Convection and Dynamo Action in Massive Stars

Abstract

Contact between numerical simulations and observations of stellar magnetism is sought, with an emphasis on those stars that are the most readily observed and those that may have magnetic activity cycles: the Sun, F-type, and B-type stars. Two approaches are taken in studying stellar dynamos and dynamics, utilizing three-dimensional MHD simulations run on massively parallel supercomputers with the full spherical geometry and employing a new compressible code in the spherical wedge geometry. A 3-D MHD simulation of the solar dynamo that utilizes the Anelastic Spherical Harmonic (ASH) code is presented. This simulation self-consistently exhibits four prominent aspects of solar magnetism: activity cycles, polarity cycles, the equatorward field migration, and grand minima. The ASH framework and this simulation's ability to capture many aspects of the solar dynamo represent a first step toward a more complete model of the Sun's global-scale magnetic activity and its cycles. The dynamics and dynamos of F-type stars are studied through global-scale ASH simulations, with significant contact made between the observed differential rotation and magnetic cycle periods of these stars and those achieved in the simulations. Separately, ASH simulations of core convection in the massive B-type stars show that generation of superequipartition magnetic fields with peak strengths above 1 MG is possible within their cores, which has implications for the evolution of these stars as well as for the properties of their remnants. The internal waves excited by overshooting convection and rotation in these star's radiative exteriors are assessed for their asteroseismic signatures. The results of 3-D compressive MHD simulations of the solar near-surface shear layer with the Compressible Spherical Segment (CSS) code are shown, with such layers arising in the coupled dynamics of ASH and CSS as well as in a more rapidly rotating, thin convective envelope of an F-type star