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(Â鶹ÒùÔº)—An international team of astronomers, led by Markus Janson of Stockholm University in Sweden, has discovered mysterious sharp symmetric features around young double star, named AK Sco. The discovery is baffling scientists as they are still unsure about the nature of these features, pondering the possibilities that they are highly eccentric rings or two separate spiral arms in the disk around the star. Moreover, these features may have been caused by circumbinary planets interacting with the disk. The results were published on Dec. 14 in the .

AK Sco, located about 460 light years from Earth, is a spectroscopic binary star in the Upper Centaurus–Lupus (UCL) stellar association. It is a relatively young system, at least in astronomical terms, as scientists estimate it to be from 10 to 20 million years old. Spectroscopic binaries are systems in which the stars are so close together that they appear as a single star even in a telescope. The only evidence of a binary star comes from the Doppler effect on its emitted light. Periodic Doppler shifts of the wavelengths of lines are seen in the spectrum, as the stars move through their orbits.

Janson and his colleagues made the discovery using the European Southern Observatory's (ESO) Very Large Telescope (VLT), located in Chile. The observations were conducted in April 2015 as a part of the Search for Planets Orbiting Two Stars (SPOTS) program. The scientists made use of the Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE) instrument recently installed on VLT. SPHERE is a powerful planet finder and its objective is to detect and study new giant exoplanets orbiting nearby stars using a method known as direct imaging.

Detecting the sharp features in near-infrared imaging of AK Sco's disk was a surprise for the scientists as they expected to find rather exoplanets in the neighborhood. However, what they found, might not be planets at all.

"We report the discovery of resolved scattered light emission from the circumbinary disk around the well-studied young double star AK Sco. The sharp morphology of the imaged feature is surprising, given the smooth appearance of the disk in its spectral energy distribution," the astronomers wrote in the paper.

They researchers have noticed that the central binary star has a semi-major axis of approximately 0.16 astronomical units or AU and that the disk appears to have a gap with an inner rim at 0.58 AU. The images of AK Sco obtained by the SPHERE instrument reveal that the system has 'arms' extending from each side of the central star almost symmetrically. The scientists also found out that these features constitute scattered radiation from off-axis material in the circumbinary disk.

The mysterious sharp features could represent an eccentric ring of material surrounding a gap. The scientists note that such structures are often found in disks that contain rings of material with gaps inside them. However, other observations conducted by ESO's Atacama Large Millimeter Array (ALMA) don't support this theory.

Another explanation taken into account by Janson and his colleagues is that these structures are spiral arms that could be induced through gravitational instability or through the influence of a planet or binary companion. These two spiral arms are wound in opposite directions—one unwinding clockwise and the other counter-clockwise. But the fact that the features are so apparently symmetric, speaks against this hypothesis.

What is worth noticing, either of these scenarios mentioned earlier, may point to circumbinary exoplanets in the disk. The features could be created by one or several planets interacting with the disk.

Whichever proposed theory is true, the authors of the paper highlight the importance of the new generation adaptive optics systems - like this installed on the SPHERE instrument - in the search of disks around stars. They hope that these features will detected more often in near future as a result of implementing new technology of observations.

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