Friday, May 22, 2015

Astronomers Nickname Cannibalized Star 'Nasty 1'

Astronomers using NASA's Hubble Space Telescope have uncovered surprising new clues about a hefty, rapidly aging star whose behavior is so unusual that astronomers have nicknamed it "Nasty 1" - a play on its catalog name of NaSt1. 

First discovered several decades ago, Nasty 1 was identified as a Wolf-Rayet star, a rapidly evolving star that is much more massive than our Sun. A Wolf-Rayet star loses its hydrogen-filled outer layers quickly, exposing its super-hot and extremely bright helium-burning core.

But astronomers say that Nasty 1 doesn’t look like a typical Wolf-Rayet star. When using Hubble, they had expected to see twin lobes of gas flowing from opposite sides of the star, similar to other Wolf-Rayet stars. Instead, Hubble revealed a pancake-shaped disk of gas encircling Nasty 1. 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.  

This artist's illustration reveals a vast disk of gas surrounding a massive, bright Wolf-Rayet star (shown at center). A close companion star is pulling gas from the Wolf-Rayet, 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.  
Artist's Illustration Credit: NASA, ESA, and G. Bacon (STScI)

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

In the team’s proposed scenario, 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.

Credit: NASA, ESA, and Z. Levay (STScI)

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 Nathan Smith of the University of Arizona in Tucson, who is a co-author on the new NaSt1 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.”

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,” Mauerhan said. “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.”