New model helps explain youthful supernovae

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 09 April, 2009

A new computer model shows how the most youthful type of Ia supernovae could occur within just 100 million years of their formation.

Type Ia supernovae occur when a white dwarf - the superdense core of a once Sun-like star - draws matter from a companion star, acumulating mass until it reaches a critical limit of 1.4 solar masses. At this point, known as the Chandrasekhar limit, further collapse is triggered and within a few seconds the core undergoes a runaway nuclear fusion reaction, exploding and releasing energy as a type Ia supernova.

Type Ia supernovae have a high and consistent luminosity, which makes them useful cosmological distance indicators, used to measure the distances to other galaxies and constrain ideas about the Universe. However, the nature of their progenitor systems and explosion mechanisms are not well constrained.

Artist impression of a white dwarf drawing matter from a nearby star, which eventually triggers its collapse and supernova explosion. Image: NASA.

Previous models suggested that type Ia supernovae events occur more than 100 million years into the star’s lifetime, but as scientists confirmed more and more type Ia events, around fifty percent of them were found to explode less than 100 million years after their host galaxy’s main star formation period.

On the case to find out how and why this is so is a team of astronomers led by Dr Bo Wang from the Yunnan Observatory of the Chinese Academy of Sciences. Wang and colleagues have developed a computer model to track stellar evolution. They performed calculations for 2,600 binary systems consisting of a white dwarf and a hot, blue helium star, that is, a stars which has lost most or all of its hydrogen, leaving an exposed helium core.

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 well within 100 million years of its formation, early on in the life of the galaxy they formed in.

“Type Ia supernovae are a key tool to determine the scale of the Universe so we need to be sure of their properties,” says team member Zhanwen Han. “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 eventually be verified by future observations from the Large Sky Area Multi-Object Fiber Spectral Telescope (LAMOST). LAMOST is a Chinese venture and will combine a large aperture with a wide field of view and be capable of collecting light from objects down to magnitude 20.5, at a rate of tens of thousands of spectra per night of observations.

A paper describing the results of the computer modelling by Wang et al is published in the Monthly Notices of the Royal Astronomical Society.

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