discovered_01_2016 - The Link Between Simulation and Experiments

WWW.HZDR.DE discovered 01.16 PORTRAIT "Astrophysicists are sometimes a tad skeptical about our experiments," says Frank Stefani, grinning about his experiences with his colleagues. That is one of the reasons why the Dresden researcher is even more delighted that he has been able to welcome an expert in the field to the HZDR Institute of Fluid Dynamics: since the beginning of December 2015, George Mamatsashvili from Tbilisi State University has been in residence thanks to a Georg Forster Research Fellowship awarded by the Alexander von Humboldt Foundation. The Georgian astrophysicist will spend two years at HZDR – bringing the stars to Rossendorf – because he wants to elucidate how stars acquire their mass. Or rather: he is interested in magnetorotational instability (MRI). This magnetic effect plays an important role both in the formation of stars and planets as well as in the emergence of black holes in the center of galaxies. "MRI describes how magnetic fields can cause turbulence in what are basically stable flows," George Mamatsashvili explains. "And it is precisely this phenomenon that occurs in the universe." Around black holes and young stars, so-called accretion disks orbit – ring-shaped disks of gas and dust. "Because according to Kepler’s laws of planetary motion the angular momentum increases towards the periphery, these disks are actually very stable. That is why the particles of matter shouldn’t accrete onto black holes or stars." How do black holes and young stars get their mass? Only when they are decelerated does the centrifugal force become too weak to keep them in orbit, the Humboldt Fellow explains. "And this is what MRI does. It de-stabilizes the disk and at the same time causes mass to be transported inwards and angular momentum outwards." However, for this, the matter in the disks must at least be able to conduct minimal amounts of electricity. In dead zones or at the outermost edges this is not always the case which is why the effect of standard MRI – with a purely vertical magnetic field being applied – is controversial. One solution was delivered by Frank Stefani and colleagues at HZDR, together with the Leibniz Institute for Astrophysics in Potsdam back in 2006, when they complemented the vertical magnetic field of the standard MRI with a circular one. By doing so, using even just low magnetic field strength and rotation speeds, researchers were able to create a special variant of the phenomenon in the lab for the first time: helical MRI. "But there is a flaw," Stefani admits. "Helical MRI only destabilizes rotation profiles that drop relatively steeply towards the outside, and this is not initially the case with accretion disks." A couple of years ago, the Dresden researcher managed to rebut this argument with an expanded theory. As he and his colleagues showed, helical MRI can also operate in cases such as this – at least it will, provided that some small part of the circular magnetic field has been generated in the disk itself: which is a perfectly realistic assumption. Unique opportunities It was these theories, and especially the experiments, that attracted George Mamatsashvili to Dresden. "Apart from Dresden, there are only a couple of labs in the world that are investigating this phenomenon experimentally and not just numerically." His stay at the Dresden research center // Thanks to a Georg Forster Research Fellowship, George Mamatsashvili is spending two years at the HZDR. He is not only building a bridge between Georgia and Germany but also between astrophysics and fluid dynamics. _TEXT . Simon Schmitt THE LINK BETWEEN SIMULATION AND EXPERIMENTS George Mamatsashvili

Seitenübersicht