Categories: Astronomy

Researchers Propose New Model for the Most Eager Supernova Explosions

2005ke, a Type 1a supernova. Credit: NASA/Swift/S. Immler

Type 1a supernovae like 2005ke, above, are known to go off when one member of a star pair exceeds critical mass and kickstarts a runaway fusion reaction.

Researchers have long puzzled over why some of the explosions happen so fast. Now, a team of Chinese astronomers believes they’ve arrived at a probable cause for the earliest of the blasts.

A team of astronomers, led by Bo Wang from the Yunnan Observatory of the Chinese Academy of Sciences, have shown how the transfer of material from a ‘helium star’ to a compact white dwarf companion causes these cataclysmic events to take place. The new results appear in Monthly Notices of the Royal Astronomical Society.

Most type Ia supernovae are believed to occur when a white dwarf (the superdense remnant that is the end state of stars like the Sun) draws matter from a companion star orbiting close by. Previous theories for the origins of a Type Ia include an explosion of a white dwarf in orbit around another white dwarf, or an explosion of a white dwarf in orbit around a red giant star.

When the white dwarf mass exceeds the so-called Chandrasekhar limit of 1.4 times the mass of the Sun, it eventually collapses and within a few seconds undergoes a runaway nuclear fusion reaction, exploding and releasing a vast amount of energy as a type Ia supernova. Due to their high and remarkably consistent luminosities, astronomers use these events as ‘distance indicators’ to measure the distances to other galaxies and constrain our ideas about the Universe.

Scientists have confirmed more and more type Ia supernovae, and found that about half of them explode less than 100 million years after their host galaxy’s main star formation period. But previous models for these systems did not predict that they could be this young — so Wang and his team set out to solve the mystery.

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Employing a stellar evolution computer code, they performed calculations for about 2600 binary systems consisting of a white dwarf and a helium star, a hot blue star which has a spectrum dominated by emission from helium. They found that if the gravitational field of the white dwarf pulls material from a helium star and increases its mass beyond the Chandrasekhar limit, it will explode as a type Ia supernova within 100 million years of its formation.

“Type Ia supernovae are a key tool to determine the scale of the Universe so we need to be sure of their properties,” said research team member Zhanwen Han, also from the Yunnan Observatory. “Our work shows that they can take place early on in the life of the galaxy they reside in.”

The team now plans to model the properties of the companion helium stars at the moment of the supernova explosions, which could be verified by future observations from the Large sky Area Multi-Object fiber Spectral Telescope (LAMOST).

LEAD IMAGE CAPTION: Supernova 2005ke shown in optical, ultraviolet and X-ray wavelengths. When it was captured, this was the first X-ray image of a Type 1a, and it provided observational evidence that Type Ia come from the explosion of a white dwarf orbiting a red giant star. Credit: NASA/Swift/S. Immler

Source: Royal Astronomical Society. The paper is available here.

Anne Minard

Anne Minard is a freelance science journalist with an academic background in biology and a fascination with outer space. Her first book, Pluto and Beyond, was published in 2007.