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Mysterious ripples racing through planet-forming disk

An international team of astronomers, including Leiden’s Christian Ginski, has discovered previously unobserved structures in a debris disk around a nearby star. Article published in Nature on 8 October.

Het SPHERE-instrument
The SPHERE instrument


The astronomers noticed fast-moving, wavelike arcs in the disc around the star AU Microscopii, 32 light years away from earth. For their research they used the SPHERE instrument, which was partially built in the Netherlands.

AU Microscopii, or AU Mic for short, is a young, nearby star surrounded by a large disk consisting of dust and debris. This is the sort of debris disk in which planets are created. In 2014, astronomers first studied the star using the new instrument on ESO’s Very Large Telescope in northern Chile. The results from those observations have now been processed. In the shots taken in 2014, five wavelike arcs can be seen at various distances from the star. They are reminiscent of ripples in water, but on an enormous scale.

Moving at over 40,000 km an hour

When the researchers saw these odd structures, they decided to take a look at older images taken by the Hubble Space Telescope in 2010 and 2011. The unusual structures could also be seen on those images. The astronomers laid out all the images in a row and were able to see that the ripples had moved, And very quickly, at that! The arcs move away from the star at a speed of more than 40,000 kilometres an hour. Speeds that high exclude the possibility that the ripples were created, for example, by a planet making its way through the disk like a vacuum cleaner. 

One of the explanations for the strange ripples is that AU Mic’s enormous solar flares may have removed material from a planet. Under that scenario, that material, propelled by the power of such an outburst, is now flying through the debris disk.

Further research

The team of astronomers is going to continue studying AU Mic using SPHERE, as well as other observatories, such as ALMA. The scientists think that this research provides insight into how debris disks and planets are created

Dutch contribution

The SPHERE instrument was partially developed by the Netherlands. The acronym stands for Spectro-Polarimetric High-contrast Exoplanet REsearch. The instrument also has the nickname ‘the Very Large Telescope’s planet finder’. Dutch astronomers, brought together in the Dutch Research School for Astronomy (NOVA), have been involved with SPHERE from the outset, first in feasibility studies and later in the design and construction. The instrument has been in use on the Very Large Telescope since 2014.

What is special about SPHERE is that it can pick up faint light in the vicinity of bright light. Astronomers sometimes compare this to photographing a firefly by a lamppost. SPHERE has three special cameras, as well as filters and software, that filter the bright starlight out of the faint light emanating from planets and debris disks.

Anthony Boccaletti et al. Fast-moving features in the debris disk around AU Microscopii. 526, 230–232, 08 October 2015

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