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Center for Nonlinear Studies |
Accretion is an important process in astrophysics. It is invoked to explain the origin of compact objects like planets, stars and massive black holes. The gravitational energy released during accretion powers some of the most energetic phenomena in the universe. In many cases, the accreting material is in the form of a disc orbiting the central compact object. The most remarkable thing about these types of accretion discs is that they accrete at all. It is easy to show that in order for the material to fall onto the central object it must loose angular momentum. Thus, in a disc, the accretion rate is controlled by the outward transport of angular momentum. Transport by collisional processes like viscosity is too inefficient to explain the observed (or inferred) accretion rates therefore it is commonly assumed that the discs must be turbulent. The origin of disc turbulence however is only partially understood. It is known that magnetic fields play an important role in destabilizing the disc. The resulting magneto-rotational turbulence can develop in discs with rotational profiles that are stable to hydrodynamics perturbations. Furthermore the resulting motions do indeed transport angular momentum in the right direction—namely outward. One interesting possibility is that the turbulence may regenerate, by dynamo action, the very magnetic fields that are needed to sustain itself. That being the case, discs could self-magnetize to a level that only depends on the disc properties, but not on externally imposed quantities, like the magnetic flux threading the disc. In this talk, I will formulate this problem within the framework of two idealized configurations, shearing boxes, and cylindrical Couette flow. I will review the results of various numerical efforts, and assess to what extent we can currently answer the question about the self-magnetization of accretion discs.
Host: Robert Ecke, CNLS, 667-1444, ecke@lanl.gov