Free Astronomy Magazine November-December 2015

SPACE CHRONICLES these strange fea- tures over a four- year period,” ex- plains team mem- ber Christian Thal- mann (ETH Zürich, Switzerland). “By doing this, we found that the arches are racing away from the star at speeds of up to about 40,000 kilo- metres/hour!” The features fur- ther 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 spe- eds rule out the possibility that these are conventional disc features caused by objects — like planets — disturbing material in the disc while orbiting the star. The edge-on view of the disc complicates the interpre- tation of its three-dimensional struc- ture. There must have been some- T his image shows the sky around the nearby young star AU Microscopii. It was created from images forming part of the Digi- tized Sky Survey 2. AU Mic appears just below the centre of the image as an orangish star of moderate brightness. Because the photographs through dif- ferent coloured filters that were used to make this picture were taken many years apart, AU Mic ap- pears double, as the star’s own proper motion has moved it a small distance across the sky in the inter- vening time. [ESO/Digi- tized Sky Survey 2] thing else involved to speed up the ripples and make them move so quickly, meaning that they are a sign of something truly unusual. “Everything about this find was pretty surprising!” comments co-au- thor Carol Grady of Eureka Scientif- ic, USA. “And because nothing like this has been observed or predicted in theory we can only hypothesise when it comes to what we are see- ing and how it came about.” The team cannot say for sure what caused these my- sterious ripples around the star. But they have con- sidered and ruled out a series of phe- nomena as expla- nations, including the collision of two massive and rare asteroid-like objects releasing large quantities of dust, and spiral waves triggered by instabilities in T his short timelapse sequence shows images of the debris disc around the nearby star AU Micro- scopii in 2010, 2011 and 2014. The two earlier images were from the NASA/ESA Hubble Space Telescope and the final one from the new SPHERE instrument on ESO's Very Large Telescope. [ESO, ESA & NASA] the system’s gravity. But other ideas that they have considered look more promising. “One expla- nation for the strange structures links them to the star’s flares. AU Mic is a star with high flaring ac- tivity — it often lets off huge and sudden bursts of energy from on or near its surface,” explains co-au- thor Glenn Schneider of Steward Observatory, USA. “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.” “It is very satisfying that SPHERE has proved to be very capable at studying discs like this in its first year of operation,” adds Jean-Luc Beuzit, who is both a co-author of the new study and also led the de- velopment of SPHERE itself. The team plans to continue to ob- serve the AU Mic system with SPHERE and other facilities, includ- ing ALMA, to try to understand what is happening. But, for now, these curious features remain an un- solved mystery. n