually luminous. Understanding how magnetic fields influence these explosions could help astronomers refine their interpretation of supernova brightness and improve their understanding of cosmic expansion.
The findings come from computer simulations rather than direct observations of white dwarfs growing to such extreme masses. They nevertheless suggest a possible evolutionary route by which some stars could produce white dwarfs exceeding the traditional Chandrasekhar limit.
Strong internal magnetic fields may therefore represent an important exception to one of astronomy’s best-known mass limits, allowing some dead stars to become far more massive than previously expected.
Source: www.sciencedaily.com
