Unexpected Gas Cloud Found Around a Wolf-Rayet Star NaSt1

May 26, 2015

Using the Hubble Space Telescope, astronomers have found a hydrogen gas disk around a Wolf-Rayet class star named NaSt1, thought to be the result of a messy mass-transfer process between NaSt1 and its companion star.

This is one of those discoveries where astronomers expected to find one thing, yet they found something else that makes them abandon or change current theories and then search for more empirical evidence to support new theories.

NaSt1 is a Wolf-Rayet star 3,000 light-years from Earth, discovered a several decades ago. It has now been found that it is surrounded by a cloud of hydrogen 0.3 light-years, or almost 3 trillion kilometres, wide. The discovery of such disk around a Wolf-Rayet star like NaSt1 is the first such observation and it is most likely caused by another, companion star accreting loose hydrogen in a very inefficient way. NaSt1 is thus nicknamed Nasty 1.

"That's what we think is happening in Nasty 1," said Jon Mauerhan, of the University of California, Berkeley, USA. He is the lead author of the study published in Monthly Notices of the Royal Astronomical Society. "We think there is a Wolf-Rayet star buried inside the nebula, and we think the nebula is being created by this mass-transfer process. So this type of sloppy stellar cannibalism actually makes Nasty 1 a rather fitting nickname."

Wolf-Rayet stars age quickly by burning and losing their outer, hydrogen-filled layers and exposing their very hot and bright helium core. We are unsure how this happens exactly. One theory is that strong solar winds emitted by the Wolf-Rayet star, which have at least 20 solar masses, literally blow away the now-loose hydrogen layer. This was somewhat a prevailing theory. Another theory, supported by this new observation, is that the hydrogen layer is stripped by a more compact companion star. Having in mind that over 70% of Wolf-Rayet stars are part of a binary system, the second theory could very well be the correct one, especially since other massive, non-Wolf-Rayet stars, evolve more slowly, yet they too lose hydrogen layers via stellar winds.

"We're finding that it is hard to form all the Wolf-Rayet stars we observe by the traditional wind mechanism, because mass loss isn't as strong as we used to think," said Nathan Smith of the University of Arizona, Tucson, USA. He is a co-author on the paper. "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."

This short process, where mass exchange creates a wide hydrogen disk, lasts for about 100,000 years. This is indeed a really short period of time compared to most cosmological phenomena, so it is no wonder this kind of cloud is seldom seen in our galaxy. The hydrogen cloud is the reason why astronomers don’t know the exact mass of either star, the distance between them or the amount of material accreted onto companion star.

"We were excited to see this disk-like structure because it may be evidence for a Wolf-Rayet star forming from a binary interaction," added Mauerhan. "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."

Not only do we not know the distance between stars and their masses, we are also not sure what exactly happens inside the disk, but we know that the gas is not homogeneous. Pockets of both cold and hot hydrogen gas have been observed. By taking a look with Chandra X-ray Observatory, astronomers have found that the solar winds from both stars are colliding, creating hot plasma, then dissipating and cooling.

As the time goes by, the hydrogen gas around NaSt1 will scatter and reveal what exactly goes inside. By that time, we will hopefully find many similar stars and learn for certain how Wolf-Rayet stars lose their hydrogen, but for now, this is one-of-the-kind discovery and astronomers will keep their eye on the NaSt1 and how it further evolves.

"What evolutionary path the star will take is uncertain, but it will definitely not be boring," said 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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