‘Never before seen’ star behavior observed by Hubble

Chuck Bednar for redOrbit.com – @BednarChuck

An unusual star, so unique that astronomers gave it the nickname “Nasty 1”, is the target of new analysis conducted using the Hubble Space Telescope, shedding light on what could be a brief transitory stage in the evolutionary process of extremely massive stars.

“Nasty 1,” officially named NaSt1, is described by NASA as “a hefty, rapidly aging star whose behavior has never been seen before in our Milky Way galaxy.” First discovered several decades ago, NaSt1 is a Wolf-Rayet star (a rapidly evolving type of star that is far more massive than our sun), but it doesn’t look like your run-of-the-mill Wolf-Rayet star, the agency explained.

These stars typically lose their hydrogen-filled outer layers quickly, leaving their super-hot and extremely bright helium-burning cores exposed. The astronomers behind the new study thought that they would see twin lobes of gas flowing from opposite sides of NaSt1, but instead, Hubble showed them a pancake-shaped, two trillion mile wide disk of gas encircling the star.

Lead investigator Jon Mauerhan of the University of California, Berkeley and his colleague, who published their findings online in the Monthly Notices of the Royal Astronomical Society on May 21, believe that the disk may have formed from previously undetected companion star that dined on the outer envelope of the newly-formed Wolf-Rayet star.

Possible models of Wolf-Rayet star formation

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

He and his co-authors believe that the massive star would have evolved very quickly, swelling up as it starts to exhaust its supply of hydrogen. The outer hydrogen envelope would start becoming vulnerable to gravitational stripping by a nearby companion star. As a result, this star would start gaining mass, and the original would lose its hydrogen envelope, exposing its helium core.

This is one way that scientists believe that Wolf-Rayet stars form. Another takes place when a massive star ejects its hydrogen envelope through a strong stellar wind streaming with charged particles, NASA explained. The binary interaction model is gaining acceptance because of the realization that at least 70 percent of all massive stars are members of double-star systems, and the direct mass model along cannot account for the number of Wolf-Rayet stars.

However, the mass exchange that takes place in the binary interaction model is not always very efficient, the US space agency added, and some of the stripped matter can spill out and form a disk around the binary. Mauerhan believes that this is the what’s happening in this case, as his team things that “there is a Wolf-Rayet star buried in the nebula” and that the nebula itself “is being created by this mass-transfer process.”

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