'Nasty' Wolf-Rayet behaving unusually

A massive Wolf-Rayet star is displaying unusual properties, giving astronomers new evidence that these super-huge stars can form binary interactions.

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Artist's illustration of a gas disk surrounding a bright Wolf-Rayet star (centre), while a close companion begins pulling its gas away, as shown by the bridge of bright material connecting the two
Credit: NASA/ESA/G. Bacon (STScl)

Hubble has revealed new information about an unusual Wolf-Rayet star, the like of which has never before been observed in the Milky Way.

The star, nicknamed ‘Nasty 1’, is substantially bigger than the Sun and is thought to represent a transitory stage in the evolution of Wolf-Rayets.

Wolf-Rayet stars are notable because they are missing their outer hydrogen envelope and have begun burning helium at their core, making them incredibly bright and fiercely hot.

But Nasty 1 is not your typical Wolf-Rayet. During observations, astronomers would have expected to see twin lobes of gas flowing from opposite sides, but instead Hubble has revealed a pancake-shaped disk of gas encircling it.

This disk is almost two trillion miles wide, and astronomers think it could have formed as the result of an unseen companion star feeding on the outer rims of the Wolf-Rayet.

“We were excited to see this disk-like structure because it may be evidence for a Wolf-Rayet star forming from a binary interaction,” says study leader Jon Mauerhan of the University of California, Berkeley. “There are very few examples in the galaxy of this process in action because this phase is short-lived, perhaps lasting only a hundred thousand years, while the timescale over which a resulting disk is visible could be only ten thousand years or less.”

A scenario constructed by the team shows a massive star beginning to swell up as it runs out of hydrogen. The outer hydrogen envelope then becomes more loosely bound and the massive star begins to lose its mass to the companion star.  The hydrogen envelope is eventually lost, exposing a helium core and turning the star into a Wolf-Rayet.

However, sometimes during the mass exchange tussle, parts of stripped matter may spill out and form a disk around the binary. The team believes this may be occurring in the case of Nasty 1.

“Mass exchange in binary systems seems to be vital to account for Wolf-Rayet stars and the supernovae they make, and catching binary stars in this short-lived phase will help us understand this process,” says Nathan Smith of the University of Arizona in Tucson, who is a co-author on the paper.

Nasty 1’s name derives from NaSt1, after Jason Nassau and Charles Stephenson, the astronomers who discovered it in 1963. Previous observations have revealed the extra material surrounding the star to be travelling about 22,000 miles per hour in the outer nebula. This is much slower than similar stars and indicates that the star expelled its material relatively slowly.

“What evolutionary path the star will take is uncertain, but it will definitely not be boring,” says Mauerhan. “Nasty 1 could evolve into another Eta Carinae-type system. To make that transformation, the mass-gaining companion star could experience a giant eruption because of some instability related to the acquiring of matter from the newly formed Wolf-Rayet.

"Or, the Wolf-Rayet could explode as a supernova. A stellar merger is another potential outcome, depending on the orbital evolution of the system. The future could be full of all kinds of exotic possibilities, depending on whether it blows up or how long the mass transfer occurs, and how long it lives after the mass transfer ceases.”


 

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