Page last updated at 00:12 GMT, Tuesday, 14 April 2009 01:12 UK

Thieving dwarves cause supernovae

SN1604 supernova (Nasa)
Light from Type I supernovae is used as a standard for distances

Researchers have come up with a theory for how stars can end in a spectacular so-called Type Ia supernova in less than 100 million years.

While such early-stage supernovae are well-known, theory has been unable to explain them.

The secret, the researchers say, is that white dwarf stars steal mass from nearby "helium stars" until they have enough mass to initiate a supernova.

The research appears in Monthly Notices of the Royal Astronomical Society.

The new theory concerns white dwarf stars, the dense remains of stars like the Sun that have fused their hydrogen into helium and then the helium into carbon and oxygen.

Existing theory held that these carbon/oxygen white dwarves can gather up further mass from nearby companion stars.

When they reach a critical mass, about 40% more than the Sun, they can undergo further fusion. In just a few seconds, the white dwarf's carbon is fused into heavier elements in a runaway process that releases huge amounts of energy into the cosmos: a supernova.

Too fast

Because the process happens with a known brightness, such supernovae have been used by astronomers as a standard for distances.

Before this investigation, there was no model which could produce a large population of such young Type Ia supernovae
Xuefei Chen

However, the theory held that the mass-accreting process would take more than 100 million years to occur. However, observations have it that about half of the Type Ia supernovae observed occur in less than that amount of time.

Bo Wang and colleagues of the National Astronomical Observatories at the Chinese Academy of Sciences investigated the problem, performing calculations based on 2600 relatively nearby white dwarf/companion star pairs.

They found that if the companion star was a so-called helium star - which had fused all its hydrogen to form a helium core - then the white dwarf could steal away mass more quickly, leading to a supernova event in less than 100 million years.

"Before this investigation, there was no model which could produce a large population of such young Type Ia supernovae, and no knowledge of a way to produce such numbers," study co-author Xuefei Chen told BBC News.

Dr Chen said that the theft of mass from helium stars is likely to produce most of the "young" Type Ia supernovae that we observe, if not all of them.

"A significant population of young Type Ia supernovae may have an effect on models of galactic chemical evolution, since they would return large amounts of iron to the interstellar medium much earlier than previously thought," Dr Chen added.

"It may also have an impact on cosmology, as they are used as cosmological distance indicators."

The team now plans to study the extremely high-velocity helium stars that would be the remnants of such supernovae.

While such fast-moving stars have been spotted before, their speed was attributed to gravitational interactions, rather than supernovae. Those remnants may now serve as proof of the team's theory.



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