Wolf-Rayet Star “Nasty 1” Transitional Stage in Evolution of Massive Stars

A very rapidly evolving, supermassive star with a newly formed nebula only a few thousand years old

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Space news (supermassive stars: Wolf-Rayet stars; star NaSt1) – 3,000 light-years away on the edge of a pancake-shaped disk of gas moving at 22,000 mph – 

Astronomers using the Hubble Space Telescope have discovered new clues concerning a nearby supermassive, rapidly aging star they have nicknamed “Nasty 1”. Designated NaSt1 in astronomy catalogs, “Nasty 1” when first discovered decades ago was identified as a non-typical Wolf-Rayet star with an orbiting disk-like structure. A vast disk estimated to be almost 2 trillion miles wide astronomers now think formed due to a companion star snacking on its outer envelope. Putting NaSt1 in a class of Wolf-Rayet stars astronomers haven’t observed often during the human journey to the beginning of space and time. A star type possibly representing a transition stage in the evolution of supermassive stars. 

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“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.” 

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Study leader Jon Mauerhan of the University of California, Berkley. Credit: University of California, Berkley.

In the case of NaSt1, computer simulations show a supermassive star evolving really fast and swelling as it begins to run out of hydrogen. Its outer hydrogen envelope is loosely bound and is gravitationally stripped from the star- astronomers call this process stellar cannibalism – by a more compact, nearby companion star. In the process the more compact star gains mass, while the more massive star loses its hydrogen envelope, exposing its helium core and eventually becoming a Wolf-Rayet star. 

The mass-transfer model is the favored process for how Wolf-Rayet stars evolve at the moment and considering at least 70 percent of supermassive stars detected, so far, are members of binary star system, this seems logical. Astronomers used to think this type of star could also form when a massive sun ejects its hydrogen envelope. But the direct mass loss model by itself can’t account for the number of Wolf-Rayet stars observed relative to less-evolved supermassive suns in the Milky Way.  

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“We’re finding that it is hard to form all the Wolf-Rayet stars we observe by the traditional wind mechanism because the 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.” 

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Co-author of study Nathan Smith of the University of Arizona in Tucson. Credit: The University of Arizona.

Astronomers computer models show that the mass-transfer process isn’t always perfectly efficient. Matter can only transfer from NaSt1 at a certain rate, left over material begins orbiting, creating a disk-like structure. 

“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.” 

Observing Nasty 1 (star NaSt1) through the clock of gas and dust surrounding this star system hasn’t been easy. The intervening disk-like structure even blocks the view of the Hubble Space Telescope. Scientists haven’t been able to measure the distance between the stars, their mass, or the volume of material transferring to the smaller companion star.  

Astronomers have been able to discover a few items concerning the disk-like structure surrounding Nasty 1. Measurements indicate it’s traveling at around 22,000 mph in the outer nebula, a slower speed than recorded in other stars of this type. Scientists think this indicates a much less energetic supernova than was recorded for other events, like Era Carinae. In this case and other similar stars, the gas in the outer nebula has been recorded in the hundreds of thousands of miles per hour. Nasty 1 could be different supernova animal altogether.  

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High atop the Cerro Manqui peak at the Las Campanas Observatory in Chile the twin the Walter Baade Telescope is the first of the twin 6.5-meter Magellan telescopes to be completed. Credit: Ico.cl

Nasty 1 could also lose its outer envelope of hydrogen intermittently. Previous studies in the infrared light provided clues indicating the existence of a dense pocket of hot gas and dust close to the central stars in the region. More recent observations using the Magellan Telescope located at the Las Campanas Observatory in Chile has also detected a bigger pocket of cooler gas and dust possibly indirectly blocking light from these stars. Astronomers think the existence of warm dust in the region implies it formed just recently, perhaps intermittently, as elementally enriched matter from the stellar winds of massive stars collides, mixes, flows away, and cools. Irregular stellar wind strength, the rate at which star NaSt1 loses its outer envelope, could also help explain the observed clumpy structure and gaps noted in the outer regions of the disk.  

Astrophysicists used NASA’s Chandra X-ray Observatory to measure the hypersonic winds screaming from each star. Readings showed a scorching hot plasma, indicating colliding stellar winds producing high-energy shockwaves that glow in X-rays. This is consistent with previous data collected on other evolving Wolf-Rayet star systems. We’ll get a better view once the outer hydrogen of Nasty 1’s (star NaSt1) depleted, and the mass-transfer process completes. Eventually, the gas and dust in the lumpy, disk-like structure will dissipate, giving us a clearer view of this mysterious binary star system.   

 

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NASA’s Chandra X-ray Observatory has shown the cosmos is full of objects and events far beyond anything we imagined when we first started the human journey to the beginning of space and time. Credit: NASA/Chandra

Nasty 1’s still evolving!

“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.” 

Astronomers continue to study Nasty 1 and its peculiar, unusual disk-like structure looking for clues to explain the mysteries surrounding its origin. 

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Superstar Binaries Like Eta Carinae More Common Than First Thought

Astronomers using NASA’s Spitzer and Hubble space telescopes discovered similar superstar binaries in four nearby galaxies

Eta Carinae's great eruption in the 1840s created the billowing Homunculus Nebula, imaged here by Hubble, and transformed the binary into a unique object in our galaxy. Astronomers cannot yet explain what caused this eruption. The discovery of likely Eta Carinae twins in other galaxies will help scientists better understand this brief phase in the life of a massive star. Credits: NASA, ESA, and the Hubble SM4 ERO Team
Eta Carinae’s great eruption in the 1840s created the billowing Homunculus Nebula, imaged here by Hubble, and transformed the binary into a unique object in our galaxy. Astronomers cannot yet explain what caused this eruption. The discovery of likely Eta Carinae twins in other galaxies will help scientists better understand this brief phase in the life of a massive star.
Credits: NASA, ESA, and the Hubble SM4 ERO Team

Space news (February 15, 2016) – 7,500 light-years away in the southern constellation of Carina

Astronomers combing through data provided by the Hubble and Spitzer space telescopes looking for superstar binaries like Eta Carinae think they have finally found a few additional instances in nearby galaxies. 

The signature balloon-shaped clouds of gas blown from a pair of massive stars called Eta Carinae have tantalized astronomers for decades. Eta Carinae has a volatile temperament, prone to violent outbursts over the past 200 years. Observations by the newly repaired Space Telescope Imaging Spectrograph (STIS) aboard NASA’s Hubble Space Telescope reveal some of the chemical elements that were ejected in the eruption seen in the middle of the 19th century. Image credit: NASA/ESA
The signature balloon-shaped clouds of gas blown from a pair of massive stars called Eta Carinae have tantalized astronomers for decades. Eta Carinae has a volatile temperament, prone to violent outbursts over the past 200 years.
Observations by the newly repaired Space Telescope Imaging Spectrograph (STIS) aboard NASA’s Hubble Space Telescope reveal some of the chemical elements that were ejected in the eruption seen in the middle of the 19th century.
Image credit: NASA/ESA

We knew others were out there,” said co-investigator Krzysztof Stanek, a professor of astronomy at Ohio State University in Columbus. “It was really a matter of figuring out what to look for and of being persistent.

Astrophysicists had previously conducted a survey of data on seven galaxies provided by the pair of space telescopes between 2012-2014. During this extensive study of the data, scientists found no superstar binaries similar to Eta Carinae. They determined they needed to devise a more sensitive way to identify possible candidates. 

Astronomers devised an optical and infrared fingerprint to detect and identify these five superstar binaries similar to Eta Carinae. With Spitzer we see a steady increase in brightness starting at around 3 microns and peaking between 8 and 24 microns,” explained Khan. “By comparing this emission to the dimming we see in Hubble’s optical images, we could determine how much dust was present and compare it to the amount we see around Eta Carinae.

During the follow-up survey conducted in 2015, astronomers discovered data indicating the existence of five superstar binaries similar to Eta Carinae in four nearby galaxies. 

The nearby spiral galaxy M83 is currently the only one known to host two potential Eta Carinae twins. This composite of images from the Hubble Space Telescope's Wide Field Camera 3 instrument shows a galaxy ablaze with newly formed stars. A high rate of star formation increases the chances of finding massive stars that have recently undergone an Eta Carinae-like outburst. Bottom: Insets zoom into Hubble data to show the locations of M83's Eta twins. Credits: NASA, ESA, the Hubble Heritage Team (STScI/AURA) and R. Khan (GSFC and ORAU)
The nearby spiral galaxy M83 is currently the only one known to host two potential Eta Carinae twins. This composite of images from the Hubble Space Telescope’s Wide Field Camera 3 instrument shows a galaxy ablaze with newly formed stars. A high rate of star formation increases the chances of finding massive stars that have recently undergone an Eta Carinae-like outburst. Bottom: Insets zoom into Hubble data to show the locations of M83’s Eta twins.
Credits: NASA, ESA, the Hubble Heritage Team (STScI/AURA) and R. Khan (GSFC and ORAU)

In nearby galaxy M83, just 15 million light-years away, astronomers discovered two superstar binaries similar to Eta Carinae. They also found one superstar binary each in NGC 6946, M101 and M51, located between 18-26 million light-years away.

Researchers found likely Eta twins in four galaxies by comparing the infrared and optical brightness of each candidate source. Infrared images from NASA's Spitzer Space Telescope revealed the presence of warm dust surrounding the stars. Comparing this information with the brightness of each source at optical and near-infrared wavelengths as measured by instruments on Hubble, the team was able to identify candidate Eta Carinae-like objects. Top: 3.6-micron images of candidate Eta twins from Spitzer's IRAC instrument. Bottom: 800-nanometer images of the same sources from various Hubble instruments. Credits: NASA, ESA, and R. Khan (GSFC and ORAU)
Researchers found likely Eta twins in four galaxies by comparing the infrared and optical brightness of each candidate source. Infrared images from NASA’s Spitzer Space Telescope revealed the presence of warm dust surrounding the stars. Comparing this information with the brightness of each source at optical and near-infrared wavelengths as measured by instruments on Hubble, the team was able to identify candidate Eta Carinae-like objects. Top: 3.6-micron images of candidate Eta twins from Spitzer’s IRAC instrument. Bottom: 800-nanometer images of the same sources from various Hubble instruments.
Credits: NASA, ESA, and R. Khan (GSFC and ORAU)

An additional study indicates each of these five candidates has the same optical and infrared fingerprint as Eta Carinae. Astronomers think within each a high mass star is buried in five to ten solar masses of gas and dust, like Eta Carinae. 

More study’s needed

They plan additional study of these five candidate superstar binaries similar to Eta Carinae, to determine if they’re indeed what they were looking for. The launch of the James Webb Space Telescope, late in 2018, will enable additional and better study of these five possible superstar binaries. 

The James Webb Telescope’s Mid-infrared instrument (MIRI) has ten times the angular resolution of the Spitzer Space Telescope. It’s also most sensitive to the wavelengths needed to detect superstar binaries at their brightest. 

Combined with Webb’s larger primary mirror, MIRI will enable astronomers to better study these rare stellar laboratories and to find additional sources in this fascinating phase of stellar evolution,” said Sonneborn, NASA’s project scientist for Webb telescope operations. It will take Webb observations to confirm the Eta twins as true relatives of Eta Carinae.

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