Using images from ESO’s Very Large Telescope (VLT) in Chile and the NASA/ESA Hubble Space Telescope, a team of astronomers has discovered unique structures within the dusty disc around the nearby star AU Microscopii. According to the team, these fast-moving wave-like structures are unlike anything ever observed, or even predicted, before now.
This image shows the sky around AU Microscopii. It was created from images forming part of the Digitized Sky Survey 2. AU Microscopii is an orangish star of moderate brightness in the centre of the image. Image credit: ESO / Digitized Sky Survey 2.
AU Microscopii lies in the southern constellation Microscopium and is about 29 light-years away.
It is a young star surrounded by a large disc of dust. The disc essentially comprises asteroids that have collided with such vigor that they have been ground to dust.
The star is an optimal object to observe because the disk is tilted edge-on to our view from Earth. This allow for certain details in the disk to be better seen.
Astronomers have been searching AU Microscopii’s disc for any signs of clumpy or warped features, as such signs might give away the location of possible planets.
And in 2014 they used imaging capabilities of VLT’s SPHERE instrument for their search – and discovered something very unusual.
Five wave-like arches at different distances from the star show up in the SPHERE images, reminiscent of ripples in water.
“Our observations have shown something unexpected. The images from SPHERE show a set of unexplained features in the disc which have an arch-like, or wave-like, structure, unlike anything that has ever been observed before,” said Dr Anthony Boccaletti of the Paris Observatory, lead author on a study published in the journal Nature.
Top and middle rows show Hubble images of the AU Microscopii disc from 2010 and 2011, the bottom row is an image taken with VLT’s SPHERE instrument in 2014. The black central circles show where the brilliant light of the central star has been blocked off to reveal the much fainter disc, and the position of the star is indicated schematically. The scale bar at the top of the picture indicates the diameter of the orbit of the planet Neptune in the Solar System (60 AU). Image credit: ESO / NASA / ESA.
After spotting the features in the SPHERE data Dr Boccaletti and his colleagues turned to earlier Hubble images of the disk, taken in 2010 and 2011.
The astronomers were not only able to identify the features on the earlier Hubble images – but the team also discovered that the features had changed over time. They report that these ripples are moving – and they are moving very fast.
“We reprocessed images from the Hubble data and ended up with enough information to track the movement of these strange features over a four-year period. By doing this, we found that the arches are racing away from the star at speeds of up to about 22,000 miles per hour (10 km per second),” said co-author Dr Christian Thalmann of the Swiss Federal Institute of Technology in Zurich, Switzerland.
“Because nothing like this has been observed or predicted in theory we can only hypothesize when it comes to what we are seeing and how it came about,” said co-author Dr Carol Grady of Eureka Scientific in Oakland, California.
The features further away from the star seem to be moving faster than those closer to it. At least three of the features are moving so fast that they could well be escaping from the gravitational attraction of the star.
Such high speeds rule out the possibility that these are conventional disc features caused by objects – like exoplanets – disturbing material in the disc while orbiting the star.
There must have been something else involved to speed up the ripples and make them move so quickly, meaning that they are a sign of something truly unusual.
This set of images of a 40 billion-mile diameter edge-on disk encircling AU Microscopii reveals a string of wave-like features. Image credit: NASA / ESA / ESO / A. Boccaletti, Paris Observatory.
Dr Boccaletti and co-authors have also ruled out a series of phenomena as explanations, including the collision of two massive and rare asteroid-like objects releasing large quantities of dust and spiral waves triggered by instabilities in the system’s gravity. But other ideas that they have considered look more promising.
“One explanation for the strange structure links them to the star’s flares,” said co-author Dr Glenn Schneider of Steward Observatory.
“AU Microscopii is a star with high flaring activity – it often lets off huge and sudden bursts of energy from on or near its surface.”
“One of these flares could perhaps have triggered something on one of the planets – if there are planets – like a violent stripping of material which could now be propagating through the disc, propelled by the flare’s force,” he said.
The astronomers plan to continue to observe the AU Microscopii system with SPHERE and other facilities to try to understand what is happening.
But, for now, these curious features remain an unsolved mystery.
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Anthony Boccaletti et al. 2015. Fast-moving features in the debris disk around AU Microscopii. Nature 526, 230–232; doi: 10.1038/nature15705
