Dimming star: Cometary commentary

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  • Published: Dec 1, 2015
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Dimming star: Cometary commentary

The WTF star

This illustration shows a star behind a shattered comet. Observations of the star KIC 8462852 by NASA's Kepler and Spitzer space telescopes suggest that its unusual light signals are likely from dusty comet fragments, which blocked the light of the star as they passed in front of it in 2011 and 2013. The comets are thought to be traveling around the star in a very long, eccentric orbit. Credit: Illustration by NASA/JPL-Caltech

Data from the Infrared Array Camera of NASA's Spitzer Space Telescope reported in Astrophysical Journal Letters suggest that fragments of a shattered comet are the explanation of the dimming of the mysterious "double" star KIC 8462852 that seemed to defy the laws of physics.

A star known by the name KIC 8462852 had astronomers all aglow because it seemed to be dimming. They, and other observers, cast around for a theory to explain the phenomenon. Was it a catastrophic collision in the star's asteroid belt? Was there a giant impact that disrupted a nearby planet? A dusty cloud of rock and debris entering the system? One theory even suggested that it was a megastructure built by an alien intelligence to harvest the star's energy?

KIC 8462852 is less formally known as Tabby's star, after the lead author of the discovery paper, Tabetha S. Boyajian of Yale University, or even less formally as the WTF star ("Where's The Flux?"). The star lies in the constellation of Cygnus, the swan, 1480 light years from Earth. In that paper, now published in Monthly Notices of the Royal Astronomical Society, citizen scientists, working as part of the Planet Hunters project, are cited as having tagged the star's deep and irregular dips in brightness as "bizarre" and "interesting". The unusual light fluctuations caused quite a stir in September 2015 when they were reported in the media. Now, Massimo Marengo of Iowa State University has taken a closer look at the observations and amid all that alien speculation suggests that the evidence points to a shattered comet as being the culprit.

Bright stars of citizen science

The original citizen scientist team had highlighted measurements of star brightness recorded by NASA's Kepler spacecraft. Tiny dips in a star's brightness can hint at the presence of a planet in orbit around the star. But Tabby's star had much bigger dips in its brightness than would be due to an orbiting planet eclipsing the star as viewed from Earth; the dips were up to 22 percent. "The unusual features of this star are the large dips, which happened with an irregular frequency and lasted from days (in the first occurrence) or from weeks (in the second occurrence)," Marengo told SpectroscopyNOW. "Apart from this behaviour the star does not show irregular change of brightness, but only a very small modulation due to star-spots and other phenomena expected for a star of this kind." A search of the more than 150,000 stars in Kepler's database at the time revealed nothing else like it.

Now, Marengo and colleagues have looked at star data recorded with the Infrared Array Camera of NASA's Spitzer Space Telescope and reckon they have an explanation that does not hinge on the existence of alien technology, as first suggested by Jason Wright and colleagues at Penn State University . "The scenario in which the dimming in the KIC 8462852 light curve were caused by the destruction of a family of comets remains the preferred explanation," Marengo and colleagues Alan Hulsebus, an Iowa State doctoral student; and Sarah Willis, a former Iowa State graduate student now with the Massachusetts Institute of Technology’s Lincoln Laboratory, write. Boyajian and colleagues had suggested several plausible explanations with their "most promising theory" being a barrage of crumbling comets passing in front of the star.

Cometary conclusion

When the Iowa State astronomers studied the star with Spitzer infrared data from January 2015 – two years after the Kepler measurements – Marengo said they didn't see much. If there had been some kind of catastrophe near the star, he said, there would be a lot of dust and debris. And that would show up as extra infrared emissions. Marengo explains that their study looked at two different infrared wavelengths: the shorter was consistent with a typical star and the longer showed some infrared emissions, but not enough to reach a detection threshold. The astronomers concluded there were no excess infrared emissions and therefore no sign of an asteroid belt collision, a giant impact on a planet or a dusty cloud of rock and debris. The most likely explanation must be the destruction of a family of comets near the star. The fast-moving comet fragments approach the star in their steep, elliptical orbit and could create a big debris cloud that could dim the star. Then the cloud would move off, restoring the star’s brightness and leaving no trace of excess infrared light.

Marengo admits his team didn't look for the alien megastructure. "We can’t really say it is, or is not. But what the star is doing is very strange. It's interesting when you have phenomena like that - typically it means there's some new physical explanation or a new concept to be discovered."

Related Links

Astrophys J 2015, 814, L15: "KIC 8462852: The Infrared Flux". Open access preprint is available here.

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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