Astronomers using the Wide Field Camera 3 on NASA’s Hubble Space Telescope have discovered a pancake-shaped disk of gas around an extremely bright star in the Milky Way Galaxy. The star is nicknamed ‘Nasty 1,’ derived from its catalog name of NaSt1.
This illustration reveals a vast disk of gas surrounding the Wolf-Rayet star Nasty 1, shown at center. A close companion star is pulling gas from the star, shown by the bridge of bright material connecting the two stars. This act of celestial cannibalism exposes the massive star’s hot, helium core. Some of the material, however, is escaping into space, forming the huge disk. This disk structure has never been seen before around a Wolf-Rayet star. Image credit: NASA / ESA / G. Bacon, STScI.
Nasty 1 is also known as Wolf-Rayet 122 or WR 122. The star’s catalogue name, NaSt1, is derived from the first two letters of each of the two astronomers who discovered it in 1963, Jason Nassau and Charles Stephenson.
The star lies at a distance of about 3,000 light-years and is thought to be a Wolf-Rayet star – a massive, rapidly evolving star weighing well over 10 times the mass of our Sun. It is losing its hydrogen-filled outer layers quickly, exposing its super-hot and extremely bright helium-burning core.
But Nasty 1 doesn’t look like a typical Wolf-Rayet star. The astronomers using Hubble had expected to see twin lobes of gas flowing from opposite sides of the star, perhaps similar to those emanating from the massive star Eta Carinae, which is a Wolf-Rayet candidate.
Instead, they revealed a pancake-shaped disk of gas encircling the star. The vast disk is nearly 2 trillion miles wide, and may have formed from an unseen companion star that snacked on the outer envelope of the newly formed 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. 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,” said Dr Jon Mauerhan of the University of California, Berkeley, lead author of the paper reporting the results in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).
In the scenario proposed by Dr Mauerhan and co-authors, a massive star evolves very quickly, and as it begins to run out of hydrogen, it swells up. Its outer hydrogen envelope becomes more loosely bound and vulnerable to gravitational stripping, or a type of stellar cannibalism, by a nearby companion star. In that process, the more compact companion star winds up gaining mass, and the original massive star loses its hydrogen envelope, exposing its helium core to become a Wolf-Rayet star.
Compass and scale image of the Wolf-Rayet star Nasty 1. Image credit: NASA / ESA / Z. Levay, STScI / J. Mauerhan, University of California, Berkeley.
Another way Wolf-Rayet stars are said to form is when a massive star ejects its own hydrogen envelope in a strong stellar wind streaming with charged particles. The binary interaction model where a companion star is present is gaining traction because astronomers realize that at least 70 percent of massive stars are members of double-star systems. Direct mass loss alone also cannot account for the number of Wolf-Rayet stars relative to other less-evolved massive stars in the galaxy.
“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 co-author Dr Nathan Smith of the University of Arizona in Tucson.
“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.”
But the mass transfer process in mammoth binary systems isn’t always efficient. Some of the stripped matter can spill out during the gravitational tussle between the stars, creating a disk around the binary.
“That’s what we think is happening in Nasty 1. 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,” Dr Mauerhan said.
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Jon Mauerhan et al. 2015. Multiwavelength observations of NaSt1 (WR 122): equatorial mass loss and X-rays from an interacting Wolf–Rayet binary. MNRAS 450 (3): 2551-2563; doi: 10.1093/mnras/stv257
