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Stellar cannibal hides it age well

Artist's impression of the star BP Piscium

Artist's impression of the star BP Piscium. Astronomers think it has eaten another star, with the leftover material forming a surrounding gas and dust cloud, within which new planets might form. Some of the leftover stuff is being shot out in "jets" from near the poles of the star.

  • Young looking star is actually probably quite old
  • Seems to have recently devoured a companion star
  • Leftover crumbs could be forming into planets

An astronomer may have caught a cannibalistic star in the act of devouring a companion and making a new generation of planets from the resulting cloud of leftover crumbs.

Using data from NASA’s Chandra X-ray Observatory, Joel Kastner, professor at Rochester Institute of Technology (RIT), has found evidence that a star in the constellation of Pisces—called BP Piscium, or BP Psc for short—is not the young star it appears to be, but is more likely a one billion-year-old red giant that has gobbled up a star or planet in its vicinity.

The star’s extreme properties have puzzled astronomers since Kastner and Ben Zuckerman, professor at the University of California, Los Angeles, first looked at BP Psc 15 years ago. The star is about 1,000 light-years from Earth.

False-colour image of BP Piscium

False-colour image of BP Piscium, put together using X-ray and optical wavelength observations. The two jets blasting out of the star are several light-years long.

Conflicting characteristics have caused confusion as to whether the star is young or old.

Kastner attributes the star’s potentially deceptive youthful appearance to two things: an surrounding cloud of gas and dust that resembles the sort that forms planets around young stars; and prominent “jets” extending from the poles of the star that eject material at high velocity.

A typical young star sucks in material from the surrounding cloud, incorporating about 90 percent of the material and spitting out the rest through jets or geysers shooting out in opposite directions.

Kastner and his colleagues were doubtful about the youth of the star. For one thing, the star is isolated, whereas most young stars form in clusters.

“As hard as people have looked, they have not been able to find [another] young star near BP Psc,” says Kastner, a professor in RIT’s Chester F. Carlson Centre for Imaging Science. “That was one of several things that made Ben [Zuckerman] and me suspect that it wasn’t actually young.”

Second, this enigmatic star in the Pisces constellation lacks the large abundance of lithium on its surface that is typical of young stars. Older stars lose their lithium in nuclear reactions when mixing and churning folds the gases into the centre of the star. According to Kastner, other key spectral features involving the star’s radius and surface gravity also point to the star’s advanced age.

Stellar cannibalism

Kastner is ready to close the debate with data obtained from the Chandra X-ray Observatory.

“The last piece of evidence, which, to me, is the nail in the coffin that BP Psc is old rather than young, is that its rate of X-ray production is very similar to old, yet rapidly spinning, giant stars that have surface temperatures similar to BP Psc,” Kastner says.

If BP Psc were a young star, it would emit X-rays in the hundreds, even up to a few thousand, in a day’s observing time with Chandra, Kastner notes. Instead, it is a weak X-ray source.

Artist's impression of the Chandra X-ray Observatory.

Artist's impression of NASA’s Chandra X-ray Observatory.

“We stared at BP Psc for one day with Chandra and only detected about 18 X-rays,” Kastner says. “We could almost name them.”

The rate of X-rays coming from the star are in keeping with a class of rapidly rotating old stars having similar temperature to BP Psc, Kastner says. This class is thought to be the result of one star swallowing another close companion star.

“Our working speculation is that we are observing the star right at the point at which it has swallowed its companion and hence formed a [surrounding cloud from the leftover bits],” Kastner says. “Some of the material that used to be its companion has fallen onto the star and some has been shot out at high speeds, and that’s what we’re seeing.”

The enigmatic star is likely about a billion years old and just entering the red giant stage in its life cycle in which it swells to digest its star or planet companion.

“It could be a small star or a large planet,” Kastner says. “We don’t know which it could be, but we’re very interested in finding out.”

“In order to understand the extrasolar planets that are now being discovered by the dozen, we need to figure out how planets might be forming and therefore where we should go look for them,” Kastner says. “I think this object is especially interesting because it gives us a good shot at finding young planets around an old star.”

Image credits: (X-ray) NASA / CXC / RIT / J. Kastner et al; (optical) UCO / Lick / STScI / M. Perrin et al; (illustration) CXC / M. Weiss.

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Today’s recipe: Roasted planet

Artist's illustration of the gas giant planet HD 209458b

Artist's illustration of the gas giant planet HD 209458b, as seen from a hypothetical nearby planet. The planet orbits so close to its star that its heated atmosphere is escaping into space in a comet-like tail.

  • Super-hot planet probably has a comet-like tail
  • 100 times closer to its star than Jupiter is to the Sun
  • Hubble has studied the planet’s atmosphere

As if the debate over what is and what is not a planet hasn’t gotten confusing enough, Hubble Space Telescope astronomers have now confirmed the existence of a tortured, baked object that could be called a “cometary planet.”

The gas giant planet, dubbed HD 209458b, is orbiting so close to its star that its heated atmosphere is escaping into space.

Now, observations by the new Cosmic Origins Spectrograph (COS) aboard Hubble suggest that powerful “winds” from the star are sweeping the cast-off material behind the scorched planet and shaping it into a comet-like tail.

“Since 2003 scientists have theorised that the lost mass is being pushed back into a tail and have even calculated what the tail looks like,” says astronomer Jeffrey Linsky of the University of Colorado in Boulder, leader of the COS study.

“We think we have the best observational evidence to support that theory. We have measured gas coming off the planet at specific speeds, some coming toward Earth.”

“The most likely interpretation is that we have measured the velocity of material in a tail.”

Hubble's Cosmic Origins Spectrograph

Hubble's Cosmic Origins Spectrograph can split starlight into its spectrum, enabling astronomers to detect the signatures of chemical elements.

Atmosphere escaping into space

HD 209458b has a mass slightly less than that of Jupiter, but it orbits 100 times closer to its star than Jupiter does. This means the roasted planet zips around in a mere 3.5 days. (In contrast, our Solar System’s speedster, Mercury, orbits the Sun in a leisurely 88 days.)

The planet is one of the most intensely scrutinised exoplanets (ones in other star systems) because it is one of the few known alien worlds that can be seen passing in front of, or transiting, its star. The transit causes the star’s light to dim slightly.

In fact, the gas giant is the first alien world discovered to transit its parent star. It orbits the star HD 209458, located 153 light-years from Earth.

Linsky and his team used COS to analyse the planet’s atmosphere during transiting events. During a transit, astronomers can study the structure and chemical makeup of a planet’s atmosphere by sampling the starlight that passes through it.

The dip in starlight due to the planet’s passage, excluding the planet’s atmosphere, is very small, only 1.5 percent. When the atmosphere is added, the dip jumps to 8 percent, indicating a bloated atmosphere.

COS detected the heavy elements carbon and silicon in the planet’s super-hot (1,100-degree-Celsius) atmosphere. This detection reveals that the parent star is heating the entire atmosphere, dredging up the heavier elements and allowing them to escape the planet.

The COS data also showed that the material leaving the planet was not all travelling at the same velocity.

“We found gas escaping at high velocities, with a large amount of this gas flowing toward us at [35,000 kilometres per hour],” Linsky explains.

“This large gas flow is likely gas swept up by the stellar wind to form the comet-like tail trailing the planet.”

Artist's illustration of the gas giant planet WASP-12b

Artist's illustration of the gas giant planet WASP-12b, whose atmosphere is spilling onto its parent star.

The power of Hubble

Hubble’s newest spectrograph, with its ability to probe a planet’s chemistry at ultraviolet wavelengths that are not accessible to ground-based telescopes, is proving to be an important instrument for probing the atmospheres of “hot Jupiters” like HD 209458b.

Astronomers have also used COS to sample the atmosphere of another baked planet, WASP-12b, whose puffy atmosphere is spilling onto its star.

Another Hubble instrument, the Space Telescope Imaging Spectrograph (STIS), studied HD 209458b in 2003. The STIS data showed an active, evaporating atmosphere, and a comet-tail-like structure was suggested as a possibility.

But STIS wasn’t able to obtain the spectroscopic detail necessary to show an earthward-moving component of the gas during transits.

Because of COS’s unique combination of very high ultraviolet sensitivity and good spectral resolution, the earthward moving component of the gas—the tail—could be directly detected for the first time.

Although this “extreme” planet is getting roasted by its star, it won’t be destroyed anytime soon. “It will take about a trillion years for the planet to evaporate,” Linsky says.

Adapted from information issued by the Space Telescope Science Institute / NASA, ESA, and G. Bacon (STScI).

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