According to the classical understanding of newly-born neutron stars they should rotate more than 1000 times per second. Observations tell, however, that they rotate "only" 10 to 100 times per second and that their specific angular momentum is with 1014 cm2/s only 1% of the predicted values for angular momentum conservation. Neutron stars share this strong discrepancy with white dwarfs for which new spectroscopic observations give a maximum surface rotation rate of 10 km/s. This also leads to low values of the specific angular momentum, much less than expected. How is it possible?
Neutron stars and white dwarfs are the condensed remnants of stars after their giant phases. To understand their relatively slow rotation it is necessary to assume a transport mechanism for the angular momentum from the core outwards during the lifetime of the stars. It is not possible to explain this process without magnetic fields because of the low value of the gas viscosity. The current-driven magnetic Tayler instability has been proposed as a hypothetic candidate to enhance the effective viscosity, whose existence, however, was an open question until now. The laboratory experiment "GAllium Tayler Experiment" (GATE) successfully attacked this question and reveals the Tayler instability as a new powerful member of the family of magnetic instabilities.
Scientists from AIP and Helmholtz-Zentrum Dresden-Rossendorf realized a small part of the stellar interior in a cylindrical column with a diameter of only 10 cm filled with liquid gallium (Fig. 3). A strong electrical current along the axis leads to strong toroidal fields expected to exist also inside the stars. For all electric currents weaker than 2,600 Ampere the produced magnetic field simply follows the classical Maxwell relation (Fig. 1, left) but stronger currents become unstable and generate nonaxisymmetric magnetic field patterns (Fig. 1, right). The measured growth rates of the field perturbations are shown in Fig. 2 compared with the theoretical prediction. For weakly supercritical currents the data agree almost perfectly. Deviations for stronger currents indicate that the unstable system tends to reach another more stable state. It is clear that the experimental results will lead to new and more detailed computer models for the interior of stars and also of the Sun. A better understanding of important transport processes in magnetized stars such as heat transport and mixing of chemicals will be the consequence of this experiment. Also technical applications to stabilize strong electric current in liquid conductors are possible.
The GATE experiment as successor of the awarded "PROMISE" experiment, that proved the existence of the so-called magnetorotational instability (MRI) in 2010, represents the second successful team work of astronomers from AIP with scientists at HZDR to shed more light on stars in the lab.
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| Figure 1. Computer simulation of the magnetic field structure (toroidal field) for subcritical (left) and supercritical (right) electric currents. The resulting non-axisymmetric magnetic pattern is reminiscent of the so-called kink instability in plasma physics. Credit: Leibniz Institute for Astrophysics |
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| Figure 2. Comparison of the observed (cross symbols) and calculated (solid line) growth rates of the GATE experiment. The measured value has always to stay below the theoretical limit. Credit: Leibniz Institute for Astrophysics |
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| Figure 3. Set up of the GATE experiment. The electric current passes through the cylinder with liquid gallium from top to bottom. From a certain critical current on, the induced magnetic field starts to deform and the liquid gallium developes a non-axisymmetric flow pattern. Credit: Leibniz Institute for Astrophysics |
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| Figure 4. Turbulent flow pattern of a strongly supercritical configuration. The liquid metal starts to move irregularly. Additionally to the flow surface field lines of the magnetic field are drawn. Credit: Leibniz Institute for Astrophysics |
Sources:
- GATE: Why do neutron stars rotate so slowly?, Leibniz-Institut für Astrophysik Potsdam, June 15, 2012
- G. Rüdiger, M. Gellert, M. Schultz, K. Strassmeier, F. Stefani, T. Gundrum, M. Seilmayer, G. Gerbeth: Critical field and growth rate in a columnar Gallium-Tayler experiment, Astrophysical Journal, submitted (Preprint: arXiv:1201.2318 )
- M. Seilmayer, F. Stefani, T. Gundrum, T. Weier, G. Gerbeth, M. Gellert, G. Rüdiger: Experimental evidence for Tayler instability in a liquid metal column, PRL, in press (Preprint: arXiv:1112.2103)
- M. Gellert, G. Rüdiger: Eddy diffusivity from hydromagnetic Taylor-Couette flow experiments, Phys. Rev. E 80, 046314
