Thursday 24 Mar 2016: Joys of Convective Stars and Their Magnetic Dynamos (Joint Astrophysics & GAFD Seminar)
Prof. Juri Toomre - JILA & Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, USA
The rich magnetism displayed by many stars, including our sun, must have their origin in dynamo action proceeding within their convection zones involving highly turbulent flows influenced by rotation and stratification. The striking advances in seismic probing of stars is providing guidance about rotation states, and possibly of magnetism, deep within a range of stars. This is complemented by supercomputing advances that now permit 3-D global simulations, such as with our anelastic spherical harmonic (ASH) code, of solar and stellar convection to study the nature of magnetic dynamo action and differential rotation that can be achieved by complex flows involving a hierarchy of scales and patterns. In rotating sun-like stars with convective envelopes, we have found using ASH modeling that the resulting magnetic fields can exhibit remarkable large-scale structure involving wreaths of strong magnetism amidst the turbulent convection. These wreaths can involve temporal flips and even quite regular cycles. In one prominent case that cycling with reversals was interrupted by a more quiescent interval, after which the cycling resumed, not unlike a Maunder Minimum. In more massive B-type rotating stars that have convective cores and radiative envelopes, we find with our ASH studies that core dynamo action can yield super-equipartition magnetic fields with mega-gauss strengths. These intense fields may have implications in later stages of evolution, whether in the cores of red giants or in the survivors of supernova explosions. These overlaps in observations and computational theory make for a period of major adventures in stellar astrophysics.