A team of mainly Dutch astronomers has observed moving shadows on a dust disk around a star. On multiple days they took a 'photo' of the star and its disk. They used the SPHERE instrument, partially built in the Netherlands, on the Very Large Telescope in Chile. Probably, processes in the inner disk cast their shadows at the outer disk. The astronomers publish their findings in The Astrophysical Journal.

The discovery builds on an earlier publication in which the researchers made one image of the disk. By making multiple images, the astronomers clearly saw variations in the shadows. As a result, they could study the shadows in more detail.

The astronomers observed the shadows near the star HD 135344B. That's a young star at a distance of about 450 light-years from Earth. The dust disk around the star shows striking spiral arms. The researchers suspect that they are caused by one or more heavy protoplanets that will evolve into Jupiter-like worlds.

The astronomers saw subtle variations of brightness in the outer dust disk. They presume this is because the gas and dust in the inner disk quickly turn around the star. The astronomers do not know yet which process causes the quick turning of the dust.

"It may be winds, or swirls or clashes of pebbles." says Tomas Stolker who is the first author of the paper about the shadows. Stolker is now postdoc at ETH in Zurich (Switzerland). At the time of the observations, he was a PhD student at the University of Amsterdam.

SPHERE is one of the newest instruments on ESO's Very Large Telescope at Cerro Paranal in northern Chile.

The instrument has been partially developed and built in the Netherlands. It uses adaptive optics to correct for the vibrant Earth atmosphere. Furthermore, it has a coronagraph that blocks most of the starlight. In addition, polarization filters remove the last residue of star light. Finally, an image remains of the dust disk around the star.

Stolker: "Two years ago, we already expected that the shadows on the outer disk were caused by processes in the internal disk. Unfortunately, we cannot see that part of the disk directly with SPHERE. But due to additional observations with SPHERE, we observed the shadows on the outer disk better, and therefore we now know more about the inner disk."

In the future, researchers would like to make an image with SPHERE every few days. Stolker: "And if we also do photometric and spectroscopic observations at the same time, we can exclude certain scenarios."

Research Report: "Variable Dynamics in the Inner Disk of HD 135344B Revealed with Multi-Epoch Scattered Light Imaging," Tomas Stolker, Mike Sitko, Bernard Lazareff, Myriam Benisty, Carsten Dominik, Rens Waters, Michiel Min, Sebastian Perez, Julien Milli, Antonio Garufi, Jozua de Boer, Christian Ginski, Stefan Kraus, Jean-Philippe Berger and Henning Avenhaus. To appear in the Astrophysical Journal

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