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Twin stars could doom planets

planetary collision

This artist's concept illustrates an imminent planetary collision around a pair of double stars. NASA's Spitzer Space Telescope found evidence that such collisions could be common around certain types of tight binary star systems.

  • Dust cloud spotted circling stellar twins
  • Could be the remains of planetary collisions

Tight double-star systems might not be the best places for life to spring up, according to a new study using data from NASA’s Spitzer Space Telescope.

The infrared observatory spotted a surprisingly large amount of dust around three mature, close-orbiting star pairs. Where did the dust come from? Astronomers say it might be the aftermath of tremendous planetary collisions.

“This is real-life science fiction,” said Jeremy Drake of the Harvard-Smithsonian Centre for Astrophysics, Cambridge, Mass. “Our data tell us that planets in these systems might not be so lucky—collisions could be common.”

“It’s theoretically possible that habitable planets could exist around these types of stars, so if there happened to be any life there, it could be doomed.”

Drake is the principal investigator of the research, published in the August19 issue of the Astrophysical Journal Letters.

The particular class of binary, or double, stars in the study are about as snug as stars get. Named RS Canum Venaticorums, or RS CVns for short, they are separated by only about 3.2 million kilometres (2 million miles), or two percent of the distance between Earth and our Sun.

The stellar pairs orbit around each other every few days, with one face on each star perpetually locked and pointed toward the other.

binary star

Artist's concept of a tight pair of stars and a surrounding disc of dust—most likely the shattered remains of planetary smash-ups. Using NASA's Spitzer Space Telescope, scientists found dusty evidence for such collisions around three sets of stellar twins.

Colliding planets

The close-knit stars are similar to the Sun in size and are probably about a billion to a few billion years old—roughly the age of our Sun when life first evolved on Earth.

But these stars spin much faster, and, as a result, have powerful magnetic fields, and giant, dark spots. The magnetic activity drives strong stellar winds—gale-force versions of the solar wind—that slow the stars down, pulling the twirling duos closer over time.

And this is where the planetary chaos may begin.

As the stars cosy up to each other, their gravitational influences change, and this could cause disturbances to planetary bodies orbiting around both stars. Comets and any planets that may exist in the systems would start jostling about and banging into each other, sometimes in powerful collisions.

This includes planets that could theoretically be circling in the double stars’ habitable zone, a region where temperatures would allow liquid water to exist.

Though no habitable planets have been discovered around any stars beyond our Sun at this point in time, tight double-star systems are known to host planets; for example, one system not in the study, called HW Vir, has two gas-giant planets.

“These kinds of systems paint a picture of the late stages in the lives of planetary systems,” said Marc Kuchner, a co-author from NASA Goddard Space Flight Centre. “And it’s a future that’s messy and violent.”

Spitzer space telescope

An artist's impression of the Spitzer Space Telescope, which studies the cosmos at infrared wavelengths.

Not a fluke

Spitzer spotted the infrared glow of hot dusty discs, about the temperature of molten lava, around three such tight binary systems. One of the systems was originally flagged as having a suspicious excess of infrared light in 1983 by the Infrared Astronomical Satellite.

In addition, researchers using Spitzer recently found a warm disc of debris around another star that turned out to be a tight binary system.

The astronomy team says that dust normally would have dissipated and blown away from the stars by this mature stage in their lives. They conclude that something—most likely planetary collisions—must therefore be kicking up the fresh dust.

In addition, because dusty discs have now been found around four, older binary systems, the scientists know that the observations are not a fluke. Something chaotic is very likely going on.

If any life forms did exist in these star systems, and they could look up at the sky, they would have quite a view. Marco Matranga, first author of the paper, from the Harvard-Smithsonian Centre for Astrophysics and now a visiting astronomer at the Palermo Astronomical Observatory in Sicily, said, “The skies there would have two huge suns, like the ones above the planet Tatooine in Star Wars”.

Adapted from information issued by SAO / NASA / JPL-Caltech.

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Big stars aren’t special

Artist's impression of the dusty cloud surrounding the star IRAS 13481-6124

Artist's impression of the dusty cloud surrounding the star IRAS 13481-6124, and the jets of particles spat out by the star. Observations suggest big stars such as this one—20 times the mass of the Sun—are born in the same way as smaller stars.

  • Dusty cloud discovered encircling a young star
  • Star is 20 times as massive as the Sun
  • Big stars form the same way as small ones

Astronomers have obtained the first image of a dusty disc-shaped cloud closely encircling a massive baby star, providing direct evidence that massive stars form in the same way as their smaller brethren.

The discovery, made thanks to a combination of the European Southern Observatory’s (ESO) telescopes, is described in an article in this week’s issue of Nature.

“Our observations show a disc surrounding an embryonic young, massive star, which is now fully formed,” says Stefan Kraus, who led the study. “One can say that the baby is about to hatch!

The team of astronomers looked at an object known by the cryptic name of IRAS 13481-6124. About twenty times the mass of our Sun and five times its radius, the young central star, which is still surrounded by its pre-natal cocoon, is located about 10,000 light-years away.

From archival images obtained by the NASA Spitzer Space Telescope as well as from observations done with the APEX 12-metre submillimetre telescope, astronomers discovered the presence of a jet, or focused stream of particles.

“Such jets are commonly observed around young low-mass stars and generally indicate the presence of a disc,” says Kraus.

Circumstellar discs are an essential ingredient in the formation process of low-mass stars such as our Sun. However, it is not known whether such discs are also present during the formation of stars of more than about 10 times the mass of the Sun, where the strong light emitted might prevent mass falling inwards towards the star.

For instance, it has been proposed that massive stars might form when smaller stars merge.

A view of the dust cloud surrounding IRAS 13481-6124

A real view of the dust cloud surrounding the star. Combining the light from multiple telescopes enabled a more detailed view to be made of it.

Telescope team effort

In order to discover and understand the properties of this disc, astronomers employed ESO’s Very Large Telescope Interferometer (VLTI). By combining light from three of the VLTI’s 1.8-metre Auxiliary Telescopes with the AMBER instrument, the facility enables astronomers to see details equivalent to those a telescope with a mirror of 85 metres in diameter would see.

The resulting resolution is about 2.4 milliarcseconds, which is equivalent to spotting the head of a screw on the International Space Station from the ground, or more than 10 times the resolution possible with current visible-light telescopes in space.

With this unique capability, complemented by observations done with another of ESO’s telescopes, the 3.58-metre New Technology Telescope at La Silla, Kraus and colleagues were able to detect a disc around IRAS 13481-6124.

“This is the first time we could image the inner regions of the disc around a massive young star”, says Kraus. “Our observations show that formation works the same for all stars, regardless of mass.”

The astronomers conclude that the system is about 60,000 years old, and that the star has reached its final mass. Because of the intense light of the star—30,000 times more luminous than our Sun—the disc will soon start to evaporate.

The flared disc extends to about 130 times the Earth–Sun distance—or 130 astronomical units (AU)—and has a total mass similar to that of the star, roughly 20 times the Sun.

Adapted from information issued by ESO / Spitzer / NASA / JPL / S. Kraus.

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